How to buy the best boiler

A new boiler is likely to be one of the most expensive household purchases you'll make, so it's important to get the right one for your home, from a brand you can rely on.

But depending on the age of your boiler, replacing it with a newer model can save you money and reduce carbon emissions. Replacing an old, inefficient, non-condensing boiler with a modern, efficient, condensing system can improve your boiler's efficiency by more than 30% and save you anywhere from £45 to £300 a year on your heating bill (Energy Savings Trust, 2021)

Already know what type of boiler you need? Head straight to our list of Best Buy boilers, or read all our boiler reviews from the very best to the worst.

Video: how to buy the best boiler

When buying a new boiler, you need to consider what type of boiler you want, what size of boiler you need, whether your heating system needs anything doing to it while you're installing a new boiler, and which brand of boiler will be most reliable. 

Watch our video to help you decide which type of boiler is right for you.

Not sure if you need a new boiler? Look out for these five signs you need a new boiler.

What type of boiler should I get?

Gas, oil and LPG boilers

Most UK households are connected to the UK's gas network and so have a gas heating system equipped with a gas boiler. 

But for the 15% of homes not on the gas network (Energy Savings Trust, May 2021) there are alternatives such as fitting an oil boiler, liquid petroleum gas (LPG) boiler, or a wood-burning stove with a back boiler (which come with another set of environmental implications).  

Know the type of fuel you need? Read our gas boiler, oil boiler, LPG boiler reviews or see our best gas and oil boiler brand guide for the most reliable brands.

Combi, heat-only and system boilers

There are typically three different types of gas boiler to choose from: combination (combi) boilers, heat-only (also called conventional or regular) boilers and system boilers.

• Combi boilers provide hot water on demand. Best suited for small flats or homes, with low water demands (eg one bathroom).
• Heat-only (regular) boilers require a cold water feed tank and hot water storage tank, usually installed in the loft. Best suited for larger homes with high water demand (eg multiple bathrooms being used at the same time).
• System boilers are like regular boilers but don't require a cold water feed tank and have more components built into the boiler body. Best suited for larger homes, with poor water pressure, and high water demand (eg multiple bathrooms being used at the same time)

Your heating engineer will be able to talk you through the different types of boiler and make a recommendation on the best type for your needs. 

Learn more about boiler pros and cons in our different types of boiler advice guide.

Why is boiler efficiency important?

In 2017, a typical household generated around 2,745kg of carbon emissions (Energy Savings Trust, 2021). The UK is heading towards net zero carbon emissions by 2050. To achieve this household emissions will need to be reduced to just 138kg a year per household by 2050 (Energy Savings Trust, 2021).

At the moment, switching to a non-fossil-fuel boiler isn't necessarily cheap or easy. Most of us will be getting gas and oil boilers for the foreseeable future.  

This makes your choice of boiler a key priority, helping to lower your carbon footprint and and cut your energy bills.

You will see the biggest savings if you're replacing an old-fashioned non-condensing boiler with a modern condensing system, but since 2005 all gas boilers have had to be condensing boilers.

Moreover, since 2018, the government's 'Boiler Plus' legislation has required that all newly installed boilers have a minimum ErP efficiency of 92%, plus time and temperature controls fitted.

You can filter our boiler reviews by efficiency rating, but in reality there's not much difference between modern condensing boilers – all are efficient.

Which boilers are most energy efficient? Learn more about different efficiency ratings

Low-carbon heating systems

In order to reach the UK's net zero target by 2050, we will all need to eventually install low-carbon heating systems. Many are available to install now, with schemes such as the Renewable Heat Incentive designed to financially reward people who use renewable energy to heat their homes.

You can learn more about low-carbon heating, including how it works, the costs and if it's suitable for your home, in our expert guides on:

• Solar water heating Solar thermal panels that use heat from the sun to heat water to use in your home.
• Ground source heat pumps A network of water pipes buried underground, extracting natural heat from the ground that is then transferred to your home heating system. 
• Air source heat pumps Works in a similar manner to a ground source heat pump, but instead extracts heat from the air, which is then boosted to a higher temperature using a compressor.

You can still reduce your carbon footprint and heating bills with a gas or oil boiler. Discover our top tips to reduce your heating bills.

Choosing the right size boiler for your home

Choosing the right boiler size includes two things: its physical size (whether it will fit in the space you have designated for it) and its heating/hot water capabilities. The latter is measured in kilowatts (kW) and is often just referred to as 'boiler size'.

The correct boiler size will be something unique to you and your home, as it depends on many things: how many bathrooms do you have? How many people do you live with? Do you have large gatherings at your home? How many radiators do you have? 

However, the general rule is the more heat and hot water you need, the higher a kW boiler you will need.

Choosing the right size of boiler is something your heating engineer can help you with, and they will be key to making sure you achieve your hot water and heating requirements. 

We give each boiler's hot water and heating capabilities and give an indication as to what type of home it's suitable for. We also display the boiler's measurements, so that you can figure out if it will fit in the space you have for it.  

In the tables below you can see which is the right size boiler for you, according to The Heating Hub that provide independent and expert advice on heating systems¹.

It's always good practice to ask your heating engineer what size boiler they think best fits your requirements. Choose a Which? Trusted Trader for an unbiased, expert opinion.

Buying a reliable boiler

Reliability is by far the most important factor that will influence which boiler you'll buy. So we go to great lengths each year to separate the good from the bad brands in terms of reliability.

In 2021, we surveyed 8,001 people who bought a boiler in the last six years and asked what year their boiler first developed a fault (if at all). From this we calculate how many boilers remain fault-free over the first six years.

As you can see in the graph above, there's a big difference between the best and worst brands. After six years, you're almost twice as likely to have experienced a fault with the worst brand than the best.

Best boiler for a small flat

If you live in a small flat with only one bathroom, then a combi boiler is the ideal choice for you. Combi boilers don't need space for a hot water cylinder to store the hot water, or a large cold water feed tank, which means the boiler doesn't eat away at limited space.

We would recommend a small combi boiler that has an output of less than 24kW. Make sure you buy the best by selecting from one of our Best Buy small combi boilers.

How to get a free boiler or grant – find out if you're eligible

Best boiler for a two or three-bedroom house

For a mid-sized family home, the choice will be between a mid-sized combi boiler or a small heat-only boiler. If you have multiple bathrooms and a large family, with room to store a hot water cylinder and cold water storage tank, then the benefits of a heat-only boiler may start to outweigh the benefits of a combi. 

A trained heating engineer will be able to help you with your decision after conducting a survey. But make sure you get a Best Buy mid-size combi boiler or small heat-only boiler.

Best boiler for a four-bedroom house

For a larger home, the best choice is likely to be a heat-only boiler. You will need space for a hot water storage cylinder and a cold water feed tank in the loft, but for a larger home with multiple bathrooms, a heat-only boiler is nearly always the best option.  

Depending on the size of the home and how well insulated it is, you will need either a small or mid-sized heat-only boiler.

Best boiler for a home with low water pressure

If you live in an area with low water pressure, or if you live in a large home where you've converted the loft and don't have space for a cold water feed tank, then a system boiler could be your saving grace.  

System boilers don't require a cold water feed tank and they also make use of a pressurised system, which makes low mains water pressure much less of an issue. 

Make sure you pick one of the very best brands by heading to our system boiler reviews.

Keep your boiler in tip-top condition by getting the best boiler service.

The best boiler brands

With the average price of a single repair costing £200 (Which? boiler survey, 2021), getting a boiler from a reliable brand will cost you less to maintain over the years.

We ask boiler owners how satisfied they are with their boiler, and if they would recommend it to a friend. We also survey 153 Which? Trusted Trader heating engineers to get their expert opinion on each boiler brand. These are then combined to create a boiler brand's overall score.

Knowing where to start in your search for the best boiler can be tricky, so we've highlighted a few models below: the top three boiler brands overall, as well as high scoring, affordable boiler brands.

Become a Which? member to unlock the tables below and discover the best boiler brands.

Tables last updated 14 September 2021.

Find out how boiler brands performed in our best gas and oil boiler brands guide.

How to get the best boiler installation

Leave installing a new boiler to the professionals. You can find a qualified heating engineer on the Gas Safe Register, or use Which? Trusted Traders to find a local, trustworthy heating engineer. 

However, even though you won't be the one physically installing the boiler, there are still big decisions that need to be made. 

Read on to find out what you need to ask and tell your engineer when they visit to give you a quote, what you should expect from a proper boiler installation quote, and checks you can carry out after the new boiler is installed.

Getting a boiler installation quote

When you get a heating engineer to assess your home and give you a quotation for a new boiler, it shouldn't be a quick conversation. A good installer should take the time to conduct a full survey of your home, learn about your heating and hot water requirements, and only then give you recommendation on the boiler type and brand for your home. 

In order to get the best advice and quote, here are six questions to think about before you have a conversation with your installer:

1. How do you use your hot water? Do you regularly need hot water to be supplied to more than one tap or shower at the same time?
2. Do you have, or plan to have, a pumped power shower? Combi boilers are not usually compatible with a shower that has a powerful electric pump. So you will have to ditch the power shower if you go for a combi boiler, or get a heat-only or system boiler.
3. Do your hot water demands change through the year? Do you frequently have friends stay over, or do you host large parties or Christmas gatherings? If there are going to be large spikes in your hot water demand, make sure you get a boiler that can handle this.
4. Are you planning any home improvements? Let your installer know if you're thinking about adding an extension, loft conversion, new bathroom, underfloor heating. Any home improvements you have in the pipeline may change their recommendation
5. Are you having any problems with your current heating system? Are radiators taking too long to warm up? Is the flow rate of your water poor? Tell your installer about any concerns or performance issues you have with your current system.  
6. What boiler brand do you want? Our 2021 boiler survey found the worst brand to be nearly twice as likely to develop a fault in its first six years than the best brand. Some engineers are incentivised to install boilers from particular brands, so be sure they are recommending a boiler brand that benefits you, rather than them. 

We think a good installer will mention some or all of the following below. If they don't come up, ask them yourself. 

1. How will you dispose of condensate? This pipework takes water from the boiler away to the drain. External pipework should be kept minimal, but if there needs to be some the installer should ensure its protected from freezing temperatures, in accordance with manufacturer's instructions
2. How is the boiler water treated? Heating systems build up sludge over time causing cold spots on radiators and long heat up times. Systems require cleaning and flushing: a gravity flush for most, but a power flush if the system is heavily sludged. A power flush typically costs around £350 (Checkatrade, 2021). When refilling your heating system post-clean, ensure the installer adds a chemical inhibitor to slow down the formation of sludge, corrosion and scale build-up.  
3. Do I need system filters/scale reducers? Installing a new boiler into a dirty heating system will lead to future reliability problems. Filters typically cost between £70 and £200 (Screwfix, 2021) and prevent sludge and debris entering your boiler, prolonging its life. Scale reducers are useful for hard-water areas, and again help prolong the life of your combi boiler. Ask your installer if you need them.
4. How big will the plume be? This is the water vapour leaving your boiler flue. Ask the installer how big a plume you are likely to get, and where it will be situated. Will it bother neighbours? If needed, the plume can be taken away using an additional pipe.
5. Do I need radiators replacing? Older radiators may be inefficient, or have internal corrosion, meaning they may need replacing. Ask your installer for their advice.
6. Any additional energy-saving measures? Since 2018, 'Boiler Plus' legislation requires each new gas combi boiler installations to include at least one of four energy-saving measures: flue gas heat recovery, load-compensating thermostats, weather-compensating thermostats or smart heating controls. Get your installer to advise on the most appropriate one for you, or read more about boiler controls and thermostats.

New boiler installation costs can typically be anywhere from £1,000 to £5,000 (Which?, November 2019), so we recommend getting at least three quotes to ensure you're getting the best deal. Don't be taken in by any headline discounts, look at the total price for the boiler and labour charges.   

You may find that some installation companies will suggest a replacement boiler for you rather than allowing you to choose your preferred model. This is because many heating engineers are incentivised to install particular brands.

So it's vital that you check your heating engineer's recommendation against the views of their peers – which is where we can help. We surveyed 153 Which? Trusted Trader heating engineers in June 2021, to find out what they really think of the different boiler brands on the market. Read our boiler brand reviews to find out what they said.

What is Boiler Plus?

The UK government introduced new legislation in 2017 called 'Boiler Plus' to govern new gas boiler installations. Since 2018, the regulation requires that all new gas boiler installations in England:

• regardless of type, must have a minimum ErP efficiency of 92%
• must have timer and temperature controls included
• if it is a combi boiler, an additional energy-saving measure must be installed:

1. Flue gas heat recovery system This reuses heat from your boiler that would otherwise be wasted, increasing the boiler's efficiency.
2. Load-compensating thermostat This adjusts radiator temperature to be hotter when your home is cold, and cooler when your home is close to the desired temperature.
3. Weather-compensating thermostat The same as load-compensating, but instead based on the temperature outside the home.
4. Smart heating controls With automation and optimisation functions so you can control your heating remotely through a smartphone

This is all to make gas boilers as efficient as possible, reducing their environmental impact, and saving you money on your heating bills. 

Heating controls also let you take charge of when, where and at what temperature your heating is operating. It’s a good way of managing your heating costs, as you can ensure that your heating is only on certain rooms at certain times. Read more about boiler controls and thermostats in our expert guide.

Thinking of getting smart heating? Found out if smart thermostats are worth it, and read our smart thermostat reviews and smart radiator valve reviews to get started.

What should be included in a boiler installation quote?

Quotes should be clear and correctly itemised so you can clearly see what your money is paying for. However, we have seen many installation quotes that are bewildering and difficult to understand. 

Here is what you should look for in a correctly itemised installation quote:

1. The brand, model and price of the boiler being installed There should be an explanation on why this boiler has been recommended, with reference to its type (combi, heat-only or system) and size.
2. Extra heating controls These should be costed for and listed separately to the boiler.
3. The location of the boiler There should be an explanation of whether the new boiler will remain in the same place as the previous boiler, or if it needs to be moved. For the latter it should explain why it's moving.
4. Water treatment Cleaning is required, but the type of cleaning will depend on how dirty of heating system is. Your installer should advise on the most appropriate cleaning method for you.
5. An overview of labour costs This can vary depending on the amount of work that needs doing. Expect it to be higher if the new boiler needs installing in a new location. 
6. System balancing/Post-installation costs It should be mentioned by the engineer that they will make the necessary checks after installation, like checking your radiators are balanced and are working.
7. Explanation of the warranty This should detail what needs to be done to maintain the terms of the warranty your boiler comes with, for example getting annual service

Checks after your boiler installation 

After installation, your engineer should complete a series of safety and performance checks to make sure your new boiler is working correctly. 

The results should be recorded in the back of the boiler's instruction manual; this forms part of the benchmark process that's important for the warranty on your new boiler. You should also make sure your engineer registers your boiler with the manufacturer to activate the warranty.

If, after these checks, you notice cold spots on your radiators or they are taking a long time to heat up, this could be a sign that your system hasn't been balanced or cleaned properly. You should contact the installer immediately.

Ready to get a new boiler installed? Read our expert guide on how much a new boiler costs.

Which? Trusted Trader boiler installers

Getting the best boiler installation depends a lot on the installer you choose for the job, so pick a Which? Trusted Trader, all of whom have passed our stringent standards to become accredited. 

To replace a gas boiler, the installer must be Gas Safe registered, so ask to see a registration card, or check the Gas Safe Register. 

But we also think that you, the customer, have an important role in getting across to the installer exactly what it is you want from your new boiler.