What Size Boiler Do I Need? A Simple Guide

Picking a new boiler that is too small leaves you short of hot water, and too big wastes money and gas. So what size boiler do I need for my home? The answer comes down to output measured in kilowatts, and how much hot water and heating your home actually demands.

What boiler size really means

Boiler size is not physical size, it is power output in kilowatts, written as kW. A higher output heats more water faster. The aim is to match the output to your home, not simply to fit the biggest boiler you can afford.

A rough guide by home size

• Small flats and one or two bedroom homes with one bathroom usually suit a combi around 24 to 27 kW
• Average three bedroom homes tend to need roughly 28 to 34 kW
• Larger homes with two or more bathrooms often need 35 kW or more, and sometimes a system boiler instead of a combi

Combi or system?

The number of bathrooms matters as much as floor space. If two showers might run at once, a combi can struggle and a system boiler with a cylinder may suit you better. Our guide on combi vs system boilers explains the difference.

The honest answer to what size boiler do I need is that these figures are a starting point, not a final spec. Water pressure, radiator count and pipework all play a part, so the right size is best confirmed with a proper assessment. A free new boiler installation quote includes sizing it correctly for your home.