The size of boiler that you will need will vary depending on the number of bedrooms and bathrooms that you have, how well your property is insulated and finally if you are buying a system boiler or a combi boiler.

The tables below will help you determine what kw boiler that you will need.

System boiler guidance

No of bedrooms Poorly insulated propertyModerately insulated propertyWell insulated property
1 bed8kW7kW5kW
2 bed8kW7kW6kW
3 bed10kW9kW6kW
4 bed12kW10kW7kW
5 bed15kW12kW9kW
6+ bed18kW15kW12kW

Combi boiler guidance

No. Bedrooms & BathroomsPoorly Insulated PropertyModerately Insulated PropertyWell Insulated Property
1 bed & 1 bath20 kW20 kW18 kW
2 bed & 1 bath20 kW20 kW18 kW
2 bed & 2 bath24 kW20 kW20 kW
3 bed & 1 bath28 kW26 kW24 kW
3 bed & 2 bath28 kW28 kW24 kW
4 bed & 1 bath30 kW30 kW28 kW
4 bed & 2 bath32 kW32 kW30 kW
5 bed & 2 bath40 kW34 kW30 kW

Why is it important to get the right size boiler?

Your new boiler will be providing your heating and hot water for your central heating for the next 10-15 years. Therefore you need to be careful when choosing it to ensure it can meet the hot water demands your household will place on it. Not doing so will mean less home comfort!

The kW must also be correct to ensure your energy bills aren’t any higher than they need to be.

Picking one too small means much of the boiler heat will be lost. Picking one too large will ensure high utility bills and plenty of wasted energy. This wasted energy will also mean unnecessary emissions, which is harmful to the environment. 

The majority of boilers come with an “output” range between 24kW and 42kW. Let us start making an estimate for which boiler size is right for you.

Do I need to oversize my boiler?

In short, no. Modern technology and boiler development has designed them to ensure they are more efficient than ever. Most operate above 90% energy efficiency! Old boilers were far less efficient, usually around 70%.

Once upon a time, the opposite advice was often given. Homeowners were advised to oversize the kW of their boiler to compensate for heat loss. Heat loss was usually as a result of poor home insulation, which isn’t often the case anymore.

As mentioned earlier, to oversize would just mean wasted energy nowadays. The only reason to justify one would be if you are planning to have an extension!

Many modern boilers are designed to “modulate”. This means the boiler can change its output (kW) to match heating demand. Many combi boilers have a wide modulation range.

How do I find out the right boiler size?

The first thing to note about getting a boiler is the physical size does not need to be considered. It is about picking the boiler with the best kW for you. A kilowatt is simply a unit measuring how much energy your boiler can output. 

This can be determined by examining a wide range of factors and parameters. Let’s dive right in!

Look at the size and type of your current boiler 

Taking into account your current boiler’s kW is a good place to start. Has anything changed in your household since it was installed? This will help initially determine if the new boiler output is going to be bigger. 

You must also note if it is a combi, system or regular boiler. No type is necessarily better than the other, it’ll depend on what your household needs. 

If you are looking to change from perhaps a conventional boiler to a combi boiler, you’ll have to check a few things. Firstly, that the boiler kW will match the pressure of the mains. Secondly, that the pipework durability is good enough due to the switch to a pressurised system. 

As different areas have different water pressure levels, you will need to check if you need a higher kW boiler. This would be to ensure you don’t suffer a drop in pressure if more than one tap or shower was operating. 

Check out our blog posts for combi and system boilers on what they are, and what they can offer.

Calculating your hot water and heat demand

One of the most important factors for determining your boiler size is the demand you will place on it. 

It may be you have a large or growing family, and find yourself running out of water too often.  Alternatively, it may be that you have more than one bathroom or have recently installed a new one. 

For example, the most popular type of boiler in Britain is the combi boiler. The hot water flow rate of a combi boiler is usually counted as “litres per minute”, or LPM. The litre per minute rate usually varies from 9LPM up to 25LPM. This can tell you how quickly a combi boiler can heat water. 

The higher the flow rate you require to heat water, the higher the kW output you are going to need. 

If your house is going to have a large water demand, it may be good to look at getting a system boiler. As well as the boiler, this also comes with a hot water cylinder to store extra hot water. 

Look at your home size

When we say home size, we don’t mean the actual square footage of your home. This has become an all too common mistake for boiler measuring.

Instead, the recommended procedure is to make note of how many bedrooms, bathrooms and radiators you have. 

Mainly focusing here on bedrooms and bathrooms, here are some general kW sizing estimates (for a combi boiler):

24-27kW1-2 bedrooms, 1 bathroom, up to 10 radiators
28-34kW4 bedrooms, 1-2 bathrooms, up to 15 radiators
35-42kW4+ bedrooms, 2+ bathrooms, up to 20 radiators

This statistical guide is for normal hot water usage. If there are more people in the house, the higher the demand, the higher, in turn, the kW.

A house with as many 5 bedrooms and up to 20 radiators is very big, by boiler measuring standards. A large 35kW-42kW combi boiler may provide enough sufficient heat. 

But it is worth noting that a house with 3 or more bathrooms could push a combi boiler to the limit. The boundary would be especially pushed if multiple showers were being used at once. 

Again, as a result of this, it would be worth considering a different type of boiler for large water demand. A system or conventional heat only boiler with a tank would probably be better. 

Here are some general kW sizing estimates (for a system boiler), with additional appraisal for insulation levels. As a rule of thumb according to heatable, you should do the following. Add 1.5kW for every radiator, and 3kW for your water cylinder (at home):

No of bedrooms and bathroomsPoorly insulated propertyModerately insulated propertyWell insulated property
1 bed8kW7kW5kW
2 bed8kW7kW6kW
3 bed10kW9kW6kW
4 bed12kW10kW7kW
5 bed15kW12kW9kW
6+ bed18kW15kW12kW

Table Credit: Warmzilla

Radiators play an important role in working out your ideal kW. This is because your boiler needs to provide hot water for them. To keep your house at your desired temperature, especially during winter, the boiler has to work hard and often.

If you have just a few radiators (supposing you live in a small flat) then a small kW boiler would likely suffice to save you money. 12-24kW would be about right. 

If you are getting a new boiler with a higher kW, you may need to look at getting new radiators installed. This is to ensure the heat is being spread efficiently!

According to a post from The Heating Hub, the vast majority of UK households have a maximum heat demand of: 

  • 6-8kW (very few properties exceed 10kW). This is smoothly accommodated by system and heat only boilers, as small boilers are available. 
  • The majority of combi boilers are producing an output of 18kW on heating, and upwards of 24kW for hot water. This is far too high! 
  • As the majority of modern boilers can be “range rated”, ensure at installation the output is adjusted to 6-10kW.

Stats courtesy of Viessmann

Does my fuel type affect my kW boiler?

Fuel type can actually affect what kW boiler you need to get. There are premium, high-end boilers that can automatically convert to any fuel type. But many boilers are suited to a particular type.  

Currently, natural gas is considered the most efficient type of fuel for the modern boiler. However, gas is only provided to a household if it is linked on to the national gas grid. 

As some homes are off the grid, they will need to rely on LPG (liquefied petroleum gas) or an oil boiler. These are not as efficient.

Less efficient fuel types, therefore, will need a boiler with a higher kW output.

And whilst we’re on this topic, remember many boilers are compatible with solar water heating systems. If this is the case, you won’t need such a high kW boiler. This is because it won’t need to work so hard to produce heat. 

We hope that this sizing guide to help you find the right kW boiler was helpful. Obviously getting a boiler is costly and can be a stressful process!

But as explained it is crucial you make the right decision, as it will have a long term impact, financially and domestically.

If you have any specific question you need help with, get in touch! Please continue to check out the rest of the blog for other queries.

Archie Everard

Archie is a content writer at Homesage, who has enjoyed freelance writing before and after his time at Oxford Brookes. Obviously he loves boilers most of all, but also enjoys reading, football and going to the pub!