What size boiler do I need? Try our power calculator

There are three main types of boiler: Combi, System, and Heat Only, but within these categories, homeowners must choose from a range of sizes. In the heating industry, size has nothing to do with physical dimensions; it is all about the kilowatt (kW) output, or the amount of energy the boiler can provide to your heating and hot water system.

As a general rule, the larger your house and the more radiators, taps, and showers you have, the more powerful the boiler you need. However, bigger isn't always better. Choosing a capacity much higher than you need leads to wasted money on both the initial purchase and ongoing running costs. Conversely, an underpowered boiler will leave you with tepid radiators and a lack of hot water.

At Ideal Heating, we want to help you find the perfect balance of power and efficiency for your current home and any future plans, such as adding an en-suite or extending your property.

Read our full gas boilers guide for expert advice on choosing your system

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 the 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 ratio of output to input. No boiler is 100% efficient, as it 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 perfect job.

Our boiler 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

What are the different 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 they 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.

Want to know more? Head to our guides below for more information:

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 needs to deliver to keep them all piping hot. As a ballpark 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.

Real-world example

To give you an idea of how these factors come together, here are three common household profiles:

Single-occupant flat

Property type: flat
Bedrooms: 1
Bathtubs: 0
Showers: 1
Gas: natural

Recommended: Vogue MAX Combi 26 kW

Average family

Property type: semi-detached
Bedrooms: 3
Bathtubs: 1
Showers: 1
Gas: natural

Recommended: Logic MAX System 24 kW

Large family

Property type: detached
Bedrooms: 5
Bathtubs: 2
Showers: 3
Gas: natural

Recommended: Vogue MAX System 32 kW

Leave it to the Ideal Heating Professionals

While these estimates are a helpful starting point, a Gas Safe registered engineer will provide a precise recommendation. They factor in insulation quality, ceiling height, and local water pressure to ensure your system is optimised for maximum energy efficiency.

Ideal Boilers has a list of engineers we trust to give honest advice and to 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.

The Future of Heating: Air Source Heat Pumps

Selecting the right size for your air source heat pump is essential for peak performance and energy efficiency. Factors such as the size of your property and its heating requirements play a key role in determining the system size. For more information about heat pumps, get in touch with our experts.

Learn more about heat pump size

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