07th February, 2022

What Size Combi Boiler Do I Need?

Find out whether the size of your combi boiler will make any difference to its capabilities.

What Size Combi Boiler Do I Need?

How is a boiler sized?

Contrary to popular belief, size does make a difference...... When it comes to your boiler.

When we talk about boiler size, we’re often referring to the output in kilowatts (kW) as opposed to the physical dimensions of the boiler itself.

Kilowatts are the measurement of energy that is output by the boiler, in the form of heat. We know it can be confusing when it comes to buying a new boiler, which is why we've not only provided our best combi boilers of 2021, but will also explain how to determine which size boiler you need in the article below.

Generally the bigger the home the bigger kW combi boiler you will need.

Which size boiler is right for my home?

Boiler Sizes

The right kW boiler for your home depends on a number of factors, including your demand for hot water, the number of bedrooms, bathrooms and radiators. In general, the higher your demand for heat and hot water, the higher the output you will need.


Small single flat home

If you live alone or have a small family and live in a flat or bungalow with one to two bedroom(s), one bathroom and up to ten radiators then a boiler with an output between 24-29kW is ideal for your home.

Average sized family

If you have an average sized family and live in a medium terrace, bungalow or semi detached house with three to four bedrooms, one to two bathrooms and up to fifteen radiators, a boiler with an output between 30-35kW is ideal for your home.

Larger family home

If you have a large family with a high demand for hot water and live in a large semi-detached or detached house with over four bedrooms, over two bathrooms and up to twenty radiators, a boiler with an output of 35-43kW is generally ideal for your home, or you may require a much larger system boiler if your family needs lots of hot water at any given time.

Count your radiators

One way of identifying the right boiler size for you and your home is by counting the number of radiators you have. The more radiators you have, the bigger the boiler you’ll need due to the higher amount of power that is required to heat the entire home. Generally, you can expect the following number of radiators to require the following power outputs:

  • Up to 10 radiators: 24–27 kW
  • 10 to 15 radiators: 28–34 kW
  • 5 to 20 radiators: 32–40 kW
  • Up to 10 radiators: 12–15 kW
  • 10 to 15 radiators: 18–24 kW
  • 15 to 20 radiators: 30–40 kW

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Other factors

Another factor that plays a part in deciding what size of boiler you require is how well insulated your home is. If you don’t have cavity wall insulation, loft insulation, double/triple glazed windows and decent sealing around doors and windows, then you could be losing a lot of heat from your home and therefore require a higher output if you are not intending to make those changes.

The type of fuel used to heat your home can also affect the efficiency of your boiler. Gas boilers offer the greatest levels of efficiency and are the most common type of boiler in the UK, whilst oil or LFG (liquified petroleum gas) are less popular and less efficient.

Another major factor when it comes to deciding which boiler is best for you is the price. None of us want to overspend! For more information on the cost of a new boiler, read our advice to ensure you are as informed as possible.

Should I oversize to be on the safe side?

Unfortunately, a boiler is not like a school jumper that you can grow into. Oversizing on your boiler could lead to wasted energy and is likely to make you pay more than you need for your fuel bills.

Leave it to the experts

To find out which boiler size is best for you, get a bespoke, fixed boiler quote here with installation available as soon as the next day. If you'd prefer to speak to one of our friendly team over the phone please don't hesitate to give us a call on 0333 305 6880.

Whichever boiler size you choose, keep in mind that its efficiency and operation will be at their best if your boiler is serviced annually by one of iHeat’s Gas Safe registered engineers.

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