What Size of Boiler Do I Need?

The size of the boiler you require is also known as the output. It’s measured in kW (kilowatts) and, in basic terms, the higher the number of kW, the better the supply of hot water you would receive.

What Size of Boiler Do I Need?
What Size of Boiler Do I Need?

However, this doesn’t mean everyone should be aiming to buy the highest output available. The size of your boiler should suit the size of your home, number of bathrooms and number of radiators. You should also take other factors into consideration such as the flow rate and mains pressure.

Why is it so important to get the right size boiler for your home?

Without meaning to sound too ‘Goldilocks’ about the whole thing, you need to find a boiler that is just right.

A boiler that is too small will not be able to heat your home adequately or produce enough hot water. A boiler that is too big will produce more heat than you need which will waste energy, increase your bills and produce unnecessary emissions. Neither is a great way to live, so how do you find the solution?

How to calculate the right size boiler for your home

In general, boilers range from 24kW to 42kW in size.To get an idea of the size of boiler you need, there are a few things to consider.

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Current boiler size

It’s a good idea to know the size of your current boiler as a starting point but don’t assume you need to stick with the same size going forward. Several things may have changed since it was installed such as the household’s water demands, the level of insulation or the addition of an extra bathroom. Or you may have plans for the near future such as an expanding family, a loft conversion or an extension.

If you find your home is regularly running out of hot water it could mean that your current size of boiler is not up to the task and you need to think bigger. However, if your current boiler is more than 8 years old then it may just be a case of inefficiency rather than a lack of capacity.

It’s also possible that the current boiler was never the ideal fit for your home and may have been mis-sold.

Looking for a new Combi boiler? Find out which combi is the best.

Size of your home

Generally, the more radiators and bathrooms you have the higher the boiler’s output will be.

For example, in the case of getting a Combi boiler*, here is a rough sizing guide:

  • 24-27kW = small house (1-2 bedroom) or flat, up to 10 radiators, 1 bathroom
  • 28-34kW = medium house (3-4 bedroom), up to 15 radiators, 1-2 bathrooms
  • 35-42kW = large house (4 or more bedrooms), up to 20 radiators, 2 or more bathrooms

*Combi boilers are not generally recommended for larger homes with 2 or more bathrooms as they only provide a single flow of water that weakens when split between outlets.

It’s also worth noting that an electric shower doesn’t take rely on the boiler for hot water so doesn’t need to be included in any calculations.

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Mains flow rate and water temperature

While Conventional or System boilers take their water supply from a large hot water cylinder, a Combi is fed directly from the mains. For this reason, if you are replacing an older system with a Combi boiler, the size you need will be influenced by the flow rate, or speed, of water coming from the mains. You need to make sure the Combi boiler you choose is big enough to cope with the amount of water passing through it to be heated.

You can get a rough idea of your flow rate with the help of a bucket with measurements. Run cold water from one of your taps for 1 minute and measure how much you collect. The more you collect, the faster the flow of water and the harder your boiler is going to need to work to heat the water as it flows through. The harder it needs to work, the higher the output needs to be. Many boiler manufacturers will specify the maximum flow of water in litres per minute so you will be able to see if your new boiler is a good match.

Similarly, the colder the temperature of the mains water the harder the boiler needs to work to heat it to the right temperature. A professional installer will be able to measure the flow rate and temperature of your mains water for you. Get in touch and up to 3 qualified installers will be in touch to arrange a visit to your home.

Hot water demand

Most replacement boilers fitted today are Combi boilers. They heat water on demand so there’s no waiting around for a cylinder to refill. However, what this does mean is that you need to give some thought as to how much hot water your household gets through on an average day. If you have lots of showers, sinks or baths running at the same time then a Combi boiler that is not powerful enough may struggle to cope. In fact, a professional may advise you that a System or Conventional boiler may be more suitable.

DIY Calculations

There are many websites offering boiler size DIY calculators which can help to give you a rough guide. They will ask for basic details about your home and possibly your existing boiler, but they should never be relied on as they can only provide estimates. We recommend getting multiple quotes from professionals who have actually visited your home and assessed your needs. Boiler Guide can provide you with up to 3 quotes from registered installers in your area.

Some other ways to keep your home as cosy as possible…


If your insulation is not adequate the you could be losing a lot of your heating through the roof, walls, floors or windrows. To make sure you’re giving your heating system the best chance to do its job you should think about investing in some more effective insulation.


Not all radiators are created equal as some have higher outputs than others. This is measured in British Thermal Units (BTUs). Each radiator should be adequate for the size of room that it is in.

Smart Controls

Boiler controls have moved on considerably in recent years and it’s now possible to have a greater level of control than ever before. You can now adjust the temperature of your home from any location via smartphones or tablets so you can reduce the amount of energy you waste. Some can even automatically compensate for the weather outside.

Annual boiler servicing

To keep a boiler running at its best for as long as possible it’s really important to get it serviced by a professional on an annual basis. This will catch problems early, before they can develop into long term damage.

Need more help?

Calculating the right boiler size is something that installers will be able to advise you on so it’s important to get multiple quotes so you can compare their advice. Unfortunately, some less reputable installers may try to sell bigger boilers than you need to maximise profit, so getting second and third opinions is invaluable.

Boiler Guide has a network of over 5000 reputable and highly recommended installers ready to provide you with a quote. Send us your enquiry today and we will provide you with up to 3 quotes from Gas Safe registered installers.

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