Oil boilers are reliable when serviced and temperamental when not. Here are the 10 faults we see most often at Belfast callouts, what’s actually happening, and whether it’s a DIY fix or an engineer visit.
1. Boiler locks out, won’t restart
Symptom: amber lockout light on, or fault code showing. Cause: flame failure, dirty nozzle, air in line, empty tank, photocell fouling. Fix: reset ONCE. If it locks out again, stop and call. See Oil boiler not firing up and Oil boiler fault codes.
Never reset more than twice: repeated resets flood the chamber with unburned oil.
2. Smoke from the flue
Symptom: visible dark smoke or soot marks around the external flue terminal. Cause: poor combustion, usually a dirty nozzle or baffle plates. Fuel isn’t burning cleanly. Fix: engineer. Annual service with full combustion analysis.
Untreated, poor combustion wrecks the heat exchanger (£480-£850 to replace) and can void warranty.
3. Oil smell indoors
Symptom: kerosene smell near the boiler, cupboard, or anywhere inside. Cause: leaking flexible oil line, failed gasket, loose union, oil tray overflow. Fix: stop using the boiler, ventilate, call immediately.
This is a fire risk. Call Emergency Plumbing.
4. Kettling (rumbling noise)
Symptom: rumbling, boiling-kettle noise when the burner fires. Cause: scale or sludge in the heat exchanger (combustion side carbon, or water-side magnetite). Fix: on water-side sludge, a power flush. On combustion-side carbon, a deep baffle clean during service.
Kettling on a 12+ year old oil boiler often signals end-of-life. See When should I replace my boiler.
5. Pressure keeps dropping
Symptom: you keep topping the boiler up, pressure keeps falling. Cause: leak somewhere in the system: radiator valve, pipe joint, expansion vessel failure, boiler body leak. Fix: inspect accessible joints for water marks. If nothing visible, call for leak tracing. Pressure loss is never “nothing”, don’t keep topping up indefinitely.
See Boiler Repair.
6. No hot water, heating works
Symptom: radiators heat, hot taps don’t (or vice versa). Cause: diverter valve failure (combi boilers), hot water cylinder thermostat failure (system boilers), immersion fault. Fix: engineer. Diverter valve replacement £220-£320. Cylinder thermostat £95-£160.
7. Frozen outside pipework
Symptom: boiler was running fine, temperature drops overnight, now no ignition. Cause: condensate pipe (plastic, runs to a drain outside) has iced up. Very common in December-February. Fix: thaw gently. Wrap a warm (not boiling) water bottle or towel around the external pipe. Boiler restarts once water flows.
Preventively: insulate the external pipe before winter. Or have an engineer reroute it internally.
8. Oil pump failing
Symptom: motor runs, no ignition, sometimes a low whining from the pump. Cause: pump can’t build pressure to atomise fuel at the nozzle. Fix: engineer. Pump replacement £140-£220.
9. Thermostat problems
Symptom: boiler never fires, or fires constantly, regardless of demand. Cause: thermostat batteries dead, timer programmed wrong, wireless thermostat unpaired, or thermostat itself failed. Fix: DIY first: new batteries, check timer, factory-reset wireless stats. If that doesn’t fix it, replacement thermostats £60-£180 fitted.
10. Boiler fires, radiators stay cold
Symptom: boiler runs, pump hums, radiators stay cold. Cause: pump seized, airlock in the system, all TRVs closed, diverter valve stuck on hot water side. Fix: check TRVs are open, bleed any airlocks, if still nothing engineer visit.
Preventive maintenance
Two-thirds of the above we diagnose because the boiler hasn’t been serviced in 2+ years. An annual oil boiler service at £95-£130 is cheaper than even a single emergency callout at £150+ parts.
Service schedule for oil boilers:
- Annual service (mandatory for warranty, catches most faults early)
- Nozzle replacement every service
- Filter replacement every service
- Tank check every service
See How much is a boiler service.
When to replace rather than repair
If you’re on a second fault this winter, or the boiler is 12+ years old and locking out regularly, the honest conversation is replacement. See When should I replace my boiler and Oil to Gas Conversion for switching fuel.
Book a repair or service
Tell us the boiler make, age, fault code (if any), and when the problem started. We come back with a likely cause and a slot inside the week.
Related services: Oil Boiler Servicing · Boiler Repair · Emergency Plumbing · Oil to Gas Conversion
Related guides: Oil boiler not firing up · Oil boiler fault codes · When should I replace my boiler