The average London boiler dies on the coldest weekend of the year, usually with an unhelpful fault code flashing on the display. This guide is the playbook we give our own dispatchers: how to read the signs before you call, when a repair is worth it, when it isn't, and what a Gas Safe registered boiler engineer should actually charge.
Every brand we mention — Worcester Bosch, Vaillant, Baxi, Ideal, Glow-worm — is one our engineers carry spares for on the van, so most of the faults below get fixed on the first visit.
The 5 most common boiler faults in London homes
- Pressure dropping below 1 bar — a slow leak somewhere on the system, often a corroded radiator valve. Re-pressurising will only buy a few days if the leak isn't found.
- No hot water, heating works fine — nine times out of ten, a failed diverter valve. £180–£280 fitted for most combi boilers.
- Kettling / rumbling noises — limescale build-up on the heat exchanger. London's hard water is brutal on boilers; a magnetic filter and annual power flush prevent it.
- F.22 / F.28 fault codes (Vaillant) or EA / F3 (Worcester) — ignition or flame-loss faults, usually a dirty electrode or a bad spark generator.
- Boiler cycles on and off — a failed thermistor or a pump problem, easy diagnosis with a multimeter.
Repair or replace? The 50% rule
Rough rule: if the repair quote is more than 50% of a new boiler, and the boiler is over 10 years old, replace it. A new Worcester Bosch 2000 combi fitted in a straightforward swap runs £1,900–£2,600 in London and comes with a 10-year manufacturer warranty. Old boilers under 8 years old are almost always worth repairing.
Other signals it's time to replace: rusty water in the system, a boiler that's been repaired twice in one year, or a G-rated inefficient model that's doubling your gas bill.
What a Gas Safe boiler repair should cost
A fair Gas Safe boiler engineer in London quotes a fixed hourly rate on the phone (typically £85–£120 for the first hour, £60–£90 subsequent hours) plus parts at trade price with a modest markup. Diverter valves £110–£180, PCBs £180–£320, pumps £120–£180, expansion vessels £90–£140.
Red flags: 'We'll need to condemn your boiler' without showing you the fault. Refusal to leave a Gas Safe registration card. A £400+ 'diagnostic fee' before any work starts. Any of those and you show them the door.
What to do while you wait
- Note the fault code on the display — photograph it if it's intermittent.
- Check the pressure gauge. If it's below 1 bar, that's usually your problem right there.
- Try one full reset — hold the reset button for 5 seconds, wait 30 seconds, retry. That fixes about 15% of call-outs before the van arrives.
- Don't keep re-pressurising a leaking system — you're just watering the leak faster and risking a saturated ceiling.
- Turn off the gas at the meter if you smell gas at all, open windows, and leave the property. Call the National Gas Emergency line on 0800 111 999.
Annual servicing — why it's worth it in London
London's hard water and old cast-iron radiator systems combine to kill boilers early. An annual service (£85–£120) picks up worn diverter seals, dirty burners and pressure vessel failures before they become £600 breakdowns. If you're a landlord, an annual CP12 Gas Safety Certificate is legally required under the Gas Safety (Installation and Use) Regulations 1998 — the service and CP12 can be done in the same visit.