What size boiler do I need? A simple sizing guide
Getting the boiler size right matters for your bills and your hot water. Here is how to work out the kW output your home needs.
Bigger is not better when it comes to boilers. Get the size right and you get strong hot water and lower bills. Get it wrong and you either run out of hot water or pay to heat water you never use. Here is how to size it sensibly.
It is measured in kW
Boiler output is measured in kilowatts (kW). A higher kW means more heating and hot water power. For combi boilers, the flow rate matters most, because a combi heats water on demand:
- 1 to 2 bedrooms, 1 bathroom: around 24 to 28kW
- 3 bedrooms, 1 bathroom: around 28 to 30kW
- 4+ bedrooms or 2 bathrooms: around 32 to 35kW
Browse the Norvera boiler catalogue and you will see the kW output listed on each model.
System boilers work differently
With a system boiler and a hot water cylinder, the cylinder size matters as much as the kW, because hot water is stored rather than made on demand. Larger homes with two or more bathrooms often do better with a system setup. Our combi vs system vs regular guide explains which suits which home.
Radiators and heat loss
A proper sizing also considers how many radiators you have and how well insulated your home is. The Energy Saving Trust has useful advice on insulation, which can let you fit a smaller, cheaper-to-run boiler.
Why not just fit the biggest?
An oversized boiler short-cycles, firing on and off repeatedly. That wastes gas, wears the boiler out faster and can actually feel less comfortable. Independent testers at Which? consistently make the same point: the right size beats the biggest.
Let us size it for you
The safest way to get it right is a proper assessment. Answer a few questions about your home and our Gas Safe engineers will recommend the right size and give you a fixed price.
Frequently asked questions
What size combi boiler for a 3 bed house?+
A 3-bed home with one bathroom is usually well served by a 28 to 30kW combi. Add a second bathroom and you may want 32 to 35kW for a strong flow when taps run together.
Is a bigger boiler always better?+
No. An oversized boiler cycles on and off, wears faster and costs more to run. The right size for your home is more efficient than the biggest one available.
How do I know what size I have now?+
The output in kW is usually on a sticker inside the boiler door or in the manual. Your engineer will confirm the right size for your home during the assessment.
Related guides
GuideHow to bleed a radiator (a 5-minute job you can do yourself)
Cold at the top but warm at the bottom? Your radiator needs bleeding. Here is how to do it safely in five minutes.
GuideHow often should a boiler be serviced? (And why it matters)
How often to service your boiler, what a service actually involves, and why skipping it can void your warranty.
AdviceBoiler Pressure Too High? How to Bring It Down
If your pressure gauge is sitting in the red, here's how to safely bring your boiler pressure back down without calling an engineer.
Need a boiler replacement?
Tell us about your boiler and we will get you a fixed price. No obligation.