What Size Boiler Do I Need?

a woman wondering: "what size boiler do i need?"

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Flats usually need a 20-24kW boiler

Three-bedroom houses typically have a 30-35kW model

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Buying boilers is a tricky process.

You want the best boiler possible, but you don’t want to overpay by getting a model that’s too big. A 60kW machine isn’t necessary unless you’re heating Downton Abbey.

But you also don’t want to buy a machine that fails to supply your home with enough heat or hot water. No-one wants to be left in the cold, especially when British winters last so long.

A boiler’s size refers to its output – the number of kilowatts per hour (kW) it uses to heat your home. So whether you have a flat or a five-bedroom manor, you need to pick the right number to fill your home with warmth.

If you choose poorly, your boiler can break down before its time, cost more than it needs to, provide insufficient heat, and/or take up precious extra space in your home.

If you’re ready to take the plunge, fill in this form to find out the best boiler deal for you.

a woman wondering: "what size boiler do i need?"

What size boiler do I need?

It depends on the number of bedrooms and radiators you have to heat, but in general: the bigger your home, the bigger the boiler.

If you have a flat, you’ll want a machine with a 20kW to 24kW output – or, if your flat is particularly small, a 15 to 18kW boiler will do. 

If you have a two-bedroom house, look at boilers with outputs between 24kW and 30kW.

For a three-bedroom house, we recommend considering 30kW to 35kW boilers.

And if your house has four or more bedrooms, you’ll be looking at a giant 40kW boiler, so your heat and hot water never runs out on you.

Type of home

Size of boiler (kW)

Flat

20-24

Two-bedroom house

25-30

Three-bedroom house

30-35

Four-bedroom house or bigger

40

If you’re looking at the box which applies to you, not sure which end of the scale you should be looking at, still desperately asking: “What size boiler do I need?” – you’re in luck.

We’ve made a calculator to help you answer this difficult question – and here it is, in all its glory.

If you got a ‘999’ result in the calculator, that is a cry for help from your home. It means you don’t have the recommended number of radiators for the amount of bedrooms you have.

Please check out this handy table, which will let you know how many extra radiators we suggest you invest in before replacing your boiler. It’ll also provide a neat recap of the calculator’s other results.

Home type

1-4 radiators

5-9 radiators

10-14 radiators

15-19 radiators

20+ radiators

Flat

8kW

12kW

24kW

28kW

35kW

2 bedroom house

10kW

14kW

26kW

28kW

35kW

3 bedroom house

11-14 radiators

6-10 radiators

1-5 radiators

30kW

35kW

4 bedroom house

16-19 radiators

11-15 radiators

6-10 radiators

1-5 radiators

40kW

How can I tell what size I need?

As shown above, there are two main factors. So before you buy your new boiler, consider how many radiators are in your home, and what type of home you live in.

Make sure to also think about your current boiler’s performance. How does its output suit you? Are you warm enough, or could you do with a higher output?

The calculator and table above and the advice below should be treated as estimations, to be factored into how warm or cold your home feels.

How many radiators do you have?

With that mind, let’s have a look at how the number of radiators in your home impacts the size of the boiler you’ll need.

Number of radiators

Boiler size needed (kW)

1-4

8-10

5-9

12-22

10-14

24-26

15-19

28-35

20+

40

Does a higher kW boiler cost more to run?

No, it doesn’t.

Bear with us here. It’s true that a 36kW boiler will cost more to run than a 24kW model – after all, you pay for each kW you use. A 36kW machine will use 12kW more per hour than a 24kW one, so you’ll pay 50% more.

But your home’s size isn’t negotiable.

If you have a house with four bedrooms, you may well need a 36kW boiler. A 24kW boiler just wouldn’t do.

You’d need to run a 24kW for 50% longer for it to have the same impact as its 36kW counterpart – meaning you’d pay exactly the same, but would have to wait longer for your home to heat up.

In the long run, you’ll save money by getting a bigger boiler, as long as you have the big home to match it.

This is because you won’t have to use it as much, reducing wear and tear and prolonging the machine’s lifespan.

Boiler breakdowns are disastrous, so help avoid them by choosing the right boiler size for you.

Is a bigger boiler more expensive to buy?

Yes, it typically costs more initially.

The following table of average boiler prices shows that you’ll spend hundreds more on a bigger boiler.

But as we’ve outlined above, getting a model with a higher kW output means cutting back on costs in the long term – and not having to wait for your heating or hot water.

Kilowatts

Price

24

£600

30

£850

35

£1,150

Summary

Now you know what size of boiler you need, and why getting a bigger boiler is the only good choice, if that’s what your home requires.

All you need to do is work out which type you want – whether it’s a gas combi, system, conventional, oil, electric, or biomass – and which of the latest models is the best boiler for your home.

If you want to receive a free quote from trusted experts in the field, just fill in this form.

Josh Jackman Writer

Josh is The Eco Experts’ main man for home security, smart devices, and boilers. If it can make your life better and help the environment, he’s on it.