What size boiler do I need?

If your boiler has broken down for the umpteenth time this year, it may be time to consider getting your hands on a new one. But, how on earth do you calculate boiler size? With lots of different types of boilers out there, which come in all sorts of sizes (although nearly always rectangular in shape), working out what size boiler you need can be a minefield. Luckily, the boiler experts at Toasti are here to help!

Types of boilers

There are three main types of boiler that are suitable for the home. Before asking yourself, “What size boiler do I need?”, familiarise yourself with the different types and decide which one is best for you.

Heat-only boilers

A heat-only boiler, almost known as a regular boiler or traditional boiler, is one of the oldest types of boilers around. A heat-only boiler provides the heat for your central heating systems and also warms up water which is then transferred to a hot water storage cylinder where it is kept until the hot water tap or your shower is turned on. 

Heat-only boilers require a tank of water that is usually stored in your loft to provide the water to heat the home, as well as the hot water cylinder that is usually found in an airing cupboard. These types of boilers are great for homes with low water pressure or those with multiple bathrooms. The main benefits of a heat-only boiler are that hot water can be supplied to multiple taps at once, making them a great choice for large families who need to use multiple bathrooms at the same time.

System boilers

A system boiler works very similarly to a heat-only boiler in that it provides the heat for your central heating and hot water, before storing it in a cylinder. However, unlike heat-only boilers, system boilers take the water directly from the mains. This makes them much more compact than heat-only boilers, but they are still able to deliver a high volume of domestic hot water making them perfect for large families who may lack space. By taking their water supply directly from the mains, system boilers eliminate the need for a tank of water in the loft, freeing up valuable space. 


Combi-boilers provide heat and hot water simultaneously from one unit, on-demand. Combi-boilers are typically more economical and cheaper to install making them a popular and budget-friendly option for many families. Unlike heat-only boilers, combi-boilers provide heat and hot water instantly, eliminating the need for water storage tanks. This makes them the ideal boiler solution for smaller homes, who do not have the capacity for large cold or hot water cylinders. 

Why is choosing the right boiler size important?

When it comes to choosing a new boiler, choosing the right size is critical if you want to ensure you can heat your home efficiently and have enough hot water to meet the demands of your family.

How many times in your childhood did you come to have a bath and find that all the hot water had been used? By making sure you have the right size boiler, lukewarm baths and showers can be a thing of the past, all while making sure your energy bills are cost-effective and you’re being as environmentally friendly as possible too. 

How is boiler size measured?

When you start thinking about what size boiler you need, you may be thinking about storage space and dimensions, however this is not how we measure boiler size. All boilers come in very similar dimensions, however they all output different amounts of energy – and this is what we mean by boiler size. The more power the boiler can output in kilowatts (kW), the bigger it is in terms of size. 

When you calculate boiler size you may notice two powers in the specifications of any given boiler. One is the power input, the other is power output.

Power input

This is how much energy your boiler consumes in order to heat your home and hot water.

Power output

This is how much energy your boiler can output of the input amount showing how efficient it is. No boiler will be 100% efficient as the boiler itself requires some energy to operate, however anything above 90% is deemed to be very efficient.

The output is measured in kilowatts (kW) and can come in a range of sizes between 12kW to 40kW. There are a number of factors which will determine what sort of output is right for your home. 

What size boiler do I need?

There are a number of things to consider when deciding on what size boiler you need. Firstly, the right sized boiler is smaller than you think. Don’t be tempted to get a bigger boiler than you need as this will cause inefficiency and make your energy bills much bigger than they need to be – and nobody wants that!

Let’s look at all the other things you need to consider when determining what size boiler your home requires:

Number of radiators

The number of radiators in your home is a key factor when calculating boiler size. For example, if you live in a small one-bedroom flat with only a handful of radiators, you’ll need a much smaller boiler than a five-bedroom detached home which could potentially have twenty radiators or more.

A smaller home with up to 10 radiators could benefit well from a 12kW radiator, whereas a larger home with over 20 radiators may need a boiler that can output 40kW.

Hot water demand

If the demand for hot water is high in your home, for example, if you have teens who need three showers a day or you constantly need hot water for endless washing up, then a combi-boiler which heats water on demand may not be sufficient for your needs. The more hot water you need, the higher output combi-boiler you’ll require, making them oversized for the central heating requirements, meaning they’ll be inefficient.

A system or heat-only boiler uses a storage tank to store hot water so will be much more economical for houses that use a lot of hot water. 

Ask the professionals

Still confused about how to calculate boiler size? Don’t worry, the boiler experts at toasti are here to help. Our professional boiler engineers can help you decide which size boiler will be the perfect fit for your home, taking all the hassle out of you wondering what size boiler you need.

Contact the team now, or send us an email to info@toasti.co.uk and find out more about the best sized boiler for your needs.