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What size of boiler do I need?

Posted on 12/6/2021

When it comes to getting a boiler, it’s important that you get one that’s the right size, so it meets the needs of your household. You don’t want to invest in a pricey new boiler only to find out it’s not going to give you enough hot water for everything you need. On the other hand, you don’t want to waste money and energy on a boiler that’s too powerful for what you actually need.

Understanding what size boiler I need for my house

You don’t want a boiler that’s too small that it struggles to heat up your home. Likewise, a boiler that’s too big could see your energy bills go through the roof. So how big should your boiler be? The simple rule you might have heard is, the bigger the house, the bigger the boiler. But this isn’t the actual size of the boiler – it’s the power output they produce.

Boiler output is measured in kilowatts (kW) and ranges from 24kW to 42kW for domestic homes. A heating engineer will calculate what you need based on how many radiators and bathrooms you have, as well as how many people living in your home. If you want to work out what size boiler you need for your house, here are some guidelines to help.

Small terraced house, flat or bungalow

  • One or two bedrooms
  • One bathroom
  • Up to 10 radiators
24 to 27kW output boiler.

Medium semi-detached or terraced house, or a larger bungalow

  • Three to four bedrooms
  • One or two bathrooms
  • Up to 15 radiators
28 to 34kW output boiler.

Large semi-detached or detached house

  • Four or more bedrooms
  • Two or more bathrooms
  • Up to 20 radiators
35 to 42kW output boiler.

What type of boiler do I have?

As well as knowing what output you need for a boiler, it’s also handy to know what type of boiler you have too. The best way to find out is by taking a quick look at your boiler. You should see two copper pipes coming from the top and one from the bottom. If you have a conventional (regular) boiler then you’ll have hot water cylinders, a pump, and a cold water storage tank in the loft – head up there to double check and then you know you definitely have a conventional boiler!

A system boiler will be similar to a regular one but it won’t have a tank in the loft.

If there’s no tank or cylinders and the hot water’s coming out as soon as you turn the tap on, then you’ve got a combi.

You can read up more about each boiler type here.

Talk to an expert

The best person to talk to would be a qualified engineer that’s Gas Safe registered. After all, it’s a legal requirement for any engineer installing a boiler to be on the register, to make sure it’s fitted correctly and safely. So if you want to know more about what size of boiler you need, who better to ask?

More help and advice

For more helpful tips about buying boilers, check out our guides, 5 Things To Think About Before Buying a Boiler article, and 6 Reasons Why Most People Overpay for a New Boiler.

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