Which? asked 225 of its Trusted Trader heating engineers what they would charge for parts and labour to fix the 10 most common boiler faults, in a survey carried out in May and June 20251. The consumer body published the findings on 17 September 20251.
The faults covered include diverter and zone valves, printed circuit boards, water pumps, combustion fans, gas valves, overheat thermostats, automatic air vents, pressure relief valves, thermocouples and ignition parts1. The average price the engineers quoted for each part, including installation, is available to Which? members only and has not been published openly1.
Which? separates the three most common faults from the rest. It says two of those three can be handled by a competent DIYer and do not always require a heating engineer1. Losing pressure can be corrected by opening the filling loop taps until the pressure reaches the desired level, usually around one bar, then closing both taps1. A frozen condensate pipe, a feature of condensing boilers, shuts the boiler down for safety reasons; external pipes should be insulated with foam to stop them freezing1. Water leaking from the boiler is usually the result of old and corroded washers and needs a trained heating engineer to open the boiler and replace them1.
On dangerous faults, Which? reports that faulty boilers can leak carbon monoxide, a colourless and odourless gas produced by incomplete burning of carbon-based fuels1. According to the NHS, carbon monoxide poisoning kills around 60 people a year in England and Wales1. Owners of old-fashioned non-room sealed boilers and systems with hidden flues are at higher risk1. In Scotland, a carbon monoxide detector is a legal requirement in the home1. Which? adds that its tests have found some detectors fail to alarm even in the presence of large amounts of CO1.
| Item | Figure |
|---|---|
| Engineers surveyed | 225 Trusted Trader heating engineers1 |
| Survey period | May and June 20251 |
| Boiler service cost | £70 to £110, excluding any resulting repairs1 |
| Households not servicing annually | Around half, per Which? survey figures1 |
| CO deaths, England and Wales | Around 60 people a year, per the NHS1 |
The service cost range comes from an online survey in June 2025 of 10,064 UK adults aged over 18 responsible for the maintenance and repair of boilers installed since 20191. Which? says failure to service a boiler annually will in most cases invalidate its warranty1.
Why it matters for households
A boiler that stops working takes heating and hot water with it, and the cost of putting it right depends on which part has failed. Knowing which faults sit outside the boiler casing and which do not is the difference between a householder restoring pressure or thawing a pipe themselves and paying a call-out charge during a cold snap1. The faults that require the casing to be opened sit with a trained engineer, and Which? states plainly that untrained work of that kind is dangerous and voids remaining warranty cover1.
For energy independence, the practical points are the ones that keep a boiler running and safe without an emergency call-out. An annual service is the stated route to catching problems early and limiting costly repairs, and Which? links skipped servicing to lower efficiency and higher energy bills1. Carbon monoxide is the safety side of the same question: it cannot be seen or smelled, and the risk is higher in older non-room sealed systems and those with hidden flues1. Fault codes and the boiler manual are the first diagnostic step, with most manufacturers offering an online copy1. Our guides on common boiler faults and fault codes and on home heating running costs by fuel cover the wider picture, and households weighing a replacement can compare options in our boilers and heating hub.
What happens next
No further steps or dates have been reported1.
Sources1 cited
- Common boiler problems and repair costs - Which?, which.co.uk