What Size Boiler Do I Need For My House?

Picking the perfect boiler for a home can be a minefield. 

Picking the perfect boiler for a home can be a minefield. 

It goes without saying that larger properties with higher heating and hot water demands require larger boilers, but that’s not all there is to it. Picking a boiler suitable for a property requires an evaluation of insulation, the number of radiators and average hot water demands at least. 

Our heating engineers will help you, but it’s handy to have some idea of what type of boiler you’ll need and how powerful it needs to be for your home. 

This is a guide to boiler size and power ratings. 

How do you calculate boiler size?

Boilers have different power outputs typically measured in kilowatts (kW). 

Kilowatts are a measure of power and essentially gauge how much energy a boiler can output as heat. 

Every boiler on the market has a kW rating, but you can’t compare one type of boiler to another based on the kW value only. For example, combi boilers have considerably higher kW ratings than system or conventional boilers as they provide hot water on demand.

System boilers and conventional boilers gradually heat the water in a hot water tank and thus have lower ratings despite being able to provide the same net power. To learn about the different types of boilers, check out our guide here.

Combi boilers often have two ratings, one for hot water and one for heating. 

kW is not the only thing that matters! 

What size boiler do I need for my house?

So, what size boiler should you consider for different size homes and hot water demands? 

It depends on the following factors: 

1: Number of radiators 

room with  radiators below 3 windows

Flats might have just 6 radiators, whereas a large house can have over 30. The more radiators, the more powerful the boiler has to be. An underpowered boiler won’t be able to heat all radiators efficiently.

2: Insulation and energy performance 

Model house wrapped in Scarf

Highly efficient homes with great insulation are cheaper and easier to heat up. A well-insulated home requires a lower-rated boiler, decreasing heating bills and energy usage. 

Conversely, a poorly insulated home which loses a lot of heat will require a larger boiler to adequately heat the atmosphere. 

3: Number of bathrooms


Higher hot water demands require more powerful boilers. Combi boilers especially lack the teeth to provide over two bathrooms with regular hot water in larger households. The hot water tanks provided by system and conventional boilers help meet higher hot water demands. 

What size combi boiler does my home need?

Above: Combi boiler provide both heating and hot water 

First off, let’s discuss combi boilers. Combi boilers provide heating and hot water from a single unit without needing a separate hot water tank. That means they need more power and are rated higher in kW than equivalent system and conventional boilers.

The average combi boiler size required for different homes is below. Note that, even though combi boilers extend up to 43kW and above, it’s usually better to install a system or conventional boiler in homes that large. 

Bedrooms Radiators Bathrooms Power rating
1 to 2 0 to 10 1 24 to 27kW
3 to 4 10+ 2-3 28 to 34kW
More than 4 20+ More than 3 35 to 43kW

What size system or conventional boiler do I need?

Above: Conventional boiler 

System and regular boilers are broadly analogous in terms of power. They don’t require nearly as much power as combi boilers. That doesn’t necessarily mean they’re cheaper to run, though, as they warm up hot water on an ongoing basis rather than on demand like a combi boiler. 

The average system or regular boiler size required for different homes is below. It’s worth mentioning that smaller homes or flats with fewer than 10 radiators are well-provided by combi boilers. System and conventional boilers are at their best when installed in larger homes. 

Plus, they both require additional space to mount the hot water tank (and space in the loft for the conventional boiler’s cold water feed).

Bedrooms Radiators Bathrooms Power rating
1 to 2 0 to 10 1 9 to 18kW
3 to 4 10+ 2-3 18 to 26kW
More than 4 20+ More than 3 27 to 40kW

What size boiler by house type

Boilers vary from small heat-only boilers rated 9kW to massive 150kW+ boilers for very large homes and commercial building.

A heating engineer will help you choose the best boiler for your home, as there are many options available, and specialist knowledge is indispensable. Plus, boilers must be fitted by Gas Safe engineers anyway, so gaining their opinion on what boiler you need prior to installation makes sense. 

Here is a blow-by-blow of boiler size by house type: 

What size boiler do I need for a flat?

Flats are almost invariably heated by combi boilers these days unless they’re very large with 3 bedrooms and two bathrooms, in which case a system boiler could be advantageous. 

Above: Most flats benefit from small combi boilers 

The majority of flats in the UK have one or two bedrooms and are ideally provided by a small or standard-size combi boiler. 

Combi boilers have higher kW ratings as they have to heat water on demand. That doesn’t mean they’re less efficient or more expensive to run, as they don’t have to maintain hot water in a separate tank, like system or conventional heat-only boilers. As such, there are few combi boilers rated below 20kW, if any. 

Verdict: To heat a flat with a combi boiler, you’ll need a boiler rated around 24 to 27kW at minimum. If you want a system boiler instead (e.g. for a very large flat), this drops to around 12 to 15kW. 

What size boiler do I need for a 1 bedroom house?

1 bedroom houses are ideally powered by compact combi boilers, but small system boilers are also an option. 

Since 1 bedroom flats are likely occupied by single people or couples, hot water demands are low, and combi boilers are more than powerful enough. 

Verdict: To heat a 1-bedroom home with a combi boiler, you’ll need a boiler rated around 24 to 27kW at minimum. If you want a system boiler instead, this drops to around 14 to 18kW. 

What size boiler do I need for a 2 bedroom house?

Things remain steady for homes with 2 bedrooms. 2 bedroom houses could be occupied by as many as four people, e.g. two parents and two children sharing a bedroom, or even two couples. 

In this scenario, system boiler could be a good shout to cater to higher water demands. In most cases, however, a compact combi boiler will suffice. 

Verdict: To heat a 2-bedroom home with a combi boiler, you’ll need a boiler rated around 25 to 27kW at minimum. 30kW will be more than enough if the home has two couples. System boilers could be a good choice if hot water demands are high, in which case a 15 to 20kW boiler would be easily sufficient. 

What size boiler do I need for a 3 bedroom house?

You can still heat 3-bedroom homes with combi boilers, but we’re now getting into system and conventional boiler territory. 

Above: Homes with 3 or more bedrooms and two or more bathrooms may benefit from hot water tanks

Most 3-bedroom or more houses built pre-2000s come with conventional boilers and have an airing cupboard with a hot water tank and feed tanks in the loft. Combi boilers rated up to 34kW can replace them, but system boilers might be more appropriate. 

Verdict: To heat a 3-bedroom home, both combi, system and conventional boilers have their pros and cons. For combi boilers, look for something around the 28 to 34kW mark. For system or conventional boilers, 18 to 24kW should suffice. 

What size boiler do I need for a 4 bedroom house?

4 bedroom homes often have over 20 radiators and three bathrooms. If two or more people are using bathrooms at once, system and conventional boilers definitely have an advantage over combi boilers. You can get large combi boilers rated up to 45kW, but if more than three or four people live in the house, system and conventional boilers are better choices. 

Verdict: To heat a 4-bedroom home, combi boilers rated between 30 and 45kW are generally able to deal with most heating and hot water demands. But a 27 to 40kW conventional or system boiler will likely handle high demands more efficiently. 

What size boiler do I need for a 5 bedroom house?

It’s often said that combi boilers are unsuitable for large houses, but it depends on hot water demands.

For example, a family of five living in a large 5-bedroom home might have very high hot water demands (e.g. 3 or more simultaneous showers). In this case, it’s definitely better to look at conventional and system boilers. 

Generally speaking, 5-bedroom houses are thorough in system and conventional boiler territory, but some powerful modern combi boilers can handle it. 

Verdict: To heat a 5-bedroom home, look for a conventional or system boiler rated above 40kW for best results. 

Ask for advice

Working out the appropriate boiler size for different houses is not an exact science. Different boilers excel at different things, and some old pieces of advice like ‘combi boilers are not suitable for homes with more than one bathroom’ are redundant. 

Ask us for advice. We'll take your home layout and size, the number of radiators and average hot water and demands and provide you with the best options from top brands like Viessman, Worcester Bosch, Ideal, Baxi, etc. 

There are many choices when it comes to new boilers, but the standard is very high, and there isn’t much splitting some of the main brands. One thing is for sure: there is a boiler out there for you, discover our boiler quote page for more info.

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