What size boiler do I need?

Let’s help you pick the right size boiler for your home.

Choosing a boiler size.

Why is it important to choose the right size boiler?

Put your measuring tape away. When we talk about the right boiler size, we are not referring to how tall or wide it is. We mean its power output (measured in kilowatts: kW). The higher its kilowatt output, the more heat your boiler will produce.

If you choose a boiler that is too small, you may not have enough hot water when you need it and your home may not get warm enough.

On the flip side, choose a boiler that is too large and you could end up wasting energy (and money) by using extra fuel you don’t need. That’s why it’s so important to spend time working out the right boiler size.

Factors to consider when choosing the right boiler size.

How much hot water you need.

This depends on the number of people in your home, the number of baths, showers, sinks and so on, how often you use hot water, and whether you have “hot-water-hungry” appliances like a dishwasher or washing machine.

Number of rooms with radiators.

Each radiator requires a certain amount of heat to warm up a room, so the more radiators you have, the larger the boiler you will need.

How well insulated your home is.

A well-insulated home will lose less heat, so you will need a smaller (i.e. less powerful) boiler than a poorly insulated home.

The type of fuel in your home.

If you have gas central heating, you will need a gas boiler. If your home is not on the gas mains you may have to opt for oil central heating. In which case, you will need an oil fuelled boiler (which needs more space for things like an oil storage tank).

Of course, you can always get a professional assessment from a qualified heating engineer via video call. They will be able to take into account all of the factors listed above and recommend the best boiler for your needs.

Which boiler is best for you?

Here’s a quick guide to the three main types of gas boilers to help you choose the right sized one for your home.

Combi boiler

Combi boilers.

These are great if you live in a smaller home. They don’t need a separate hot water cylinder or cold water storage tank, so they could easily fit into a cupboard and don’t take up any of your valuable space under the stairs or in the loft (for example).

More on combi boilers.

Regular boiler

Regular boilers.

If you often use hot water around your home, then a regular - or “condensing” - boiler is probably best for you. They can take up more room than a combi as they often come with a cold water tank as a kind of back up but you’ll have a plentiful supply of water ready to be heated regardless of water pressure or any other issues.

More on regular boilers.

System boiler

System boilers.

Does your home have more than one bathroom? Then a system boiler is a good choice. They’re like a condensing boiler but with a large tank for storing extra hot water. Again, they take up more room than a combi boiler, but you have a constant supply throughout your home when you need it.

More on system boilers.

How to calculate the boiler size you need?

How many radiators do you have?

The number of radiators in your home is a key factor. Generally, the more radiators you have, the larger the boiler you'll need to heat them effectively.

This is because each radiator requires a certain amount of hot water to maintain a comfortable temperature, and a larger boiler can provide the necessary flow rate to meet this demand.

Combi boilers vs regular boilers vs system boilers:

Combi boilers differ from regular and system boilers in terms of their power requirements and operation. Combi boilers are designed to provide both heating and hot water on demand, which means they need more power to heat water instantly. However, they only need to operate for a few minutes a day to meet hot water needs.

In contrast, regular and system boilers store hot water in a cylinder, which requires the boiler to heat up the entire cylinder, resulting in (you guessed it), longer operation times.

Does the size of my home affect the size of the boiler I need?

Here’s some rad advice. If you have a bigger house to heat, you’re going to need a bigger boiler. But it’s not just the size of the rooms - it’s those radiators again. As an estimate, if you have a combi boiler, you’ll need the following power outputs:

  • Up to 10 radiators: 24-27kW.

  • 10-15 radiators: 28-34kW.

  • 15-20 radiators - 32-40kW.

If you have a system or regular boiler, then we estimate you’ll need:

  • Up to 10 radiators: 12-15kW.

  • 10-15 radiators: 18-24kW

  • 15-20 radiators - 30-40kW.

You can see the range of boiler sizes here. A 12kW boiler will run only 10 radiators or so, While a 30kW boiler will heat up to 15 radiators or more.

This is why it’s so important to work out the right size boiler. Remember, these figures are only a rough guide. You may want to talk to a heating engineer to get a more expert opinion.

How many bathrooms do you have?

The number of bathrooms and the number of people taking showers or baths will affect the size of the boiler you need.

A regular or system boiler is recommended for homes with multiple bathrooms, as a combi boiler may not provide enough hot water.

For multiple bathrooms, a boiler size around 35-42kW is needed, while for one bathroom, a 10-25kW boiler should do the trick.

Other factors to consider when choosing a boiler.

As well as the number of radiators and bathrooms, you need to take into account other factors when choosing your boiler size. These include:



What insulation do you have? Is it just loft insulation or are your walls insulated too? A home with poor insulation will require a larger boiler to compensate for heat loss.


Desired temperature.

The warmer you’d like your home to consistently be, the larger boiler you’ll need.


Boiler efficiency.

The boiler's efficiency rating is also important. A more efficient boiler will use less energy to heat your home and water, saving you money on bills.


Future expansion.

If you think one day you might add a conservatory or go up into the loft, choosing a boiler with additional capacity might be a good idea.

As always, a qualified heating engineer can professionally assess your specific situation and provide expert advice on the most suitable boiler size and type for your home.

Does boiler efficiency matter?

It certainly does. Boiler efficiency is all about how much energy your boiler uses to heat water - and how much is wasted. Obviously you will want a boiler with the highest efficiency rating (A). This is an efficiency rating of about 90%. The lowest boiler rating (G) means that it has only a 70% rating - or less.

To put these ratings into some kind of context, that means that for every pound you spend heating up water in your G rated boiler with a 70% rating, 30p (or more) of it is wasted.

Fortunately, only very old boilers are G rated and in fact regulations now require that all new boilers installed have an Energy-Related Products rating of 92%.

Still, don’t assume your boiler is super efficient. Check it and see what rating it is. You may want to check its rating and consider getting a more efficient one to replace it.

Can I downsize my boiler size if I have made energy-efficient upgrades to my property?

Have you made any changes to your property such as:

  • Improved insulation?

  • Double-glazed windows?

  • A more efficient heating system?

Then you may not need such a powerful boiler as before. That’s because these kinds of changes reduce heat loss in your home - and the amount of energy needed to keep it as warm as before.

Of course, the best way to find out if you can downsize your boiler is to get a professional in. A heating engineer can work out how effective your upgrades are and help you calculate the new heat requirements for your home. They take into account other things too, like the size of your home and other heating needs like the number of bathrooms you have and so on.

With carefully calculated requirements, you may well find you can downsize to a smaller boiler. In turn, this can lead to:

  • Less energy used .

  • Lower bills.

  • And even less maintenance and repair.

But make sure these calculations are accurate. Remember, if you end up with a boiler that is too small for your home’s needs, then you may well struggle to keep it warm and have enough hot water for your needs.

How does hot water demand affect your boiler size?

Again, the simple answer is obvious: if you use more hot water for things like showering and washing then you will be better off with a boiler that can meet those needs.

But what exactly are those factors…?

Flow rate?

This is the amount of hot water that your boiler can deliver per minute, measured in litres per minute (l/min)or gallons per minute (gpm) .

The higher your demand for hot water, the larger the flow rate on your new boiler should be.

Storage or combi boiler?

A storage boiler heats up your water then stores it in a separate cylinder, so you have that reserve of hot water to hand.

A combi (combination) boiler will heat water on demand, so it does not need that separate cylinder.

Combi boilers have smaller dimensions so are great for smaller properties with less space to spare. However, a storage boiler is more suitable for those with larger hot water requirements.

Boiler output?

As we mentioned before, this is what we really mean when we talk about “boiler size”. It’s the amount of heat your boiler can produce. If you think you are going to need a lot of hot water frequently, then you should consider getting a boiler with a higher boiler output.

Calculating your hot water demands.

We can help you calculate your specific hot water requirements with a video survey. By taking into account factors like how many people use hot water in your home, when the hot water is used and how much, we can work out the best boiler for you.