Boiler Repair London

Which boiler parts fail, and what each one does

Most breakdowns come down to a short list of parts, and each fails with its own pattern of symptoms.

Condensing boilers add one weak point older boilers did not have: the condensate pipe. When the plastic pipe outside freezes, the boiler locks out on an ignition code. Our boiler not working checklist covers the checks worth doing before you book.

• Diverter valve. On a combi it sends heat either to the radiators or to the hot water. A stuck valve gives heating but no hot water, or the reverse. A worn one lets heat leak both ways, so the taps run lukewarm and the radiators warm up in summer.
• PCB. The printed circuit board runs the ignition sequence, the fan, the pump and the safety checks. A failing board can mimic almost any other fault, so it is tested last, after the parts it controls, rather than swapped first.
• Fan. It clears flue gases before the burner lights. As the bearings wear it slows and gets noisy, and the boiler locks out before ignition.
• Pump. It moves water through the boiler and round the radiators. When it seizes or weakens, the water inside the boiler overheats and the boiler cuts out, often with a hum or a very hot pump body.
• Gas valve. It lets gas through to the burner once the safety checks have passed. One that will not open gives an ignition lockout with no flame. One that will not turn the gas up leaves the boiler stuck at low output, so the hot water never gets properly hot.
• Heat exchanger. The main heat exchanger passes heat from the burner to the water. It can corrode, leak or block with sludge, and replacing it is the biggest job inside a boiler.

Combi and system boilers fail in different places

A combi heats your hot water on demand as well as the radiators, so more of its work runs through a few parts. The small plate heat exchanger for the taps is found only on combis, and a combi’s diverter valve switches every time a tap runs, so both work hard. London water is hard, and limescale lines that plate heat exchanger over the years: first the hot water runs lukewarm at full flow, later the boiler overheats and cuts out whenever a tap is on.

A system boiler heats a cylinder and the radiators through motorised zone valves, with the pump usually inside the boiler. When only one side stops, a zone valve or its wiring is the usual suspect rather than the boiler itself. An older regular boiler with tanks in the loft often has gravity hot water and a separate heating pump, so a dead pump leaves the taps hot and the radiators cold. Our guide to hot water but no heating takes each type in turn.

How the engineer finds the fault

A good diagnosis follows an order. First the supplies: is gas reaching the boiler at the right pressure, is there power at the fused spur, is the system pressure between 1 and 1.5 bar cold? Then the controls: is the thermostat or timer actually calling for heat? Only then does the engineer open the casing and test the part the fault code or symptom points at, using the manufacturer data for that model.

Many boilers keep a fault history. Reading it often shows whether tonight is a one-off or the fifth lockout this month, which changes the repair. For brand-specific faults, see Worcester, Vaillant, Baxi, Ideal and Glow-worm repairs.

What a boiler repair usually costs

Published UK cost guides put most boiler repairs somewhere between about £100 and £750. As a rough guide, a pressure relief valve or expansion vessel recharge sits at the low end, around £100 to £200. A diverter valve, pump or fan usually runs £180 to £400. A gas valve is often £200 to £500 and a PCB £350 to £700. A heat exchanger is commonly £400 or more, and on a boiler over about twelve years old that is usually the point to think about replacement.

These are market ranges, not our price. The repair price is agreed with you on site before any work starts. Our pricing page sets out how the call-out and repair price work.

Is it worth repairing, or time to replace?

Most single faults on a boiler under about twelve years old are worth repairing. Replacement comes into it when the heat exchanger has failed, when parts for the model are discontinued or hard to get, or when several major parts have gone in a short time.

On an older boiler the fan, board and diverter valve tend to wear out around the same time, so ask the engineer which other parts are close to the end of their life before you decide. Our guide to repairing or replacing a broken boiler works through examples with typical market prices.

When a boiler repair cannot wait

Book the same day if water is leaking near the boiler electrics, or if someone elderly, very young or unwell is in a cold home. A boiler that has stopped on low pressure with no leak can often wait once you have topped it up.

Some signs are not a repair call at all. If you smell gas, or a carbon monoxide alarm sounds, open windows, turn off the gas at the meter if it is safe to reach, avoid switches, get everyone outside and call the National Gas Emergency Service on 0800 111 999.

Gas work is carried out by Gas Safe registered engineers. Nothing inside the boiler casing is a householder check.