How to buy the best boiler

A new boiler is likely to be one of the most expensive household purchases you'll make – usually when your current one breaks down and needs replacing.

So it's important to get one from a brand you can rely on, and that you buy the right type and size of boiler to suit your needs. Read on for our advice on buying the best boiler for your home.

To compare boiler brands, we ask owners how satisfied they are with their boilers and survey 230 Which? Trusted Trader heating engineers to get their impartial opinion on each company. 

Compare the results in our reviews of the best gas and oil boiler brands.

Video: how to buy the best boiler

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

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

What type of boiler should I get?

Gas, oil, electric and LPG

Most households in Great Britain are connected to the gas network and so have a gas heating system equipped with a gas boiler. 

The government estimates that 4.4m households across Great Britain are not connected to the gas grid, according to its latest available data (2021). That's 15.1% of domestic properties. 

The proportion of households off the gas grid is highest in inner London and the South West (both 24%), the East of England (20%), Scotland and Wales (both 19%). 

For those not on the gas grid, there are alternatives. You can fit an oil boiler, a liquid petroleum gas (LPG) boiler or a wood-burning stove with a back boiler (which come with another set of environmental implications).  

Another option is electric boilers. These have become more popular in recent years. They have higher running costs than gas boilers, though may appeal to householders looking to cut their carbon footprint.

Know the type of fuel you need? Read our reviews of the best gas and oil boiler brands and electric boilers.

Combi, heat-only and system boilers

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

• Combi boilers provide hot water on demand. Best suited for small flats or homes, with low water demands (eg, only 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 a 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 parts built into the boiler body. Best suited for larger homes with poor water pressure and a high water demand.

Your heating engineer will be able to talk you through the different types and make a recommendation to suit your needs. 

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

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Why is boiler efficiency important?

The Government Department for Energy Security and Net Zero reported in 2022 that there were 55.7 megatonnes of greenhouse gas emissions from UK homes, accounting for around 13.4% of all national carbon emissions.

The UK is heading towards net zero carbon emissions by 2050. 

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.

ErP stands for the Energy Related Products directive. It's a recognisable label that you get when you buy any product that consumes energy, with a rating between A-G represented by a colour scale from red for the least energy efficient through to green for the most. Most modern boilers should get an 'A' grade on this scale.

But a boiler's efficiency depends on how it's working in your home. A boiler won't achieve its efficiency potential without compensation controls that adjust its flow temperature automatically, and a correctly installed and configured system. 

An efficient boiler uses less gas to create the same amount of heat, which is good for your carbon footprint. Also, when energy prices are still sky high, it makes it cheaper to run.

See our guide to which boilers are most energy efficient. 

Choosing the right size boiler for your home

Choosing the right size boiler includes two things: the boiler's physical size (whether it will fit in the space you've designated) 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 unique to you and your home. It depends on factors such as how many bathrooms or radiators you have and how many people live in your home.

The general rule is, the more heat and hot water you use – the higher kW boiler you'll need. Choosing the right size is something your heating engineer can help with. 

In the tables below, see which is the right size boiler for you, according to the Heating Hub which provides independent and expert advice on heating systems.¹

It's always good practice to ask the engineer what size boiler they think best fits your requirements. 

Where to buy a boiler

You can buy a boiler yourself, but we strongly recommend getting an assessment from a heating engineer first. Otherwise, you could find yourself buying the wrong boiler for your needs, and/or finding yourself stuck with a boiler that no local heating engineer is ready to install. Legally, only Gas Safe-registered engineers can install boilers, so you can't do it yourself. 

In the first instance, we'd suggest contacting a Trusted Trader – we thoroughly vet the traders listed. 

Ask for a quote and details of what make and model of boiler they would recommend for your circumstances. They'll probably buy the boiler for you and arrange for it to be brought to your home to fit it.

Many boiler companies, energy companies and third parties will take you through the whole process. The following companies will assess your needs, virtually or in person, sell you a boiler and arrange installation. Depending on their affiliations, some will only supply particular brands, so it's worth shopping around for one that suits what you're after:

• ATAG
• Baxi
• Boiler Central
• BOXT
• British Gas
• EDF Energy
• E.On Energy
• Ferroli
• Glow-worm
• Heatable (a Which? Trusted Trader)
• The Heating Hub
• Ideal
• iHeat (a Which? Trusted Trader)
• Ovo Energy
• Swale Heating
• Vaillant
• Viessmann
• Warmzilla
• Worcester Bosch

It's sensible to get a number of quotes before you decide on who to go with. If you feel like you're getting a hard sell, step back and take time to decide. 

Make sure the installer fully explains how you can get your boiler running most efficiently when it's fitted. 

Buying a reliable boiler

In 2024, we surveyed 10,028 people who bought a boiler in the past 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 in the first six years.

There's a big difference between the best and worst. After six years, you're more than twice as likely to have experienced a fault with the worst brand than the best.

Compare reliability results in our reviews of the best gas and oil boiler brands.

The best boiler for your home

Best boiler for a small flat

If you live in a small flat with one bathroom, a combi boiler is the ideal choice. They don't need space for a hot water cylinder or a large cold water feed tank, which means the boiler doesn't take up much space.

We'd recommend a small combi boiler that has an output of less than 24kW. 

Find out if you're eligible for a free boiler or grant.

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 that uses a lot of hot water, and you also have room to store a hot water cylinder and cold water storage tank, 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-sized 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'll 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 your home and how well insulated it is, you'll 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 a large home where you've converted the loft and don't have space for a cold water feed tank, a system boiler could be your saving grace.  

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

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

How to get the best boiler installation

Leave installation to the professionals. You can find a qualified heating engineer on the Gas Safe Register or use Which? Trusted Traders to find a local trader vetted by Which?. 

Here's what you need to ask and tell your engineer when they visit to give you a quote, what you should expect from a proper installation and the checks you can carry out after the new one is installed.

Getting a boiler installation quote

When you get a heating engineer to assess your home and give you a quote, 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 recommend the type and brand for your home. 

Here are six questions to think about:

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 aren't usually compatible with a shower that has a powerful electric pump, so you'll have to ditch the power shower if you go for this type or get a heat-only or system boiler.
3. Do your hot water demands change through the year? Do you regularly have friends or family staying over, or do you host large parties or Christmas gatherings? If there are going to be big spikes in your hot water demand, 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 or 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? Some engineers are incentivised to install boilers from particular brands, so be sure they're making a recommendation that benefits you, rather than them. 

A good installer should mention some or all of the below.

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. 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 prevent sludge and debris entering your boiler, prolonging its life. Scale reducers are useful for hard-water areas and again help prolong the boiler's life. 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're 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 to replace any radiators? Older ones may be inefficient or have internal corrosion, so they might need replacing. Ask the installer for 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, or read more about boiler controls and thermostats.

New boiler installation costs can typically be anywhere from around £1,400 to £7,000, 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.   

Some installation companies might suggest a replacement boiler rather than letting you choose your preferred model. Many heating engineers are incentivised to install particular brands.

So it's vital you check your heating engineer's recommendation against the views of their peers and get a few different quotes.

What is Boiler Plus?

The UK government introduced the new Boiler Plus legislation in 2017 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's a combi boiler, additional energy-saving measures must be installed:

1. Flue gas heat recovery system This reuses heat from your boiler that would otherwise be wasted, increasing its 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 property.
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 money on your 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 costs, as you can ensure that your heating is only on in certain rooms at certain times. Read more about boiler controls and thermostats.

Thinking of getting smart heating? Read our smart thermostat reviews and smart radiator valve reviews.

What should be included in a boiler installation quote?

Quotes should be clear so you can clearly see what your money is paying for. However, we've seen many installation quotes that are difficult to understand. Some are fully itemised, but some engineers don't split all the costs out. But they should still be able to verbally talk through the individual costs of each element.

Here's what you should look for:

1. The brand, model and price of the boiler being installed There should be an explanation on why it's 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.
3. Location There should be an explanation of whether the new boiler will remain in the same place as the previous one, or if it needs to be moved. For the latter, it should explain why.
4. Water treatment Cleaning is required, but the method will depend on how dirty the existing heating system is. The installer should advise you on this.
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 to be installed in a different location. 
6. System balancing/Post-installation costs It should be mentioned by the engineer that they will make the necessary checks after installation, such as checking that 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 boiler is working correctly. 

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

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

Ready to get a new boiler installed? Read our guide to 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. 

Choose a Which? Trusted Trader for an unbiased, expert opinion.