How we assess boilers

When you buy a new boiler, you'll want to be sure it will give you years of reliable service. And, you'll want to know that it's highly recommended by heating engineers – experts who work with heating systems every single day.

Only boiler brands that have a good reliability record, are loved by their owners and are highly regarded by Which? Trusted Trader heating engineers can earn our Which? Recommended Brand award.  

If you just want to find out which models are most reliable, head straight to our reviews of the best boiler brands.

How we test boilers

Every year, we survey thousands of members of the public about their experiences with popular boiler brands.

Our most recent boiler brand survey was carried out in May 2025. We surveyed more than 10,000 members of the public, all of whom had a new boiler installed within the previous six years. 

We analysed their responses to give each of the main boiler brands an overall customer score and a brand reliability score.

We then combine these with an engineers score - see more on our engineers' survey below.

Which? boiler brand reliability score

In May 2025, we asked members of the public who had had a new boiler installed within the previous six years about their brand of boiler, including whether they had ever needed to have it repaired and what faults (if any) they had experienced. 

The Which? reliability score is based on the proportion of boilers from each brand that have had to be repaired within that six hear timeframe. Scores are adjusted by age so a brand isn't unfairly penalised if the sample in our survey is made up of older boilers. This ensures that all brands are compared on an equal basis. 

We need at least 50 responses per brand to calculate a reliability score and publish an overall score for a boiler brand.

Which? boiler brands customer score

We also asked boiler owners to assess how satisfied they were, and how likely they would be to recommend their brand of electric, gas or oil boiler (regardless of their experience of getting the boiler installed). These two questions form the basis of the Which? customer score, which is included in our boiler brand reviews.

Which? Trusted Trader heating engineer score

In May and June 2025, we also collected the views of 225 Which? Trusted Trader heating engineers about the boiler brands that they routinely work on.

Which? Trusted Trader heating engineers have all been through a rigorous vetting procedure to prove their expertise and the quality of their work before they qualify for Trusted Trader status.

We ask these heating engineers whether they would recommend each brand of boiler, what the build quality is like, how easy it is to fix a common fault (e.g. replacing a domestic hot water heat exchanger) and how easy it is to obtain parts and spares.

Sometimes, engineers are incentivised by manufacturers to recommend their boilers. We filter out any of these responses, so you can be sure that our heating engineer scores for each boiler brand are independent and unbiased. 

This enables us to give you unique insight into the reputation of each boiler brand among heating engineers, and to find out whether you're likely to get hit with a higher repair bill.

The total score for both gas and electric boilers is based on: 

• Reliability (40%) How many boilers bought in the past six years developed a fault, factoring in time until the first fault and severity. The fault had to be repaired by a trained plumber or engineer. 
• Engineer score (40%) Made up of 25% unbiased recommendations (barring incentivised engineers), 20% build quality, 15% ease of sourcing parts, 15% ease of replacing common components, 15% ease of servicing, 10% manufacturer support for installers.
• Customer score (20%) How satisfied owners are and likelihood to recommend a brand. 

Is your boiler in need of a repair? Check our common boiler problems and repair costs advice to check you aren't overpaying.

Why doesn't Which? test boilers?

We don't send individual boilers to a test lab for the following reasons:

• Efficiency is tightly controlled Since 2018, the government's Boiler Plus standard requires all boilers to have a minimum ErP efficiency of 92%, so you wouldn't expect to see a noticeable difference in your gas or oil consumption if you were to choose the most over the least efficient boiler.
• Efficiency claims are accurate Our previous tests found the boiler efficiency claims made by manufacturers to generally be accurate. 
• Efficiency depends on a home's central heating system Boiler performance is heavily influenced by the heating system you connect it to and how it's installed. A boiler installed incorrectly and connected to a poorly designed heating system will not work efficiently. 
• Feedback from Which? members You told us that what you really want to know is whether a boiler is reliable and which brands to consider.

However, all of our boiler reviews contain the full specification, any customer views and our own expert analysis to help you find the best boiler brand for your home.