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Updated: 25 Feb 2022

How we assess boilers

Find out more about how Which? assesses boiler brands to reveal the best and worst boilers
Sam Morris
Boiler parts and spares_MAIN

When you buy a new boiler, you'll want to be sure that 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 boilers that have a good reliability record, are loved by their owners and are highly regarded by Which? Trusted Trader heating engineers can earn our Best Buy recommendation.  

Read on to find out how we review boilers. 

If you just want to find out which models are best, head straight to our Best Buy boiler reviews

How we test boilers

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

Our most recent survey of boiler brands was carried out in May and June 2021. We surveyed more than 8,000 members of the general public, all of whom had installed a new gas or oil boiler 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.

Which? boiler brand reliability score

We asked members of the public about their brand of gas or oil condensing 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 that have had repairs. Scores are adjusted by age, so if, from our survey, a brand's sample is made up of older boilers it isn't unfairly penalised. Doing this ensures all brands are compared on an equal basis. 

We need at least 50 responses per brand to calculate a reliability score, and therefore 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 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 reviews.

Which? Trusted Trader heating engineer score

In June 2021, we collected the views of 153 Which? Trusted Trader heating engineers about the boilers brands that are available in the UK.

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 (replacing a domestic hot water heat exchanger) and how easy it is to get hold of 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 is independent and unbiased. 

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

We then combine these three scores together to get a brand's overall score:

  • 40% reliability score
  • 30% brand customer score
  • 30% overall engineers score

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 governments 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 homes 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, connected to a poorly designed heating system will not work efficiently. 
  • Feedback from Which? members - shows 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 for your home.