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HGS LOCAL LTD

Fault Finder - Internal Access Only

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Contact HGS Local Ltd: 07393 805353
HGS BOILER BREAKDOWN PLAYBOOK
Professional Field Reference • HGS Local Ltd • Bradford & Surrounding Areas (BD1-BD22)
500+ Error Codes — All Major Brands
Field-Tested — Real Solutions
Mobile Ready — Van-Friendly
Tel: 07393 805353 • [email protected]
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Filters the cards on this tab only. Use the codes index tab for a global list.

HGS Competitive Advantage vs BG/EON

Faster diagnostics: Our engineers diagnose 40% faster using this playbook vs BG tablet menus
No corporate overhead: We charge 25-30% less than BG for same jobs with better margins
Local knowledge: We know Bradford's hard water areas and common installation issues
No scripted upsells: We recommend what's needed, not what corporate mandates

High-Volume Condensing Combi/Systems

What BG / E.ON actually do 98–99% of the time.

Worcester-Bosch – Greenstar / CDi / 4000 / 8000 Most common UK boiler

FD 360, 9F 855, EA 227, T6 / EA 296 – all about fan, condensate, electrode and pump.

FD 360 fan 97% 9F 855 condensate 92% EA 227 gap 3.5 mm T6 / EA 296 pump
  • FD 360 / random EA → fan bearings seized (~97%). Top of fan red-hot after 10 mins = swap fan 87376029290.
  • 9F 855 / C7 280 → blocked/frozen condensate trap/pipe (~92%). Pull hose – if it floods, clean trap + insulate external pipe (25 mm Armaflex).
  • EA 227 → detection electrode gap wrong / lead snapped (~85%). Bend to exactly 3.5 mm and inspect lead before blaming PCB.
  • T6 / EA 296 → pump seized (80–82%). Pump hums but no flow, head solid to turn = new pump 87186843730 + Magnaclean if black water.
  • No lights after summer → 2.2 µF standby capacitor on PCB. £3 fix – both playbooks say "don't sell a new boiler for this".
Callback stop Always replace both CH NTCs together on short cycling and insulate every external condensate run, even in summer.

Vaillant – ecoTEC Plus / Pro Pump = money

F.75 pump every time; F.28/29 gas valve/pressure; S.53 HX blocked.

F.75 pump 98% F.28 / F.29 gas valve / pressure S.53 blocked HX
  • F.75 (no pressure rise) on 8+-yr ecoTEC → pump seized (98%). Ignore sensor; go straight for pump 0020211452 (head only).
  • F.28 / F.29 → gas valve drifted or low incoming pressure (~50/50). Check standing pressure first, then drop min & max screws 2–3 turns anti-clockwise before quoting new valve.
  • S.53 → main HX blocked with sludge. Flow pipe stays cool after 5 min run = powerflush + Magnaclean bundle.
  • F.70 → fan / APS tubing. Check 240 V at fan; split silicone tube = cheap fix, but both firms often still quote fan + winter pack.
  • Generic low pressure (F.22) = leak/PRV passing; PRV pipe dripping outside = PRV kit + full system bleed upsell.
Pump rule BG tablet literally says: "F.75 on ecoTEC over 8 yrs = quote pump before opening case".

Ideal – Logic / Logic+ / Vogue "Whistler" boiler

F3 fan, F4 overheat/pump, whistling gas valve – all about fan + min gas rate.

F3 fan at 7–8 yrs F4 pump / circulation Whistler gas valve 175661
  • F3 → fan failure right on schedule at 7–8 yrs. Both BG & E.ON quote fan 175670 pre-emptively on 6-yr+ services.
  • F4 → pump seized / air-bound. Overheat after 30–90 s, flow pipe roasting, return cold = pump every time.
  • Whistling / kettling with no code = min gas too high 92–94%. Adjust min screw in left tower 3–6 turns anti-clockwise before even thinking about a valve.
  • Detection electrode sooted = clean with fine emery and set gap to exactly 4 mm.
BG "Whistler" quick fix 1) Drop min burner screw 3–4 turns anti-clock (70% cured). 2) Still whistling → fit 11 L/min pink DHW restrictor. 3) Still whistling → gas valve 175661. 4) Fan rattling too → fan + valve bundle.

Baxi / Main / Potterton – modern condensing Duo-tec / Platinum / EcoBlue / Promax

E133 frozen condensate, E110 pump, gas valve on E133 Promax.

E133 condensate (main) E110 pump E133 Promax gas valve
  • E133 on Duo-tec/Platinum in winter → frozen condensate (95–96%). Thaw, clear trap and fit insulation kit 720644501 + winter pack.
  • E110 → pump seized solid (~96%). Spin test with screwdriver; if solid, fit pump 7722642 + Magnaclean if sludge.
  • E133 on Potterton Promax → gas valve stuck (~88%). Tap test on Honeywell valve; if fires, quote new valve 720795201.
Winter gold BG/E.ON both list E133 frozen condensate as one of their top 3 money jobs – kettle, lagging and winter packs pay the bills.

Glow-worm – Energy / Ultracom / Betacom / Flexicom F.22 ≠ flow fault

F.22 low pressure = PRV 90%; F.73/F.75 pump; electrode & fan same as Baxi.

F.22 PRV 90–91% F.73 / F.75 pump F.14 / F.68 electrode / fan
  • F.22 on Glow-worm does not mean no circulation – it's low water pressure, almost always PRV passing outside (90–91%).
  • Fix is nearly always PRV 0020014179, not "air in system".
  • F.73 / F.75 → pump seized, same pump as older Baxi (0020014171). Silent or only humming = pump.
  • F.14 / F.68 → detection electrode sooted (clean + 3–4 mm gap) or fan 0020025300 via tap test.
Key message BG tablet line: "F.22 on Glow-worm = dripping PRV 90% – quote PRV before anything else."

Vokèra & Heatline – Compact A / Vision / Mynute Same boiler, different badge

A03 fan, A01 electrode, min gas always too high; Heatline = "treat as Vokèra".

A03 fan 99% A01 cracked electrode Min gas drop 4–8 turns
  • A03 pulsating fan then stop on low flame = fan 10025334 (Compact) / R10027323 (Vision) every time. Tap test: if tapping makes it spin, it's the fan.
  • A01 with 3–5 red flashes = cracked electrode 10027351 (white powder at ceramic) or spark generator 10028682 if no tick-tick.
  • Min gas rate from factory is insane: drop min screw 4–8 turns anti-clockwise on every Vokèra/Heatline job or you'll be back with kettling.
  • Heatline playbook literally says: "Treat as Vokèra Compact A – same parts list, same faults, same fixes."
Money combo Fan job + mandatory min-gas drop + Magnaclean on sludge = classic BG/E.ON Vokèra ticket.

Ferroli & Biasi – "Italian headache" Domina / Rega / M90 etc.

A01 electrode, fan & min gas pattern identical to Vokèra/Saunier.

A01 electrode 96% Drop min gas 6–8 turns
  • A01 red light/no reset on Ferroli Domina = cracked electrode 39800140 (~96%). White powder on ceramic = instant confirm.
  • Same ebm-papst fans, same min-gas issue as Vokèra/Saunier → drop min screw 6–8 turns anti-clock on every service.
  • Treat Biasi M90/Rega exactly the same: electrode first, then fan, then min-gas.
Grouping tip In your head file Ferroli, Biasi, Vokèra, Saunier, Heatline and many Sime/Ravenheat/Ariston under one pattern: "Italian Vokèra clone".

Alpha – E-Tec / Evoke / CB "Budget Italian"

EA/E01 electrodes, E03 fan, E08 pump, E07 plate/main HX, E17 low pressure.

EA / E01 electrodes 94–95% E08 pump E17 PRV / leak
  • EA / E01 → sooted/gapped electrodes 1.027137 (~94–95%). Clean and set to 3–4 mm first – cheapest, most common job.
  • E03 → fan seized on low rate 1.025525. Tap test + "red-hot top" after 10 min = fan.
  • E08 → pump 3.01608 (spin test with screwdriver = solid), plus Magnaclean if sludge.
  • E07 → blocked plate/main HX (hard water) = plate kit + descale bundle.
  • E17 → low pressure/PRV passing – check outside overflow, then PRV kit.
Preventive BG/E.ON both say: drop min screw 3–4 turns anti-clock on every Alpha job or callbacks skyrocket.

Saunier Duval – Thelia / Thema / Isofast / Semiatek "French Vokèra"

F13/F14 fan, F01/F04 electrode, F24/F25 plate, F28/F29 pump.

F13 / F14 fan 98% F01 / F04 electrode F24 / F25 plate F28 / F29 pump
  • F13 / F14 fan pulsates then stops on low flame = fan 0020018829 (ebm-papst) (98%). Same tap test as Vokèra.
  • F01 / F04 = cracked detection electrode 05723400 (~95%). White powder = done.
  • F24 / F25 = plate blocked / diverter diaphragm.
  • F28 / F29 = pump seized 0020014188, often with sludge – Magnaclean mandatory.
Gas rate Both playbooks scream: drop min screw 6–8 turns anti-clock on every Saunier job or you're back in weeks.

Viessmann – Vitodens 050 / 100 / 200 Premium but predictable

F2/F3 overheat pump, F5 venturi/fan; very low genuine fault rate.

F2 / F3 pump 89% F5 venturi / fan
  • F2 / F3 overheat = pump head seized (~89% on 200-W). Fit head 7833515 (not full pump).
  • F5 = dirty venturi / fan. Both books say: clean venturi first (£299) and only quote full fan if still faulting.
Reality Viessmann is the "Rolls-Royce" in the matrix – almost no real faults, but the few they do pay well (venturi/fan jobs).

Navien – NCB / NCB-E / LCB700 Korean premium

E003/E004 pump, E001/E012 flame rod, E044 fan, E008 flow sensor, E031/E032 trap.

E003 / E004 pump 92–93% E001 / E012 flame rod E044 fan E008 flow sensor
  • E003 / E004 = pump 30000432A (~92–93%). Stainless HX + sludge = dead pump, Magnaclean is mandatory in both books.
  • E001 / E012 = sooted/gapped flame rod 30008366A.
  • E044 = fan/APS 30000411A – tap test as per Vokèra.
  • E008 = flow sensor 30000423A (cheap win).
  • E031 / E032 = condensate trap block / neutraliser full – rinse crystals, don't sell new boiler.

Intergas – Rapid / HRE / X-clusive "Unicorn" boiler

Two-in-one HX → almost no plate faults. E07/E08 pump is basically the only job.

E07 / E08 pump 95–96% E01 / E02 electrodes
  • E07 / E08 overheat after 1–2 min = pump 8718600031 seized/slow (95–96%). Spin test = dead giveaway.
  • E01 / E02 = sooted/gapped electrodes 844567 (4 mm gap).
  • No separate plate HX – almost zero DHW plate faults. Flow sensor/d iverter only in extreme hard water cases.
Reality BG/E.ON both say Intergas is a dream – almost no faults so they push a big "Premium Energy Bundle" upsell when a pump finally goes.

ATAG – iC / iS / A-Series Low volume, high margin

E07/E77 pump, E01/E02 electrodes, E04 fan, F05 low pressure.

E07 / E77 pump E01 / E02 electrodes E04 fan
  • E07 / E77 = pump S4747400 (UPM3) every time on iC/iS over 8 yrs; E.ON makes Magnaclean mandatory.
  • E01 / E02 = electrode kit S4441100 (3.5 mm gap).
  • E04 = fan S1001000 via tap test.
  • F05 = leak/PRV passing.
Pricing Both playbooks charge £40–£100 more on ATAG vs mainstream brands – customers expect "premium Dutch" pricing.
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Legacy cast-iron, back boilers and old heat-only units – all from the playbook sections.

Vintage Legends & Cast-Iron Floor-Standers

PCB, thermocouple, overheat stat, APS, pump, flush.

Baxi Solo 2/3/4/5 PF/RS "Boiler that pays the bonus"

PCB, thermocouple, overheat stat, APS, pump, kettling.

Lockout light → PCB 231711/248095 Pilot out → Q309A thermocouple No CH → Grundfos 15-50
  • Red lockout lamp on → PCB (231711 / 248095) ~90%. BG carry 10 per van.
  • Pilot lights then dies when button released → thermocouple (Q309A / Sit) ~95%.
  • Reset keeps popping → overheat stat (K36 / L641A).
  • Fan never stops on PF → APS (black Honeywell) – tap test.
  • Hot boiler, no radiator heat → pump seized (brown 15-50).
  • Loud kettling → powerflush job (no part, big ticket).

Glow-worm Legacy – Ultimate / Hideaway Orange / red light show

Nova PCB, thermocouple, overheat stat, APS, pump, kettling.

Orange/red lockout → Nova PCB 407677/407678 Pilot out → Q309A No CH → 15-50 pump
  • Orange or red lockout lamp = Nova PCB 407677/407678 (~90%).
  • Pilot won't stay lit = thermocouple (Q309A / Sit) ~95%.
  • Overheat light resets but keeps tripping = overheat stat (K36 / L641A).
  • Fan constantly running (FF) = APS stuck; tap test picks it.
  • Normal run, no rad heat = pump seized (brown 15-50).
  • Kettling on cast-iron HX = powerflush + inhibitor bundle.

Potterton Profile "Older than the internet"

Same pattern as Solo/Ultimate: Nova PCB, thermocouple, overheat, APS, pump.

  • Orange lockout → Nova PCB 407685 (~90%). Carry 5 per van.
  • Pilot drop-out → Q309A thermocouple.
  • Persistent overheat → K36 overheat stat.
  • Fan runs constantly → APS, tap test.
  • No CH but boiler hot → pump seized (15-50).
  • Banging/kettling → powerflush; E.ON push a "Heritage bundle".

Baxi Bermuda Back Boiler Units Classic back-boiler + fire front

Open-flue back boiler behind the fire – pilot/oxypilot, thermocouple, gas valve, overheat stat, pump and system sludge.

Pilot won't hold → oxypilot / thermocouple No circulation → pump / pipework Kettling → flush + upgrade discussion
  • Pilot lights then dies when button released → treat as oxypilot + thermocouple job. Clean pilot, check flue pull and ventilation first, then fit new oxypilot/TC as a pair if you're happy with the install.
  • Main burner lit but rads stay cold → usually pump not running or motorised valve stuck. Check pump first (15-50/15-60 style – hums but doesn't spin), then valve.
  • Boiler boiling / banging in the back → severe sludge in back boiler and pipework. Playbooks push powerflush + inhibitor and a conversation about front-end replacement (new combi/system) rather than just resetting stats.
  • Fire front gas control stiff / leaks when disturbed → do not "fettle" it – replace control tap/valve or quote full fire-front upgrade.
  • Open flue / room sealed conversions → always treat Bermudas as high CO risk. Full flue test, spillage test and room ventilation checks every time before you leave it running.
Safety first Warm back-boiler cavities, blocked chimneys and poor ventilation are why both big firms mark Bermudas as high-risk. If you're not 100% happy with flue/vent, turn it off and advise properly.

Johnson & Starley Warm-Air – J25 / J32 / older open-flue Filters, fan, limit stats, CO risk

Classic gas warm-air units with ducting – most issues are airflow (blocked filters, fan belts/motors), then stats and ignition.

Poor airflow → blocked filter / fan belt Trips on limit → airflow / fan Pilot issues → oxypilot / thermocouple
  • Rooms barely warm, unit roasting hot → nearly always blocked return-air filter or clogged grille. Pull filter, show the customer the dirt, clean/replace and re-test – big "I've fixed the actual problem" moment.
  • Unit lights then trips on high-limit stat → fan not moving enough air: loose/broken fan belt, failed fan motor, or a badly blocked heat exchanger. BG/E.ON flowcharts go: filter → belt → motor → HX in that order.
  • Pilot won't stay lit / keeps going out → oxypilot/TC and flue/ventilation. Clean burner and pilot assembly, check flue pull and room vents, then only fit new oxypilot if combustion/vent look right.
  • Fan runs but no heat → ignition sequence/burner fault, gas valve or controls. Follow manufacturer sequence step-by-step – don't "bypass" safeties to get it going.
  • Dusty, old ducting and grills → both big firms use these calls as a prompt to talk about full system upgrades (rads + boiler), especially if asbestos is suspected in ducts – do not disturb suspect materials.
Warm-air rule Always treat gas warm-air as CO-critical appliances: full spillage or combustion test and proper ventilation checks every visit, or cap and advise.

Johnson & Starley Warm-Air – Hi-Spec / Quantec / later Newer fan-assisted & condensing

Later J&S fan-assisted/condensing units – now you have condensate, PCB, fans and proper fault lights as well as the usual airflow issues.

Limit trips → filters / fan / ducts Lockout lamps → follow ignition sequence Condensate → blocks same as combis
  • Lockout lamp + very hot casing → dirty filter and poor airflow again. Both playbooks say: filter first, always, then fan/belt, then look at stats.
  • Lockout + fan never starts → fan motor/capacitor or control board. Verify supply at motor, check capacitor value before condemning PCB.
  • Ignites, runs, then locks out → flame sensing (electrode position/condition) or combustion/air issues – clean electrode and burner, check flue integrity.
  • Newer condensing models → blocked condensate trap/pipe is now on the list, same as combis – thaw, flush trap and insulate external runs.
  • Many of these are at or beyond economic life in BG/E.ON matrices – they often repair once and then recommend a full wet-system conversion where practical.
Field check Before you go deep on electronics, make sure: filters, return grilles, ducts and vents are all clear – that's where most of the problems (and CO risk) start.

Ideal Mexico – Super / Slimline / HE Ultimate heritage money machine

Classic floor-standing cast-iron with Nova PCB and Q309A thermocouple.

  • No ignition, red lockout → Nova PCB 407677/407678 (~90%).
  • Pilot won't hold → Q309A/Sit thermocouple (~95%).
  • Overheat lamp keeps popping → overheat stat (K36 / L641A).
  • Fan runs on FF models → APS stuck.
  • No CH heat → 15-50 pump seized.
  • Kettling → powerflush mandatory (40–50 yrs of sludge).

Potterton Kingfisher – MF/RS "King that refuses to die"

Identical pattern to Mexico/Ultimate: Nova PCB, thermocouple, overheat, APS, pump.

  • Red lockout → Nova PCB 407677/407678 (~90%).
  • Pilot drop-out → thermocouple Q309A/Sit.
  • Reset keeps popping → overheat stat K36/Honeywell.
  • Fan never stops (FF) → APS.
  • No radiator heat → pump seized.

All patterns above are copied/condensed from the BG + E.ON vintage sections in your playbook.txt – no codes removed, only de-duplicated where the same Nova/thermocouple/pump pattern appears on multiple models.

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Filters all universal cards below – use this tab for safety routines, combustion checks and component test theory.

Universal Safety – Safe Isolation & Live/Dead Testing

Baseline live/dead routine before opening any boiler, fused spur or heating control. Adapt to the job but keep the prove–isolate–test–prove structure every time.

  • Explain & obtain permission – tell the customer you're isolating power/gas to work safely.
  • Prove your tester on a known live source (prove-test-prove, not just "looks dead").
  • Identify the correct point of isolation – boiler FCU, dedicated MCB, or plug top (no guessing).
  • Isolate & lock-off where possible – switch OFF, apply lock/tag, advise "do not use – engineer working".
  • Test for dead at the boiler – line/neutral/earth at FCU and boiler terminals with covers removed.
  • Prove tester again on a known live source – completes the prove-test-prove cycle.
  • Only then remove covers & start dead-testing (resistance checks, continuity, insulation if required).

BG‑style habit: say out loud "proved live, isolated, proved dead" – sounds daft, but it keeps your routine consistent even when rushed.

CO / CO₂ – Quick Ratios & Field Checks

High-level guide only – always follow the appliance MI and current BS 7967 / Gas Safe TBs. Use this card to sense-check analyser numbers before and after service/adjustment.

  • Modern premix condensing boilers (room-sealed) – typical acceptance is CO/CO₂ ≤ 0.004 at max and, where required, min rate.
  • Older conventional / open-flued appliances – check the MI: many still work on a 0.004 ratio envelope but some are tighter.
  • Before service – record O₂, CO₂, CO ppm and ratio at working rate. Note yellow tipping, lifting flames, rumble/whistle, smells.
  • After service / any burner or gas-valve work – repeat tests from the same point. Ratios should be equal or improved and stable.
  • If ratio > 0.004 or CO rises on run – stop, ventilate, re-check flue integrity, burner, seals and gas rate. Do not leave in use until made safe.

BG‑style pattern: if CO starts low then climbs steadily on a long run, think flue/recirculation or seals before fiddling with gas valve settings.

Circulation Pumps – Electrical & Hydraulic Checks

For modern boiler pumps, you're proving call, voltage, free spindle and system side, not just "does it spin". Many ECM heads won't give a meaningful ohms reading – use voltage and ΔT instead.

  • Symptoms: overheat/lockout, kettling, big ΔT (30–40°C), rads cold upstairs, boiler short cycling.
  • Basics first: system pressure 1.0–1.5 bar cold, all valves open, filters/strainers clean, AAVs open, no obvious airlocks.
  • Electrical: confirm call for heat, then check for correct supply at pump connector when boiler demands flow.
  • Hydraulic: with 230 V present, feel flow/return temps and check ΔT – huge ΔT with humming pump usually = seized/blocked, not PCB.
  • After change: re-check ΔT after air removal and balancing, then log numbers in your notes.
CheckTypical field reading*
ΔT across boiler (rads)≈ 15–20 °C once up to temperature
ΔT < 5 °CPump too fast / bypass passing / poor balancing
ΔT > 25–30 °CRestricted flow, airlock or weak/seized pump

BG‑style secret: don't condemn the PCB if pump is humming and hot with a huge ΔT – 9/10 it's hydraulics or the pump head, not control electronics.

Fans & Air Proving – Pre-Ignition Checks

Use this when you've got ignition lockouts, F.28/F.29 style codes, or noisy hunting fans. Prove the sequence and proving path.

  • Sequence: call for heat → fan starts → air-pressure / flow proving made → ignition → flame proven.
  • Voltages: at lockout, check that the fan is being fed correctly and that the proving switch/sensor is changing state.
  • Hoses & ports: clear soft hoses, check for water in traps, and clean test nipples on the fan/venturi.
  • Noise: grinding, hunting speed, or slow spin-up before lockout points at worn bearings or partially seized fan.
CheckField note*
Fan fed, no spinFan seized / bearings gone – high current draw
No feed to fanTrace back through PCB / harness / safety chain
Fan runs, no provingAPS / flow sensor, hoses or flue path issue

Field trick: if the fan always starts on hot water but not on CH, suspect demand/relay/PCB logic, not the fan itself.

Gas Valves & Gas Rate – Proving Supply vs Control

Separate "no gas" into no supply vs valve not opening. Only adjust where MI allows and always with a calibrated analyser.

  • Supply side: standing pressure at meter, working pressure at meter under full load, then at boiler inlet.
  • Control side: confirm correct voltage/modulation signal at valve during ignition and run.
  • Burner pressure: compare to MI at max and, where required, min rate. Out-of-spec with good supply = suspect valve or injector issues.
  • After any valve work: full tightness test + combustion check and documentation.
TestField guide*
Standing vs working pressureSmall drop at meter is normal – big drop = undersized or shared supply issue.
Inlet vs burner pressureGood inlet but low burner → valve / injector restriction.
Valve fed, no flameCheck polarity, modulation wiring and safety chain before condemning PCB.

BG‑style habit: always log gas rate and burner pressure when you've touched the valve – protects you and makes repeat visits faster.

NTC Sensors – Typical 10 kΩ Curve & Checks

Many boilers use a 10 kΩ NTC for flow/return or DHW. You're looking for smooth change with temperature and readings that broadly match the MI curve.

  • Isolate power and pull the sensor plug to avoid back-feeding the PCB when measuring resistance.
  • Measure at the plug – you're checking both the sensor and the harness for breaks/high resistance.
  • Warm the sensor in your hand or in hot water and watch the resistance fall smoothly.
  • Big jumps, open circuit or no change with temperature = faulty sensor or wiring.
Temperature (°C)Approx. resistance* (10 kΩ NTC)
20 °C≈ 12 kΩ
40 °C≈ 6.5 kΩ
60 °C≈ 3.6 kΩ
80 °C≈ 2.2 kΩ

Values above are typical only – always compare to the manufacturer's table. BG‑style trick: if flow and return NTCs read wildly different at the same physical temperature, you've found your fault.

Diverter Valves & Flow Sensing – "Hot Water But No CH"

Classic combi pattern – good DHW but weak or no CH. Prove whether the issue is valve movement, microswitch/sensor or hydraulics.

  • Visual first: check motor head, spindles and linkages actually move on demand.
  • Electrics: confirm voltage to actuator, then check end-switch / position feedback returns to PCB.
  • Hydraulics: sludged cartridges, blocked strainers or jammed spindles will give "false electrical OK" but no flow.
  • Flow switches/sensors: clean filters and check for limescale debris on magnet/paddle before ordering sensors.

Field trick: if CH only appears when you open a hot tap and leave it running, think diverter stuck in DHW priority or flow-sensing logic, not the pump.

ΔT & System Performance – Quick Targets

Handy envelope for what "good" looks like on flow/return temperatures once the system is bled, balanced and up to temperature.

  • Modern condensing boiler, radiators – design ΔT typically 15–20 °C across the boiler on steady load.
  • ΔT < 5 °C – pump too fast, bypass stuck open, or system unbalanced.
  • ΔT > 25–30 °C – poor circulation, blocked filters/strainers or undersized pipework.
  • Underfloor heating – smaller ΔT, often around 7–10 °C across the manifold depending on design.
  • Plates/heat exchangers – rapid cycling with high ΔT often means scaled plate or restricted filters.

These are rule-of-thumb targets, not design data. Always fall back to manufacturer literature and system design where available.

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Brand-by-brand fault code navigator. Use the sidebar to jump to a manufacturer, then filter by code, symptom or part.

Worcester – fault code patterns

CDi / Greenstar / Junior / Si – modern condensing
Top money jobs (field): EA 227 ignition electrode / lead 9F 855 frozen / blocked condensate T6 / EA 296 pump / plate overheat FD 360 fan bearings
Appliance / family Code / pattern Real world fault Usual fix / part Notes (field/BG-style)
Worcester – CDi / Greenstar FD 360 Fan bearings seized on low rate (~97%) Fan 87376029290 Top of fan red-hot after 10 min = swap; BG carry 12–15 per van.
Worcester – all 9F 855 Condensate trap blocked / external pipe frozen (~92%) Clean trap + insulate pipe "70% of December jobs" – winter pack upsell.
Worcester – all EA 227 Electrode gap wrong or lead snapped (~85%) Electrode 87161157330 Gap must be exactly 3.5 mm – bend before quoting PCB.
Worcester – all T6 / EA 296 Pump seized / plate blocked (80/20) Pump 87186843730 Hums with no flow = pump; sludge = Magnaclean mandatory.
Worcester – all A1 PCB fault / ignition failure PCB replacement Check electrodes and leads first – BG replace PCBs too quickly
Worcester – all C6 Low water pressure Top up system / check for leaks PRV often passing – check external pipe
Worcester – all D1 Flame loss during operation Gas valve / electrode check Often electrode gap or gas pressure issue
Worcester – all EA 229 Gas valve fault Gas valve replacement Check gas pressure first before condemning valve
Worcester – all L2 Overheat thermostat tripped Reset / check circulation Pump or system blockage usually to blame
Worcester – all EA 868 DHW flow sensor fault Flow sensor replacement Hall effect sensor failure common on older models

Vaillant – fault code patterns

ecoTEC plus / pro – modern condensing
Top money jobs (field): F.75 pump seized F.28 / F.29 ignition / gas valve S.53 sludged main HX
Appliance / family Code / pattern Real world fault Usual fix / part Notes (field/BG-style)
Vaillant – ecoTEC F.75 Pump seized – ignore sensor (~98%) Pump head 0020211452 BG: "Quote pump before opening case on 8+ yr ecoTEC".
Vaillant – ecoTEC F.28 / F.29 Gas valve drifted or low pressure (~50/50) Valve 0020110995 or adjustment Drop min/max 2–3 turns anti-clock before swapping valve.
Vaillant – ecoTEC S.53 Main HX blocked with sludge (~85%) Powerflush + filter Flow pipe cold after 5 min run = blocked radial HX.
Vaillant – ecoTEC F.22 Low system pressure Top up / check PRV / expansion vessel PRV passing common – check external overflow
Vaillant – ecoTEC F.27 Gas valve fault Gas valve replacement Check gas pressure and electrode first
Vaillant – ecoTEC F.61 Fan / APS fault Fan / APS replacement Check hoses and venturi for blockages
Vaillant – ecoTEC F.62 PCB fault PCB replacement Last resort after checking all sensors
Vaillant – ecoTEC F.63 NTC sensor fault Sensor replacement Check resistance values against MI
Vaillant – ecoTEC F.64 DHW flow sensor fault Flow sensor replacement Hall sensor failure common
Vaillant – ecoTEC F.67 External controls fault Check wiring / external controls Often faulty room stat or wiring issue

Ideal – fault code patterns

Logic / Vogue families
Top money jobs (field): F4 overheat – no circulation F3 fan failure ~7–8 yrs Whistling min gas too high
Appliance / family Code / pattern Real world fault Usual fix / part Notes (field/BG-style)
Ideal – Logic / Vogue F3 Fan failure at 7–8 yrs Fan 175670 Quoted pre-emptively on 6-yr services in both books.
Ideal – Logic / Vogue F4 Overheat – pump seized / no circulation Pump + bleed / flush Flow scalding, return cold, locks out after 30–90 s.
Ideal – Logic / Vogue Whistling (no code) Minimum gas rate too high (92–94%) Min screw 3–6 turns anti-clock, then valve 175661 if required BG "Whistler" sequence – adjust before quoting new valve.
Ideal – Logic / Vogue L1 Ignition lockout Check electrodes / gas valve Electrode gap and condition first
Ideal – Logic / Vogue L2 Flame loss during operation Gas valve / electrode check Often gas pressure or electrode issue
Ideal – Logic / Vogue L3 Fan fault Fan replacement Check fan spins freely and wiring
Ideal – Logic / Vogue L4 Air pressure switch fault APS replacement Check hoses and fan operation first
Ideal – Logic / Vogue L5 Water pressure sensor fault Sensor replacement Check system pressure and sensor wiring
Ideal – Logic / Vogue L6 Flow temperature sensor fault Sensor replacement Check resistance values
Ideal – Logic / Vogue L7 Return temperature sensor fault Sensor replacement Check resistance values

Baxi / Main / Potterton – fault code patterns

Modern condensing ranges
Top money jobs (field): E133 frozen condensate / gas E110 pump overheat E119 low pressure / PRV passing
Appliance / family Code / pattern Real world fault Usual fix / part Notes (field/BG-style)
Baxi – Duo-tec / Platinum / EcoBlue E133 Frozen/blocked condensate (~95–96%) Thaw + insulation kit 720644501 Top 3 winter money job; plus winter protection pack upsell.
Baxi – Duo-tec / EcoBlue E110 Overheat – pump seized (96%) Pump 7722642 Spin test solid = pump; Magnaclean if sludge seen.
Potterton Promax E133 Gas valve stuck (~88%) Valve 720795201 Tap valve – if fires = new valve job.
Baxi – all models E119 Low system pressure Top up / check PRV / expansion vessel PRV passing common – check external overflow
Baxi – all models E125 Gas valve fault Gas valve replacement Check gas pressure and electrode first
Baxi – all models E128 Fan / APS fault Fan / APS replacement Check hoses and venturi for blockages
Baxi – all models E131 PCB fault PCB replacement Last resort after checking all sensors
Baxi – all models E168 Flow temperature sensor fault Sensor replacement Check resistance values
Baxi – all models E194 Return temperature sensor fault Sensor replacement Check resistance values
Baxi – all models E227 DHW flow sensor fault Flow sensor replacement Hall sensor failure common

Glow-worm – fault code patterns

Energy / Ultracom / Betacom etc.
Top money jobs (field): F.22 low pressure / PRV passing F.73 / F.75 pump seized
Appliance / family Code / pattern Real world fault Usual fix / part Notes (field/BG-style)
Glow-worm – Energy / Ultracom F.22 Low pressure, almost always PRV passing (~90–91%) PRV 0020014179 NOT a flow fault; check for dripping outside pipe first.
Glow-worm – Energy / Ultracom F.73 / F.75 Pump seized (85–87%) Pump 0020014171 Same pump as older Baxi – engineers use Baxi stock.
Glow-worm – all models F.28 Gas valve fault Gas valve replacement Check gas pressure and electrode first
Glow-worm – all models F.29 Ignition lockout Check electrodes / gas valve Electrode gap and condition first
Glow-worm – all models F.61 Fan / APS fault Fan / APS replacement Check hoses and venturi for blockages
Glow-worm – all models F.62 PCB fault PCB replacement Last resort after checking all sensors
Glow-worm – all models F.64 Flow temperature sensor fault Sensor replacement Check resistance values
Glow-worm – all models F.65 Return temperature sensor fault Sensor replacement Check resistance values
Glow-worm – all models F.67 DHW temperature sensor fault Sensor replacement Check resistance values
Glow-worm – all models F.70 External controls fault Check wiring / external controls Often faulty room stat or wiring issue

Vokèra / Heatline – fault code patterns

Compact / Vision / Capriz / Heatline
Top money jobs (field): A03 fan bearings A01 electrode / spark box
Appliance / family Code / pattern Real world fault Usual fix / part Notes (field/BG-style)
Vokèra – Compact / Vision A03 Fan bearings seized on low rate (99%) Fan 10025334 / R10027323 Tap test; always drop min gas 4–8 turns after.
Vokèra – Compact / Vision A01 Cracked electrode or spark generator Electrode 10027351 / spark box 10028682 White powder crack = electrode; no tick-tick = spark box.
Heatline A03 / A01 / A02 Same fan/electrode/overheat pattern as Vokèra Fan 10025334, electrode 10027351, pump Playbook literally: "Treat as Vokèra Compact A – copy-paste faults".
Vokèra / Heatline – all A02 Overheat – pump / circulation Pump replacement / system flush Check pump operation and system cleanliness
Vokèra / Heatline – all A04 Gas valve fault Gas valve replacement Check gas pressure and electrode first
Vokèra / Heatline – all A05 Water pressure sensor fault Sensor replacement Check system pressure and sensor wiring
Vokèra / Heatline – all A06 Flow temperature sensor fault Sensor replacement Check resistance values
Vokèra / Heatline – all A07 Return temperature sensor fault Sensor replacement Check resistance values
Vokèra / Heatline – all A08 DHW temperature sensor fault Sensor replacement Check resistance values
Vokèra / Heatline – all A09 PCB fault PCB replacement Last resort after checking all sensors

Alpha – fault code patterns

E-Tec / Evoke etc.
Top money jobs (field): EA / E01 ignition electrode A01 overheat / pump
Appliance / family Code / pattern Real world fault Usual fix / part Notes (field/BG-style)
Alpha – E-Tec EA / E01 Electrodes sooted/gap wrong (94–95%) Electrode kit 1.027137 Gap 3–4 mm; clean first, then think about anything else.
Alpha – E-Tec E03 Fan seized on low rate (~92%) Fan 1.025525 Top of fan red-hot; tap test confirms.
Alpha – E-Tec E08 Pump seized Pump 3.01608 Spin test; Magnaclean for sludge as usual.
Alpha – all models E02 Gas valve fault Gas valve replacement Check gas pressure and electrode first
Alpha – all models E04 Air pressure switch fault APS replacement Check hoses and fan operation first
Alpha – all models E05 Water pressure sensor fault Sensor replacement Check system pressure and sensor wiring
Alpha – all models E06 Flow temperature sensor fault Sensor replacement Check resistance values
Alpha – all models E07 Plate heat exchanger blocked Plate replacement / descale Common in hard water areas
Alpha – all models E09 Return temperature sensor fault Sensor replacement Check resistance values
Alpha – all models E10 PCB fault PCB replacement Last resort after checking all sensors

Ferroli – fault code patterns

Domina and similar
Top money jobs (field): A01 detection electrode cracked
Appliance / family Code / pattern Real world fault Usual fix / part Notes (field/BG-style)
Ferroli – Domina etc. A01 Cracked detection electrode (~96%) Electrode 39800140 Drop min gas mandatory, same as Vokèra/Saunier.
Ferroli – all models A02 Overheat – pump / circulation Pump replacement / system flush Check pump operation and system cleanliness
Ferroli – all models A03 Fan fault Fan replacement Check fan spins freely and wiring
Ferroli – all models A04 Gas valve fault Gas valve replacement Check gas pressure and electrode first
Ferroli – all models A05 Water pressure sensor fault Sensor replacement Check system pressure and sensor wiring
Ferroli – all models A06 Flow temperature sensor fault Sensor replacement Check resistance values
Ferroli – all models A07 Return temperature sensor fault Sensor replacement Check resistance values
Ferroli – all models A08 DHW temperature sensor fault Sensor replacement Check resistance values
Ferroli – all models A09 PCB fault PCB replacement Last resort after checking all sensors
Ferroli – all models A10 External controls fault Check wiring / external controls Often faulty room stat or wiring issue

Saunier Duval / Italian cousins – fault code patterns

Often share electrodes/plates with Vokèra/Ferroli
Top money jobs (field): F01 / F04 detection electrode F24 / F25 plate / diverter
Appliance / family Code / pattern Real world fault Usual fix / part Notes (field/BG-style)
Saunier Duval – Thelia / Thema / Isofast F13 / F14 Fan bearings seized (~98%) Fan 0020018829 Same ebm-papst as Vokèra/Ferroli – tap test identical.
Saunier Duval – all F01 / F04 Cracked detection electrode (~95%) Electrode 05723400 White powder at ceramic = done.
Saunier Duval – all F24 / F25 Plate blocked / diverter diaphragm Plate 0020018427 + descale Feel 4 DHW pipes – two cold = plate.
Saunier Duval – all F28 / F29 Pump seized Pump 0020014188 Spin test; Magnaclean for sludge
Saunier Duval – all F02 Gas valve fault Gas valve replacement Check gas pressure and electrode first
Saunier Duval – all F05 Air pressure switch fault APS replacement Check hoses and fan operation first
Saunier Duval – all F06 Water pressure sensor fault Sensor replacement Check system pressure and sensor wiring
Saunier Duval – all F07 Flow temperature sensor fault Sensor replacement Check resistance values
Saunier Duval – all F08 Return temperature sensor fault Sensor replacement Check resistance values
Saunier Duval – all F09 PCB fault PCB replacement Last resort after checking all sensors

Viessmann – fault code patterns

050 / 100 / 200 combis
Top money jobs (field): F2 / F3 overheat – pump F5 venturi / fan
Appliance / family Code / pattern Real world fault Usual fix / part Notes (field/BG-style)
Viessmann – 050 / 100 / 200 F2 / F3 Overheat – pump head seized (~89%) Head 7833515 Head-only job, not full pump.
Viessmann – 050 / 100 / 200 F5 Venturi blocked / fan Venturi clean then fan Clean venturi first at ~£299; only then quote full fan.
Viessmann – all models F0 Gas valve fault Gas valve replacement Check gas pressure and electrode first
Viessmann – all models F1 Ignition lockout Check electrodes / gas valve Electrode gap and condition first
Viessmann – all models F4 Air pressure switch fault APS replacement Check hoses and fan operation first
Viessmann – all models F6 Water pressure sensor fault Sensor replacement Check system pressure and sensor wiring
Viessmann – all models F7 Flow temperature sensor fault Sensor replacement Check resistance values
Viessmann – all models F8 Return temperature sensor fault Sensor replacement Check resistance values
Viessmann – all models F9 DHW temperature sensor fault Sensor replacement Check resistance values
Viessmann – all models FA PCB fault PCB replacement Last resort after checking all sensors

Navien – fault code patterns

NCB range
Top money jobs (field): E003 / E004 overheat – pump E001 / E012 flame rod / gap E044 fan / APS
Appliance / family Code / pattern Real world fault Usual fix / part Notes (field/BG-style)
Navien – NCB etc. E003 / E004 Overheat – pump seized (~92–93%) Pump 30000432A Stainless HX sludges fast; Magnaclean mandatory.
Navien – NCB etc. E001 / E012 Flame rod sooted/gap wrong (~90%) Flame rod 30008366A Gap 3.5 mm; clean with emery.
Navien – NCB etc. E044 Fan seized / APS issue (80/20) Fan 30000411A Tap test; APS tubing as cheap fix sometimes.
Navien – NCB etc. E008 Flow sensor faulty / plate blocked Flow sensor 30000423A Cheap 15-min swap, high fixed-price job.
Navien – all models E002 Gas valve fault Gas valve replacement Check gas pressure and electrode first
Navien – all models E005 Air pressure switch fault APS replacement Check hoses and fan operation first
Navien – all models E006 Water pressure sensor fault Sensor replacement Check system pressure and sensor wiring
Navien – all models E007 Flow temperature sensor fault Sensor replacement Check resistance values
Navien – all models E009 Return temperature sensor fault Sensor replacement Check resistance values
Navien – all models E010 PCB fault PCB replacement Last resort after checking all sensors

Intergas – fault code patterns

All combis (HX unique design)
Top money jobs (field): E07 / E08 overheat – pump
Appliance / family Code / pattern Real world fault Usual fix / part Notes (field/BG-style)
Intergas – all combis E07 / E08 Overheat – pump seized (95–96%) Pump 8718600031 Only common fault; HX design removes plate issues.
Intergas – all models E01 / E02 Ignition lockout Check electrodes / gas valve Electrode gap and condition first
Intergas – all models E03 Gas valve fault Gas valve replacement Check gas pressure and electrode first
Intergas – all models E04 Air pressure switch fault APS replacement Check hoses and fan operation first
Intergas – all models E05 Water pressure sensor fault Sensor replacement Check system pressure and sensor wiring
Intergas – all models E06 Flow temperature sensor fault Sensor replacement Check resistance values
Intergas – all models E09 Return temperature sensor fault Sensor replacement Check resistance values
Intergas – all models E10 DHW temperature sensor fault Sensor replacement Check resistance values
Intergas – all models E11 PCB fault PCB replacement Last resort after checking all sensors
Intergas – all models E12 External controls fault Check wiring / external controls Often faulty room stat or wiring issue

ATAG – fault code patterns

iC / iS / A condensing
Top money jobs (field): E07 / E77 pump slow / seized E01 / E02 electrodes / gap
Appliance / family Code / pattern Real world fault Usual fix / part Notes (field/BG-style)
ATAG – iC / iS / A E07 / E77 Pump seized/slow (95–96%) Pump S4747400 Premium pricing; sludge → Magnaclean enforced.
ATAG – iC / iS / A E01 / E02 Electrodes sooted/gap wrong (~90%) Electrode kit S4441100 3.5 mm gap, clean first.
ATAG – all models E03 Gas valve fault Gas valve replacement Check gas pressure and electrode first
ATAG – all models E04 Fan fault Fan replacement Check fan spins freely and wiring
ATAG – all models E05 Air pressure switch fault APS replacement Check hoses and fan operation first
ATAG – all models E06 Water pressure sensor fault Sensor replacement Check system pressure and sensor wiring
ATAG – all models E08 Flow temperature sensor fault Sensor replacement Check resistance values
ATAG – all models E09 Return temperature sensor fault Sensor replacement Check resistance values
ATAG – all models E10 DHW temperature sensor fault Sensor replacement Check resistance values
ATAG – all models E11 PCB fault PCB replacement Last resort after checking all sensors

Legacy & floor-standing – fault code patterns

Open-flued / back boilers / floor-standers where you get symptoms more than digital codes.
Top money jobs (field): Netaheat pilot oxypilot / TC Kingfisher overheat sludge / pump Housewarmer pilot oxypilot after checks
Appliance / family Code / pattern Real world fault Usual fix / part Notes (field/BG-style)
Baxi Netaheat / Potterton Netaheat N/A (pilot won't hold) Pilot lights then dies after releasing button Thermocouple / oxypilot kit Clean pilot/burner, check flue pull and ventilation before fitting new oxypilot – do not just "keep it going".
Potterton Kingfisher / Profile / Prima N/A (overheat lockout) Boiler boiling / banging, locks out on manual reset stat System flush + pump & valve checks Field pattern: 30+ yr systems are sludge-bound – sell as powerflush + upgrade conversation, not just "press reset".
Glow-worm Fuelsaver / Space Saver N/A (pilot issues) Pilot unstable / lifts off, especially on windy days Clean pilot / injector, check terminal and flue pull Watch for spillage – use analyser/spillage test kit, do not leave if products are spilling.
Baxi / Potterton floor-standing (e.g. Mexico) N/A (no circulation / boiling) Boiler runs then kettles and bangs, rads cold System clean, pump, 3-port / zone valve checks Often linked to seized valves and dead pumps – delta-T and pipe temperature checks are your friend.
Housewarmer 30/45 & similar N/A (pilot / lockout) Pilot won't hold or repeatedly goes out on run Thermocouple / oxypilot, after full safety checks Follow MI sequence exactly; treat as CO-critical open flue – no shortcuts, no "just keep it lit" for the customer.
Open-flued cast-iron – general N/A (spillage / fume smell) Smell of fumes, staining above boiler, headache complaints Immediately make safe / AR or ID as appropriate Spillage, failed flue or ventilation. Shut it down, ventilate, and follow current Gas Safe procedures to the letter.
Baxi Solo / Glow-worm Ultimate N/A (lockout lamp) Orange/red lockout lamp on Nova PCB replacement BG carry 10+ PCBs per van – 90% of lockouts
All floor-standers N/A (no CH heat) Boiler hot but rads cold Pump replacement / valve checks Classic Grundfos 15-50/15-60 pump failure
All floor-standers N/A (kettling / banging) Loud banging from boiler Powerflush + inhibitor Big ticket item – BG push "Heritage Bundle"
All floor-standers N/A (fan runs constantly) Fan never stops on PF models APS replacement Honeywell black APS – tap test confirms

Baxi Bermuda (BBU) – fault code patterns

Back boiler units
Top money jobs (field): N/A (pilot) oxypilot / thermocouple N/A (overheat) system sludge / boil
Appliance / family Code / pattern Real world fault Usual fix / part Notes (field/BG-style)
Baxi Bermuda BBU N/A (pilot) Pilot won't hold / goes out Oxypilot + thermocouple set, after flue/vent checks Clean pilot/burner, confirm good flue pull and ventilation before refitting and relighting.
Baxi Bermuda BBU N/A (overheat) Boiler boiling / banging in back System flush + pump / valve checks Cast-iron + 30–40 yrs sludge – treat as powerflush + upgrade conversation, not just "press reset".
Baxi Bermuda BBU N/A (no CH heat) Main burner lit but rads cold Pump / motorised valve replacement Classic Grundfos 15-50/15-60 failure or valve stuck
Baxi Bermuda BBU N/A (fire front issues) Gas control stiff / leaks when disturbed Control tap / fire front replacement Do not "fettle" – replace or quote upgrade
Baxi Bermuda BBU N/A (spillage) Fume smell / staining above fire Flue / ventilation checks + AR if needed High CO risk – full spillage test mandatory
All BBUs N/A (pilot lighting issues) Pilot difficult to light Clean pilot assembly / injector Often spider webs or debris in pilot
All BBUs N/A (main burner issues) Main burner won't light Gas valve / thermostat checks Follow MI sequence step by step
All BBUs N/A (room sealed conversions) Room sealed conversion needed Full flue system replacement Big ticket – often leads to new boiler install
All BBUs N/A (upgrade conversation) System beyond economic repair New boiler installation quote BG use BBU calls as new boiler leads
All BBUs N/A (safety concerns) Any safety doubt after inspection Turn off and advise properly Never leave unsafe – document everything

Johnson & Starley warm-air – fault code patterns

Warm-air units (CO‑critical)
Top money jobs (field): N/A (filter / belt) blocked filters / belt Lockout lamp ignition / airflow
Appliance / family Code / pattern Real world fault Usual fix / part Notes (field/BG-style)
Johnson & Starley warm-air N/A (limit trip) High-limit stat tripping Clean/replace filters, check fan/belt, then stats Airflow first; both big firms put "filter/belt before parts" as step one.
Johnson & Starley warm-air Lockout lamp Ignition / flame sense / airflow Follow manufacturer ignition sequence exactly Warm-air is CO-critical – no "bypassing" safeties just to get heat on.
Johnson & Starley warm-air N/A (poor airflow) Rooms barely warm, unit hot Clean filters / check ducts / fan motor Filter cleaning shows customer you've fixed actual problem
Johnson & Starley warm-air N/A (pilot issues) Pilot won't stay lit Oxypilot/TC + flue/ventilation checks Clean burner first, only replace oxypilot if combustion OK
Johnson & Starley warm-air N/A (fan runs no heat) Fan runs but no warmth Ignition sequence / gas valve / controls Follow MI step by step – don't bypass safeties
Johnson & Starley warm-air N/A (upgrade conversation) Old ducting / suspect materials Wet system conversion discussion BG use warm-air calls as new boiler leads
Later J&S models N/A (condensate) Blocked condensate on condensing models Thaw / flush trap + insulate pipe Same as combis – winter money job
Later J&S models Lockout + hot casing Lockout with very hot unit Filter first, then fan/belt, then stats Airflow always first check
Later J&S models Lockout + fan never starts Lockout, fan never runs Fan motor / capacitor / control board Check supply at motor before condemning PCB
All warm-air units CO risk assessment Any warm-air service call Full combustion / spillage test CO-critical appliances – test every time or cap and advise

All brands – generic patterns – fault code patterns

Things that crop up on most boilers
Top money jobs (field): Overheat + ΔT > 30–40 °C pump / circulation Ignition lockout electrode / gas valve Low pressure PRV / system leak
Appliance / family Code / pattern Real world fault Usual fix / part Notes (field/BG-style)
All brands Overheat + ΔT > 30–40 °C No circulation – pump seized Pump swap Clip thermometers on flow/return; ΔT 10–20 °C is normal. >30–40 = pump almost every time.
All brands Ignition lockout Electrode gap / gas valve / pressure Electrode clean/gap or gas valve Check electrode gap and condition before anything else
All brands Low pressure fault PRV passing / system leak PRV replacement / leak repair Check external PRV pipe first – 90% are PRVs not leaks
All brands Fan runs but no ignition APS / gas valve / ignition circuit APS or gas valve replacement Check APS hoses and operation first
All brands Short cycling Oversized / poor circulation System balancing / pump speed Check ΔT and pump operation
All brands Kettling noise Limescale / sludge in HX Descale / powerflush Big ticket item – BG push "system cleanse"
All brands No DHW but CH OK Diverter valve / diaphragm Diverter repair kit Check diverter movement and microswitches
All brands No CH but DHW OK Diverter stuck in DHW position Diverter repair kit Check diverter position and operation
All brands Water leaks Various seals / joints failed Seal kit / component replacement Identify source – often pump seals or HX gaskets
All brands Gas smell Gas leak at joints / valve Tighten / replace fittings Immediate action required – tightness test mandatory

These summaries are based on common domestic UK models and field patterns. They don't replace manufacturer instructions or full BG/E.ON-style black books – always confirm against the specific appliance MI and any current technical bulletins.

BG/EON Secret Sauce & Competitive Advantages

Proprietary techniques that beat competitors

Diagnostic Speed Techniques

  • Tap test fans – if tapping makes fan spin, it's seized bearings not PCB
  • Spin test pumps – screwdriver in slot, if solid = pump not sensor
  • Red-hot fan top after 10 min = fan bearings gone (Worcester/Vaillant)
  • White powder on electrodes = cracked ceramic, replace immediately
  • Humming pump + huge ΔT = pump head, not electronics

Pre-emptive Quoting (BG Style)

  • F.75 on 8+ yr Vaillant = quote pump before opening case
  • F3 on 7+ yr Ideal = pre-emptively quote fan on services
  • E133 in winter = condensate insulation kit + winter pack
  • Any sludge seen = Magnaclean quote mandatory
  • Over 10 yr boiler = "Premium Care Bundle" upsell

Competitive Pricing Strategy

  • Charge 25-30% less than BG for same jobs
  • Bundle pricing – pump + filter cheaper than separate
  • Winter packs – condensate lagging + inhibitor bundle
  • Premium brands premium – ATAG/Viessmann +£40-100
  • Legacy surcharge – +20% for pre-2000 boilers

Upsell Triggers (E.ON Style)

  • Any sludge seen → Magnaclean Professional
  • Hard water area → scale reducer installation
  • External condensate → insulation kit + trace heating
  • Over 15 yr boiler → "Efficiency Upgrade" conversation
  • BBU/warm-air → "Safety & Efficiency" replacement quote

Callback Prevention

  • Replace CH NTCs in pairs on short cycling
  • Drop min gas rate on every Vokèra/Italian job
  • Insulate every external condensate, even in summer
  • Powerflush after pump failure if sludge present
  • Log gas rates after any valve work

Van Stock Optimization

  • 10x Worcester fans – most common UK failure
  • 5x Grundfos pumps – covers 80% of models
  • 3x Magnacleans – upsell on every sludge job
  • Condensate kits – winter money maker
  • Common electrodes – quick £150-200 jobs

HGS Local vs Big 6 Competitive Edge

40% faster diagnostics using this playbook vs BG tablet menus
No corporate markup – we charge less with better margins
Bradford-specific knowledge – hard water, common install issues
No scripted upsells – honest recommendations only
Local parts network – faster than BG national depots
Same-day service – BG often 3-5 day wait in winter

BG/EON Pricing Matrix (vs HGS Local)

Job Type BG/EON Price HGS Local Price HGS Margin Competitive Edge
Worcester fan replacement £380-£450 £280-£320 45-50% £100+ cheaper, same day
Vaillant pump replacement £420-£480 £320-£360 40-45% £100 cheaper, 1hr vs 3hrs
Condensate unblock + lagging £180-£220 £140-£160 50-55% £60 cheaper, winter pack included
Magnaclean install £350-£400 £280-£320 35-40% £70 cheaper, included with pump jobs
Powerflush £650-£800 £550-£650 30-35% £100-150 cheaper, 1 day vs 2

Pricing based on 2024 BG/EON fixed price repairs and local market research. HGS Local maintains better margins through efficient diagnostics, local parts sourcing, and no corporate overhead.

🔐 Service Menu Access Codes

How to access diagnostic/service menus on all major boiler brands

Worcester Bosch Service Menu

Greenstar i (24i/28i/30i/35i)

⚡ Method 1: Standard Access
  1. Press and hold Menu button for 5 seconds
  2. Display shows "OE" (Original Equipment menu)
  3. Use + / - buttons to scroll through codes
  4. Press OK to view/edit values
  5. Press Reset to exit
⚡ Method 2: Full Service Mode
  1. Turn boiler OFF at switch
  2. Press and hold Mode button
  3. Turn boiler back ON (still holding Mode)
  4. Keep holding for 10 seconds
  5. Release - enters full service menu
  6. Can force pump, fan, valve tests

Greenstar CDi (Compact)

  1. Press i button repeatedly
  2. Shows: Flow temp → Return temp → Pressure → Error history
  3. For full service menu:
  4. Press i + OK together for 3 seconds
  5. Displays "d.XX" codes (diagnostic)
  6. Use + / - to scroll

Useful Worcester Codes

CodeParameterUse
d.20Flow temperatureLive temp reading
d.21Return temperatureCheck ΔT
d.40Actual fan speed (RPM)Fan diagnosis
d.41Target fan speedCompare with actual
d.53Flame current (µA)Ionisation check
d.71Pump speed %Pump output check
d.84Burner hoursAge/usage
d.85Burner cyclesShort cycling check

Vaillant Service Menu

ecoTEC Plus / Pro / Exclusive

⚡ Access Service Menu
  1. Turn rotary dial to SPANNER icon (service position)
  2. Press OK button
  3. Display shows "d.0"
  4. Use + / - buttons to scroll through d.XX codes
  5. Press OK to enter/edit values
  6. Turn dial back to heating position to exit

Older ecoTEC (VUW)

  1. Press i button for 3 seconds
  2. Enters info menu
  3. Shows flow temp, pressure, etc.
  4. Press i + Mode together for service codes

Useful Vaillant Codes

CodeParameterUse
d.0Flow temperatureLive reading
d.1Return temperatureCheck ΔT
d.2Outdoor temp (if fitted)Weather comp
d.3DHW actual tempHot water check
d.17Current power %Modulation level
d.25Burner starts counterCycling check
d.40Desired flow tempTarget vs actual
d.41Desired burner powerDemand level
d.71Pump power %Pump output
d.75Actual fan speedFan diagnosis
d.84Ionisation currentFlame detection
🔥 Pro Tip: Vaillant Service Test

In service menu (spanner position):

  • Press OK + Mode together for 3 seconds
  • Enters component test mode
  • Can force pump, fan, valve independently
  • Great for diagnosing without triggering full startup

Ideal Boilers Service Menu

Ideal Logic+ / Logic Max / Logic Code

⚡ Access Service Menu
  1. From standby, press Menu button
  2. Scroll to "Service" using + / -
  3. Press OK
  4. Default code: 0000 (no password)
  5. Displays various diagnostic screens
  6. Use + / - to navigate
  7. Press Back to exit

Ideal Logic Info Screens

ScreenShowsUse
TemperatureFlow / Return tempsLive readings
PressureSystem pressureCheck filling
PowerCurrent kW outputModulation check
Fan SpeedActual RPMFan diagnosis
Flame CurrentIonisation µAFlame detection
Run HoursTotal operating timeAge check
Start CountNumber of ignitionsCycling check
Fault HistoryLast 10 faultsPattern spotting
🔥 Ideal Logic Installer Menu
  1. Service menu → "Installer Menu"
  2. Can adjust: Max power, pump speed, min power
  3. Set CH flow temp, DHW temp, etc.
  4. Change parameters and press OK to save

Baxi Service Menu

Baxi 800 / Platinum / Duo-tec

⚡ Access Info Display
  1. Press i button (info)
  2. Cycles through:
    • CH flow temperature
    • CH return temperature
    • DHW temperature
    • System pressure
    • Current power %
  3. Press i again to cycle
  4. Auto-returns to normal after 1 minute

Baxi Service Mode (Full Access)

  1. Press Mode + OK together for 5 seconds
  2. Display shows "P.XX" codes
  3. Use + / - to scroll through parameters
  4. Press OK to edit values
  5. Press Reset to exit

Baxi Parameter Codes

CodeParameterUse
P.01Max CH powerReduce for oversizing
P.02Min CH powerAdjust modulation range
P.05Max DHW powerHot water output
P.07Pump overrun timeAfter burner stops
P.11Anti-cycle timeShort cycling fix
P.30Burner run hoursAge/usage
P.31Burner start countCycling diagnosis

Viessmann Service Menu

Vitodens 050/100/200

⚡ Access Service Level
  1. Press OK button for 4 seconds
  2. Display shows "Service" menu
  3. Use rotary dial to navigate
  4. Options include:
    • Status (temperatures, pressures)
    • Operating data (hours, cycles)
    • Fault memory (last faults)
    • Service parameters (adjustments)
  5. Press Back to exit

Viessmann Codierungen (Parameters)

In Service menu → "Service parameters"

  • Each parameter has format: XXXX
  • Use rotary dial to select
  • Press OK to edit
  • Common adjustments: Power levels, pump speeds, temps
ParameterFunctionTypical Value
Max heating powerReduce for oversizing100% default
Pump speedAdjust flow rateVariable
DHW temp setpointHot water target55°C typical
Anti-cycle delayPrevent short cycling5 min typical
🔥 Viessmann is Complex!

Viessmann menus are more involved than other brands. The manual is your friend - each model has different parameter codes. Download model-specific PDF from Viessmann website.

Intergas Service Menu

Intergas Compact / Rapid / Xclusive

⚡ Access Service Menu (VERY SIMPLE!)
  1. No buttons to press!
  2. Display automatically cycles through data
  3. Shows:
    • Current mode (Heating/DHW)
    • Flow temperature
    • Return temperature
    • DHW temperature
    • Fan speed
    • Current power %
  4. Just watch the display cycle

Intergas Parameter Access

  1. Remove front cover
  2. Look for jumper on PCB labeled "Service"
  3. Move jumper to "Service" position
  4. Display shows "P" codes
  5. Use + / - buttons on PCB to navigate
  6. Press OK on PCB to edit
  7. Return jumper to normal position when done
SettingAdjustmentCommon Values
Max Power CHReduce output15-30kW range
Max Power DHWHot water output15-36kW range
Pump speed3 speed settingsUsually position 2
Anti-cycle timeDelay between cycles2-5 min
🔥 Intergas = Dead Simple

Intergas boilers are the EASIEST to diagnose - display shows everything automatically. No complex menus to navigate. If you can see the display, you can diagnose it!

Glow-worm Service Menu

Glow-worm Energy / Ultracom / Flexicom

⚡ Access Service Menu

Note: Glow-worm is Vaillant group - similar to Vaillant access

  1. Turn dial to SPANNER position
  2. Press OK
  3. Display shows diagnostic codes
  4. Use + / - to scroll
  5. Turn dial back to exit

Older Glow-worm (Betacom / Fuelsaver)

  1. Press Info button (if fitted)
  2. Shows basic temps and pressure
  3. For service access:
  4. Press Reset + Mode together for 5 seconds
  5. Limited parameters vs newer models

Vokèra Service Menu

Vokèra Compact / Linea / Unica

⚡ Access Info Display
  1. Press i button (if fitted)
  2. Shows flow temp, pressure, fault codes
  3. For service parameters:
  4. Press Mode + i together for 5 seconds
  5. Display shows "P.XX"
  6. Use buttons to navigate

Vokèra Without Display (Older Models)

Many Vokèra boilers have NO user display!

  • LED fault codes only (flashing patterns)
  • Parameter changes require:
    • DIP switches inside boiler
    • Potentiometers (manual adjustment knobs)
    • Reference model-specific manual
  • Common adjustments on PCB:
    • Max power pot (turn to reduce output)
    • Gas valve adjustment pot
    • DHW temp pot
🔥 Vokèra Pro Tip

Always reduce max gas rate on Vokèra! They're Italian and often run hot in UK installations. Drop output by 10-15% to prevent nuisance lockouts.

Sime Service Menu

Sime Format (Most Common)

⚡ LED Display Models

Most Sime boilers have simple LED displays

  1. Look for buttons below LED
  2. Press Menu button (if fitted)
  3. Shows numbered codes (1-99)
  4. Press repeatedly to cycle through
  5. Common codes:
    • 01 = Flow temperature
    • 02 = Return temperature
    • 05 = Current power %
    • 20 = Fault history

Sime Parameter Adjustment

Usually via DIP switches and pots on PCB:

  • Remove case to access PCB
  • Look for:
    • DIP switch bank (0/1 positions)
    • Potentiometers (adjustment screws)
  • Common adjustments:
    • Max power pot (reduces output)
    • Min power pot (modulation range)
    • DHW priority DIP switch
  • Label switches before changing!
🔥 Sime = Italian Vokèra Clone

Sime, Vokèra, Heatline, and some Ravenheat models share very similar designs. If you know one, you know them all. Always check the actual PCB manufacturer (often the same across "different" brands).

ATAG Service Menu

ATAG iC Economiser

⚡ Access Service Menu
  1. Press Menu button
  2. Scroll to "Information"
  3. Shows live data: temps, pressure, power
  4. For service parameters:
  5. Menu → "Service"
  6. Default code: 0000
  7. Can adjust: Max power, pump speed, temperatures
Menu ItemShows/Adjusts
StatusCurrent operating mode
TemperaturesFlow, return, DHW actual
PerformanceCurrent kW output
PressureSystem pressure bar
Fault memoryLast 10 error codes
Operating hoursTotal runtime
Service parametersInstaller adjustments

Potterton / Main Service Menu

Potterton Titanium / Promax

⚡ Access Info Mode

Note: Potterton (Baxi group) = Similar to Baxi access

  1. Press i button
  2. Cycles through temps, pressure, power
  3. For service menu:
  4. Press i + Mode for 5 seconds
  5. Shows parameter codes
  6. Use + / - to navigate

Main Eco Elite / Combi

⚡ Service Access

Main is Baxi rebranded - exact same as Baxi 800

  1. Press i button for info
  2. Press Mode + OK for parameters
  3. Same P.XX codes as Baxi
  4. Identical PCB and controls

Alpha / Remeha Service Menu

Alpha E-Tec / CD

⚡ Access Service Display
  1. Alpha boilers show data automatically on display
  2. Rotate through screens using Mode button
  3. Shows: Flow temp, pressure, power, fault codes
  4. For parameters:
  5. Press Menu (if fitted)
  6. Navigate to service section

⚡ Quick Reference - Service Menu Access

Brand Button Combo Display Type Difficulty
Worcester Menu (hold 5 sec) LCD with codes 🟢 Easy
Vaillant Dial to spanner + OK LCD with d.XX codes 🟢 Easy
Ideal Menu → Service Full color LCD 🟢 Very Easy
Baxi Mode + OK (5 sec) LED with P.XX 🟡 Medium
Viessmann OK (hold 4 sec) LCD menu system 🟡 Medium
Intergas None - auto display Auto-cycling 🟢 Easiest
Glow-worm Dial to spanner + OK LCD (Vaillant style) 🟢 Easy
Vokèra Usually none LED only or none 🔴 Hard (PCB adjustment)
Sime Menu (if fitted) LED codes or DIP switches 🔴 Hard (PCB adjustment)
ATAG Menu → Information Full LCD menu 🟢 Easy

🔥 Service Menu Pro Tips

Bradford-Specific Service Menu Uses

  • Hard Water Area: Check burner hours/cycles
    High cycles + old boiler = likely scale buildup
  • Oversized Boilers (common in small terrace houses):
    Reduce max power via service menu - prevents short cycling
  • Prepay Meters: Check flame current in service menu
    Low voltage from prepay = weak flame = low µA reading
  • External Condensates: Monitor return temp in winter
    If dropping below 50°C, condensate likely freezing

What to Check in Service Menus

SymptomCheck in Service MenuWhat It Tells You
Short cycling Burner cycles count >6 cycles/hour = oversized or issue
Weak hot water Current DHW power % Should be near 100% when tap open
Noisy fan Target vs actual fan speed Big difference = failing fan
Flame loss Ionisation current µA <1µA = poor detection
Poor modulation Current kW output Stuck at max or min = valve issue
Poor flow Flow / return ΔT >25°C = pump or blockage

Common Service Menu Adjustments

  • Max Power Reduction: Fix for oversized boilers
    Reduce to 60-80% on small properties (Bradford terraces)
  • Anti-Cycle Time: Increase to 5-7 minutes
    Prevents rapid cycling in well-insulated homes
  • Pump Speed: Usually position 2 or 3
    Increase if poor flow, decrease if noisy
  • DHW Temperature: 55-60°C typical
    Don't go above 65°C (scale risk in Bradford)

⚠️ Service Menu Safety

  • Write down original values before changing!
  • Don't adjust gas pressure without analyzer
  • Don't disable safety functions
  • Some menus allow forcing components - be careful
  • Return to normal mode before leaving job
  • Test boiler operation after any changes

📐 Technical Reference Data

Resistance values, voltage specs, and component testing data for field diagnostics

🌡️ NTC Thermistor Resistance Tables

Temperature vs Resistance for common NTC types

10kΩ @ 25°C NTC (Most Common)

Used in: Worcester Bosch, Vaillant, Ideal, Baxi flow/return sensors

Temp (°C) Resistance (Ω) Tolerance ± Test Note
0°C32,650 Ω±5%Ice water test
10°C19,900 Ω±5%Cold tap water
20°C12,090 Ω±5%Room temp
25°C10,000 Ω±5%Reference point
30°C8,312 Ω±5%Warm room
40°C5,327 Ω±5%Hand warm
50°C3,602 Ω±5%Hot tap
60°C2,486 Ω±5%Hot water
70°C1,755 Ω±5%CH flow temp
80°C1,270 Ω±5%High temp
90°C936 Ω±5%Near boiling
100°C696 Ω±5%Boiling water

5kΩ @ 25°C NTC

Used in: Some Vaillant DHW sensors, older Worcester models

Temp (°C)Resistance (Ω)Tolerance ±
0°C16,325 Ω±5%
10°C9,950 Ω±5%
20°C6,045 Ω±5%
25°C5,000 Ω±5%
40°C2,663 Ω±5%
60°C1,243 Ω±5%
80°C635 Ω±5%

20kΩ @ 25°C NTC

Used in: Viessmann, some ATAG models, flue sensors

Temp (°C)Resistance (Ω)Tolerance ±
0°C65,300 Ω±5%
10°C39,800 Ω±5%
20°C24,180 Ω±5%
25°C20,000 Ω±5%
40°C10,654 Ω±5%
60°C4,972 Ω±5%
80°C2,540 Ω±5%

🔥 HGS Pro Tips - NTC Testing

  • Quick field test: Disconnect sensor, measure at room temp (20°C). Should be ~12kΩ for 10k type.
  • Warm test: Hold sensor in hand (35-37°C). Resistance should drop to ~6-7kΩ smoothly.
  • Common failures:
    • Open circuit (OL on meter) = broken wire or sensor
    • Very high resistance (>100kΩ at room temp) = water ingress/corrosion
    • Jumping readings = loose connection
    • No change with temperature = failed sensor
  • Flow vs Return test: With system running, both sensors should read similar (within 20%). Big difference = faulty sensor or blockage.
  • Don't test with power on! PCB can back-feed voltage and give false readings.

💨 Fan Motor Specifications

Worcester Bosch Fans

Model Series Voltage Current Winding Resistance Capacitor
Greenstar 24i/28i/30i 230V AC 0.3-0.5A 50-80 Ω 2.5µF ±10%
Greenstar CDi (Compact) 230V AC 0.4-0.6A 45-75 Ω 2.5µF ±10%
Greenstar Si (System) 230V AC 0.3-0.5A 50-80 Ω 2.5µF ±10%

Vaillant Fans

ModelVoltageCurrentWindingCapacitor
ecoTEC Plus 230V AC 0.35-0.55A 40-70 Ω 2.0µF
ecoTEC Pro 230V AC 0.4-0.6A 45-75 Ω 2.5µF

Ideal Logic Fans

ModelVoltageCurrentWindingCapacitor
Logic+ / Logic Max 230V AC 0.3-0.5A 50-85 Ω 2.0µF

🔥 Fan Testing Procedure

  1. Visual check: Spin by hand - should turn freely
  2. Winding test (power OFF):
    • Measure resistance across motor terminals
    • Should be 40-85Ω depending on model
    • Open circuit (OL) = burnt winding
    • Very low (<10Ω) = short circuit
  3. Capacitor test:
    • Discharge capacitor first (short with insulated screwdriver)
    • Measure capacitance (2.0-2.5µF typical)
    • ±20% is acceptable, beyond that = replace
  4. Voltage test (power ON, boiler calling):
    • Should see 230V AC at fan connector
    • If no voltage = PCB fault or wiring
    • If voltage present but fan won't run = fan fault
  5. Current draw test:
    • Clamp meter around live wire
    • Running current should be 0.3-0.6A
    • >1A = bearings seized or capacitor failed

⚙️ Pump Specifications & Testing

Grundfos Pumps (Most Common)

Model Voltage Power Winding Resistance Current Draw
Grundfos UPS 15-50 230V AC 45-60W 300-500 Ω 0.2-0.3A
Grundfos UPS 15-60 230V AC 50-80W 250-450 Ω 0.25-0.35A
Grundfos Alpha2 230V AC 5-25W (auto) N/A (ECM) 0.05-0.15A

Wilo Pumps

ModelVoltagePowerWindingCurrent
Wilo RS 15/6 230V AC 45-65W 300-500 Ω 0.2-0.3A
Wilo Yonos PICO 230V AC 4-25W (auto) N/A (ECM) 0.05-0.15A

🔥 Pump Testing - Step by Step

  1. Quick Visual Test:
    • Feel pump body - should be WARM (not cold, not burning hot)
    • Cold = not running
    • Burning hot (can't hold hand on it) = seized bearings
  2. Spindle Test (power OFF):
    • Remove front screw (bleed screw)
    • Insert flat screwdriver
    • Spin shaft - should turn freely
    • Won't turn = seized bearings (can try freeing, but usually replace)
  3. Electrical Test (power OFF):
    • Disconnect pump wiring
    • Measure winding resistance: Should be 250-500Ω for standard pumps
    • Open circuit (OL) = burnt winding
    • Low (<50Ω) = shorted winding
    • Note: ECM pumps (Alpha2, Yonos PICO) have electronics - can't test windings
  4. Voltage Test (power ON, heating calling):
    • Measure at pump terminals: Should see 230V AC
    • No voltage = wiring fault or PCB issue
  5. Current Test (pump running):
    • Clamp meter on live wire
    • Standard pump: 0.2-0.3A normal, >0.5A = problem
    • ECM pump: 0.05-0.15A variable
  6. System Test:
    • Check flow/return ΔT (difference)
    • Should be 10-20°C for most systems
    • >25°C = poor flow (pump or blockage)
    • <5°C = pump running too fast

🔥 Gas Valve Coil Specifications

Honeywell Gas Valves (Most Common)

Valve Type Voltage Coil 1 (Safety) Coil 2 (Modulation) Current
VK4105G 230V AC 3800-4200 Ω 180-220 Ω 0.15A
VK4115V 230V AC 3800-4200 Ω 180-220 Ω 0.15A
VK8515MR 24V DC 150-180 Ω 150-180 Ω 0.12A

SIT Gas Valves

Valve TypeVoltageCoil ResistanceCurrent
SIT 845 Sigma230V AC3500-4000 Ω0.12A
SIT 848 Sigma24V DC140-160 Ω0.15A

🔥 Gas Valve Testing

  1. Coil Resistance Test (power OFF):
    • Disconnect valve wiring at PCB
    • Measure coil 1: Should be 3800-4200Ω (230V) or 150-180Ω (24V)
    • Measure coil 2: Should be 180-220Ω (230V) or 150-180Ω (24V)
    • Open circuit = faulty coil
    • Short (<50Ω on 230V valve) = damaged coil
  2. Voltage Test (power ON, ignition sequence):
    • Coil 1 (safety): Should get 230V (or 24V) when boiler calls for heat
    • Coil 2 (modulation): Should get variable voltage for modulation
    • No voltage = PCB fault
    • Voltage present but no gas = valve mechanically stuck
  3. Mechanical Test:
    • Listen for TWO clicks when boiler calls (safety coil + modulation coil)
    • One click only = one coil failed
    • No clicks = no power or completely failed

🔌 PCB Circuit Voltage Testing

Standard PCB Voltages (Live Tests - BE CAREFUL!)

Circuit/Component Expected Voltage When Measured Between
PCB Input Power 230V AC Always (when boiler on) Live & Neutral terminals
24V Control Circuit 24V DC Always (when boiler on) 24V+ and 0V (ground)
Fan Output 230V AC During ignition/running Fan live & neutral
Pump Output 230V AC When heating/DHW calls Pump live & neutral
Gas Valve Coil 1 230V AC or 24V DC During ignition Valve terminals
Gas Valve Coil 2 0-230V AC (modulating) During running Modulation terminals
Ignition Transformer 230V AC (input) During ignition Primary winding
Ignition Spark 10-15kV (HT) During ignition DO NOT MEASURE - VISUAL CHECK
NTC Sensor Feed 5V DC Always Sensor + to ground
Thermostat Call 230V AC or 24V When stat calls Room stat terminals
DHW Flow Switch 5-12V DC When tap opened Flow switch terminals

⚠️ SAFETY - Live Voltage Testing

  • DO NOT touch any terminals while boiler is live!
  • Use insulated test leads and meter probes
  • Never bridge terminals with meter probes
  • One hand in pocket when testing (prevents through-chest shock)
  • Work on dry surface, no water around
  • If unsure, TURN OFF POWER and test resistances instead

🔥 Common PCB Fault Patterns

Symptom Voltage Test Likely Fault
Complete dead board No 230V at input Check fuse, incoming supply
Display dead 230V present, no 24V DC Power supply section failed
Fan won't run No 230V at fan output PCB relay or output stage
Pump won't run No 230V at pump output PCB pump relay failed
No ignition No voltage to gas valve PCB gas valve driver failed
Lights then lockouts Voltage to valve, no flame detection Ionisation circuit fault on PCB

🔋 Capacitor Specifications

Component Capacitance Voltage Rating Tolerance Purpose
Fan Motor 2.0-2.5µF 400V AC ±20% Start/run capacitor
Old Pump (pre-2005) 4-8µF 400V AC ±20% Start capacitor
PCB Filter (mains) 100-470nF 275V AC (X2) ±20% EMI suppression
PCB Smoothing 100-2200µF 35-63V DC ±20% DC power smoothing

🔥 Capacitor Testing

  1. DISCHARGE FIRST! Short terminals with insulated screwdriver
  2. Visual check:
    • Bulging top = failed (pressure buildup)
    • Leaking = failed
    • Burn marks = failed
  3. Capacitance test (with meter):
    • Remove capacitor from circuit
    • Measure capacitance
    • Should be within ±20% of rating
    • 2.5µF cap should measure 2.0-3.0µF
    • Below range = dried out, replace
  4. ESR test (advanced):
    • ESR meter shows internal resistance
    • Should be <5Ω for motor caps
    • >10Ω = replace

🔥 Flame Detection / Ionisation Specs

Flame Rectification Current

Boiler Brand Min Current Typical Current Max Current Test Method
Worcester Bosch 0.5µA 2-4µA 10µA Service tool or multimeter (µA DC)
Vaillant 1.0µA 3-5µA 12µA Service menu shows current
Ideal 0.8µA 2-5µA 15µA Multimeter (µA DC)
Baxi 0.5µA 2-4µA 10µA Multimeter (µA DC)

Electrode Gap Specifications

Boiler TypeIgnition GapFlame Sense GapPosition
Worcester Bosch 3-4mm N/A (combined) Centre of burner
Vaillant 3-4mm N/A (combined) Right side burner
Ideal Logic 3-4mm N/A (combined) Centre burner
Baxi 3-4mm N/A (combined) Left side burner
Viessmann 3mm ±0.5 N/A (combined) Use Viessmann gauge

🔥 Flame Detection Troubleshooting

  • Low ionisation current (<0.5µA):
    • Electrode dirty/corroded - clean with emery cloth
    • Electrode gap wrong - check and adjust to 3-4mm
    • Electrode not in flame - reposition
    • Poor earth connection - check case earth
  • Zero ionisation current:
    • Electrode broken wire - replace lead
    • PCB ionisation circuit failed - replace PCB
    • Electrode to earth short - check for contact with burner
  • Intermittent flame detection:
    • Loose electrode connection - secure properly
    • Condensation on electrode - dry thoroughly
    • Gas quality issues - check gas pressure

⚡ Quick Reference - Dead Testing

Component resistance checks with power OFF

Component Typical Resistance Good/Bad Indicators Common Failure Mode
NTC Sensor (10kΩ) 12kΩ @ 20°C ✅ 10-15kΩ
❌ OL or <1kΩ		 Open circuit or water ingress
Fan Motor 40-85Ω ✅ 40-100Ω
❌ OL or <10Ω		 Burnt winding or short
Pump (Standard) 250-500Ω ✅ 250-600Ω
❌ OL or <50Ω		 Burnt winding or seized
Gas Valve Coil 3800-4200Ω (230V) ✅ 3500-4500Ω
❌ OL or <1000Ω		 Open coil or water damage
Overheat Stat 0Ω (closed) ✅ 0-2Ω
❌ OL when cold		 Stuck open, needs reset or replace
Pressure Switch 0Ω (at pressure) ✅ 0Ω when pressurised
❌ Always OL		 Microswitch failed or diaphragm leak
Flow Switch 0Ω (when flowing) ✅ 0Ω when tap open
❌ Always OL		 Paddle stuck or microswitch failed
Ignition Lead High (MΩ range) ✅ >1MΩ to earth
❌ <100kΩ		 Insulation breakdown, arcing

🔥 Bradford-Specific Testing Notes

  • Hard Water: NTC sensors often fail at 60-80kΩ (scale buildup) - replace if >50kΩ at room temp
  • Prepay Meters: Check voltage at boiler inlet - should be 230V ±10V. Prepay can drop to 210V causing issues
  • External Condensate: Test discharge temp in winter - should be 40-50°C. If cold, condensate trap frozen
  • Old Properties: Check earth loop impedance - many Bradford houses have poor earth = flame detection issues