What size boiler do I need?

Why is choosing a suitably sized boiler so important?

In the world of boilers, size equals power and output. So it’s very important to have the right amount of heat output to heat and provide hot water to your home:

1. If your boiler’s too small, your property won’t provide enough warmth or hot water

If your central heating boiler is undersized for your property, then your system will have trouble getting enough hot water to your radiators and hot water cylinder (if you have one). This means your family (or tenants) won’t be warm or happy during the colder months.

2. If your boiler’s too big, you could waste hundreds of pounds

An overpowered boiler won’t provide you with any extra benefits. However, a modern condensing boiler can automatically tweak its output to meet your lower demands. The increased output could be too much for your heating system to cope with so that extra power is wasted. But keep in mind, you’ll have to spend big bucks on a new boiler you may not need.

Types of boilers

Why combi boilers are the most efficient choice for most properties

Our article on how boilers work will help you understand the process of how a boiler heats your home. A quick read will help you decide which type and size of boiler is perfect for you and your family or property. As you’ll understand from this article, a modern condensing boiler is the most efficient choice for most properties.

Combi boilers heat water directly from the mains when you turn on a tap. With a combi boiler you don’t need a hot water cylinder or a cold water tank in the loft space, roof or basement. For a 4-person family or an average-sized home, combi boilers are usually the most efficient option.

When is a conventional boiler more suitable?

However, if you have a large property with 3 or more baths or showers that could feasibly be used at the same time, a combi boiler may not be the right choice for you. A conventional boiler will likely be more suitable (with a hot water tank as this will have a volume of water heated to 60oc that can be delivered to multiple outlets at the same time without a significant drop in flow rate or pressure.

So what’s the best way to calculate the size of combi boiler your property needs?

So, if you have an average-sized home, we’ve established that you need to choose a boiler size that’s not too big or too small. You need it to be just enough to cover your heating and hot water needs, as and when you need them. No extra expense, no excess energy.

The combi boiler size really dictates the hot water output as most combis boilers have a limit of 24kW for heating. Therefore if you purchase a 35kW combi, only 24kW will be used to heat the home whereas when you open the hot tap you get the full 35kW power output. So how many bathrooms and/or showers do you have? This is a simple calculation that gives you the most accurate idea of what to buy.

No of bathrooms or showers	Minimum boiler size
1	24kW
2	30kW
3	35kW
3+	35kW*

*Again, if you have a large family or property with 3 or more baths or showers being used at the same time, a combi boiler may not be right for you. A conventional boiler will likely be more suitable (with a hot water tank).

What sizes of combi boiler are available?

Combi boilers are generally available in three size ranges;

24 kW
Suits most flats, apartments and smaller houses, with 1 bathroom.

30kW
This is perfect for a medium to large 3-4 bedroom house with 1 bathroom and an ensuite bathroom.

35kW to 42kW
This suits a large house with anything over and 2 bathrooms.

When a combi boiler won’t cut it

Warning: Combi boilers don’t fare well when there’s more than one shower running at the same time. If yours is a busy household or you’re a landlord of a multiple occupancy property, consider a system boiler with an unvented cylinder.

For more helpful pointers for landlords, our friends at the British Landlord Association are bursting with great tips and tricks – check them out!

Get a new A-rated boiler from BOXT

We hope we’ve helped you understand what type of boiler size you need for your home. If you run into problems with your current boiler, we’re working with BOXT who offer a speedy installation service – you could be enjoying your new system as soon as the very next day*.

Get an estimated new boiler quote and find the perfect boiler for your home.

FAQs

How do I choose the right size boiler?

You need to measure it against the size of your house and number of bathrooms. A small flat or house with one bathroom can run perfectly well with a 24-27kW combi boiler. A large house with 3 or more bathrooms will suit a system boiler with an unvented cylinder.

What size boiler do I need UK?

• 24 to 27kw – Suits most flats and small houses, with up to 10 radiators and 1 bathroom
• 28 to 34kw – Perfect for a 3-4 bedroom house with up to 15 radiators, 1 bathroom and an ensuite
• 35 to 42kw – Suits a large house with up to 20 radiators and 2 bathrooms

*Subject to engineer availability