Category - Boilers
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Will Scholfield, Engineer

31 Jan : Updated 1 May ● 7 min read

What size boiler do I need?

The Ultimate Guide To Choosing The Right Size Boiler For Your Home

We all know that boilers are essential to our homes. From heating to hot water - without it we’d be cold, miserable and, quite frankly, dirty without them. But, if you’re not an expert, how do you know what size boiler to choose?

Given that purchasing a new boiler is a costly investment, it’s crucial that you get this right. Too large, and you’ll be spending more than necessary on your new boiler, too small and it won't be able to produce enough hot water or heating for your property, resulting in chilly rooms and cold showers. There are a number of things to consider before you choose the right size boiler for you, from how much hot water you use in your property, to the types of fuel supplies available.

It can be difficult, to know exactly which size to choose, but if you’re asking yourself the question - what size boiler do I need? We are here to help.

Boiler size calculator

Property Type

No. of Bedrooms

No. of Radiators

Recommended Boiler Size

Apartment/Flat or House with up to 2 bedrooms


Up to 10


Apartment/Flat or House with 2-3 bedrooms and 1 shower


Up to 15


House with 4+ bedrooms and 2 showers


Up to 28


This guide is simply a recommendation for normal hot water usage, and you will need to choose a larger boiler if you use an excessive amount of hot water on an average day, or have more people living in a smaller home. This should provide you with some indication on which boiler to go for. 

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Choosing the right type of boiler

Before we get onto the subject of boiler sizes, it’s important to note that there are a range of different types of boiler, all of which have specific benefits. Much like choosing the size of your boiler, there are a few things to consider when choosing the type too, such as how much space you have available, along with what boiler system you have currently.  

So, if you’re looking to replace your gas boiler, here are your options:

Standard boilers

Also known as 'Conventional', 'Regular' or 'Traditional' boiler. No matter what you choose to call them they are all a traditional heating system. Standard boilers use hot and cold water storage tanks to heat your water.

A conventional boiler uses fuel, such as gas or oil, to warm up the water, which is then transported to the hot water storage tank through the use of a pump. When your heating or water is turned on, this hot water travels to your taps.

Combi boilers

A hugely popular choice (and a favourite for many BOXT customers), combination boilers, also known as combi boilers, heat hot water as and when it’s needed, so no hot water is stored. They are a combination of a water heater and a central heating boiler in one single system, and they work by staying on standby and heating water almost instantly when you require it. However, combi boilers can only support one outlet at a time, so can either be used for heating or hot water, but not both at the same time.

System boilers

Ideal for larger properties that require large quantities of water, a system boiler works by taking water directly from the mains and heating it in a storage cylinder for use in your home. By using the mains water supply, system boilers eliminate the need for a cold water tank and means you will have instant access to hot water.

Does boiler size really matter?

To put it simply, getting the right boiler size for your property is vital, and depends on a number of different factors. First, you need to determine how much hot water you’re likely to use, as this will have a direct impact on what size boiler you choose.

Similarly, the size of your property, along with the number of bedrooms you have will also be a contributing factor. This is because you’ll need to select a boiler that is large enough to provide heating to your entire property, without resulting in wasted energy.

The size of the boiler, also known as the output,  is measured in kilowatts (kW). To put it simply, the higher the number of kW, the more powerful the boiler. While there is no ‘one-size-fits-all’ solution when it comes to choosing the right output, we’ll help to give you a good idea on what’s right for you.  

Please note that the boiler will only be able to provide you will hot water as fast as it receives the cold water from the mains. So for example, if you have 10 litres per minutes going into your boiler and you purchase a bigger boiler which can provide 16 litres per minute of hot water, you will only be able to receive 10 litres of hot water per minute at a maximum. So, you might be better looking at a smaller boiler to save yourself some money.

There are three main groups of combi boiler sizes, all of which work best for various property and household sizes:

  • 24kw-27kw
  • 28kw-34kw
  • 35kw-42kw

So, what size boiler do I need?

We know that trying to find the right boiler for your property can be tricky, and so, here at BOXT, we can help. Here’s how we do it - we work out the best boiler for you by calculating the number of radiators, bathrooms, and people in your home, considering the following variants:

  • Property Size – It’s simple – the larger the property, the more powerful the boiler will need to be to heat it.
  • Number of Radiators – The more radiators in a home, the more water will be needed to keep them warm, and the longer they’ll take to heat up.
  • Number of Bathrooms – We’ll need to consider how many bathrooms your property has, along with how often you run baths and showers, to determine how powerful your boiler needs to be.
  • Boiler Usage – Similarly, if you have a lot of people living in your home, the demand for hot water and heating will obviously be higher - requiring a larger kW of boiler.
  • Number of Floors – If you live in a property that is spread out over a number of different floors, you’ll need a boiler that is powerful enough to ensure water pressure is maintained throughout.

Learn more about which boiler is best for a 2-3 bedroom house.

Flow rate test

If you have a combi you're concerned about your boiler's pressure you can do a quick flow rate test to double check everything is as it should be. It doesn't take very long and is probably a good idea before contacting a plumber. Here's what you do:

1. Take a measuring jug. Roughly between 1-2 litres. 

2. Turn on the tap you're worried about (or shower). 

3. Then, simply time how long it takes for the jug to be filled.  

For more information and to calculate your flow rate, check out this guide on testing water pressure.

How does BOXT work out what size boiler I need? 

Here at BOXT, we help you to find the right boiler for your property, without all the extra fuss. We’ll simply ask you a few easy questions and give you a choice of fitted boilers at a fixed price. You can then choose your new boiler and installation date online, without the hassle of pushy salespeople! What's more, we also offer annual boiler servicing, so you can rest knowing your boiler is running as it should for many years to come.

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