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What size boiler do I need? Try our power calculator

There are three main types of boiler (combi, system and conventional), but within these types, there is a range of “sizes” that homeowners have to choose from. The size has nothing to do with the physical dimensions – they’re all about the same within each type – it’s all to do with the amount of energy they can put out into your heating and hot water system.

As a general rule, the bigger your house and the more radiators and hot taps and showers you have, the more powerful the boiler you need. A single-occupant bungalow will require a much less powerful boiler than a family of six living in a five-bedroom house with three showers.

But it’s not just about choosing the biggest boiler. If you get a boiler that’s much higher capacity than you need, you’ll be wasting money not only on the initial purchase, but also on running it. And conversely, if you try to save money on a cheaper, lower-powered boiler for a large family, you’ll probably find you frequently run out of hot water and have tepid radiators.

That’s why you need to calculate the size of boiler for your particular household, either for right now or for the near future, if you anticipate changes happening (more occupants arriving or building an en suite bathroom, for example).

Power input, output and efficiency

When calculating your boiler size, you might notice two powers in the specifications of a given boiler. One is the power input, the other is power output. Input says how much energy goes into the boiler to make it work, and output says how much of that energy is converted into usable heat. Efficiency is the percentage of output from the input. No boiler is 100% efficient, as the boiler itself requires some energy to operate, and even the most efficient condenser boiler will inevitably lose a bit of heat to the atmosphere. But at around 90% efficiency, most modern boilers do a pretty good job of getting close to perfect.

Our calculator does all the hard work

By far the easiest way to work out what size of boiler you need is to use our product selector. You input some stats and facts about your home – what kind it is, and how many bedrooms, baths and showers you have – and the computer will work out what is the best boiler for you. You’ll need to know what type of boiler you want, so if you’re completely new to this, see the next section.

Click here to use our power calculator.

Types of boiler

We’ve covered the three types of boiler in this article, but to summarise:

  • Combi boilers are best for smaller households as only one hot tap can be drawn at any time. Hot water is heated instantly, so you need good mains water pressure.

  • System boilers store hot water in a cylinder, so multiple hot taps and showers can be drawn simultaneously. They are sealed units, so need decent mains water pressure to work well.

  • Heat only boilers also store hot water in a cylinder, but the system is gravity fed, which means you need extra tanks in the loft. These are best if you live in an area with low pressure water and your home has multiple occupants.

Count your radiators

A good way of determining the size of boiler you need is to count your radiators around the home. Obviously the more you have, the more power the boiler will need to deliver to make sure they are all piping hot. As a ball-park estimate, you’ll need the following power outputs for the number of radiators for a combi boiler:

  • Up to 10 radiators: 24–27 kW

  • 10 to 15 radiators: 28–34 kW

  • 15 to 20 radiators: 32–40 kW

And for a heat only or system boiler:

  • Up to 10 radiators: 12–15 kW

  • 10 to 15 radiators: 18–24 kW

  • 15 to 20 radiators: 30–40 kW

Combi boilers need more power because they heat up the hot water instantly, whereas system and heat only boilers do it more slowly by circulating hot water around the cylinder. So while combi boilers might look much less efficient, they are different systems. 

You will only ever have the combi boiler fired up to max for the few minutes of the day when you’re washing, bathing, showering or washing up, whereas system and conventional boilers will be boiling for longer as they heat up the whole cylinder.


There are dozens of possible permutations of house type and usage, so we’ll give a few examples here.

Single-occupant flat

  • Property type: flat

  • Bedrooms: 1

  • Bathtubs: 0

  • Showers: 1

  • Gas: natural

RecommendedVogue Max Combi 26 kW

Average family

  • Property type: semi-detached

  • Bedrooms: 3

  • Bathtubs: 1

  • Showers: 1

  • Gas: natural

RecommendedLogic Max System 24 kW

Large family

  • Property type: detached

  • Bedrooms: 5

  • Bathtubs: 2

  • Showers: 3

  • Gas: natural

RecommendedVogue Max System 32 kW

Leave it to the professionals

The recommendations above are rough estimates only. A Gas Safe registered engineer will be able to give you a much more precise recommendation that will take into account the size of your home, how many floors you have, how well insulated it is, what your water pressure is like and several other important factors. They will also be able to talk you through the various thermostats, thermostatic radiator valves and other accessories that are available, so you can choose the combination that best suits your lifestyle and your future needs.

If you already have a central heating system installed, don’t assume the one you’ve got is perfect for your home. It could have been installed for a completely different household, and might not be set up for optimal efficiency and effectiveness. An engineer will be able to recommend if you need to change type, install or remove cylinders or tanks, and whether your boiler would work better if it was relocated.

Ideal Boilers has a list of engineers that we trust to give honest advice and install our equipment with quality, safety and efficiency in mind. Pop your postcode into the “find an installer” box on our home page, and we’ll list those closest by.

Click here to use our power calculator.

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