What size boiler do I need?

Typical boiler sizes by home

The table below shows the ranges installers and manufacturers commonly quote. Treat it as a starting point for a conversation, not an answer. Two houses with the same number of bedrooms can need very different boilers if one is a draughty Victorian terrace with solid walls and the other is a modern, well-insulated flat.

Combi boilers are listed by their hot water output. System and regular boilers are listed by their heating output, because the hot water comes from a stored cylinder instead.

Online boiler size calculators use the same kind of rules of thumb. They are fine for a ballpark, but none of them can measure your mains flow or your home's heat loss, which are the two figures that actually decide the size.

Why combis are sized differently

A combi heats water as you use it, so it has to turn cold mains water into hot water fast enough to run a shower. That takes far more power than heating the house. The headline kW figure on a combi is almost always its hot water output. Many combis have a lower maximum output for central heating, and they turn themselves down to match what the radiators actually need.

System and regular boilers work the other way round. They heat a hot water cylinder and the radiators, and the stored water does the heavy lifting when several taps are open. Their output only needs to cover the heating load plus a sensible allowance for reheating the cylinder, which is why a 15 to 24kW system boiler can serve a house that would need a 35kW combi. Our guide to the types of boilers explains how each one is set up.

What combi kW means at the shower

A combi's output translates into a flow of hot water. The figures below assume the incoming mains water is about 10°C and you want it at about 45°C, a 35°C rise, which is roughly what a comfortable shower needs. In a cold winter the mains can be colder, so the flow at the same temperature drops.

A typical mixer shower uses somewhere around 8 to 12 litres a minute, and a large rainfall head can use more. That is why a small combi can feel fine for one shower in summer and weak in January, and why running a kitchen tap while someone showers makes the shower go cooler.

Your mains supply matters more than the boiler

A combi can only heat the water it receives. If your incoming supply delivers 10 litres a minute, a 40kW combi will not give you 16 litres a minute; it will give you 10 litres of hotter water, and you will turn the shower's mixer towards cold. Paying for extra kW that the supply cannot use is one of the most common sizing mistakes.

A simple check is to open the kitchen cold tap fully and time how long it takes to fill a bucket of known size. A survey should also look at how the flow holds up with a second tap open, because many London properties have long or narrow supply pipes, shared supplies in converted houses, or old lead or iron pipework. If the flow is poor, our guides to low water pressure and to good pressure at the boundary but poor flow indoors explain what can be done before a new boiler goes in.

• Flow at the kitchen cold tap, in litres a minute, with nothing else running
• How much the flow drops when a second tap is opened
• Whether the supply is shared with another flat or property
• The size and material of the incoming supply pipe, where it can be seen

Heat loss decides the heating output

For heating, the right size is whatever replaces the heat the home loses on a cold day. That depends on floor area, insulation, walls, windows, ceiling heights and how exposed the building is. A heat loss calculation works this out room by room and also shows whether each radiator is big enough.

Most homes need less heating output than the boiler they already have. An oversized boiler reaches temperature quickly, switches off, then fires again a few minutes later. That on-off pattern, known as boiler short cycling, wastes gas and puts more wear on the fan, ignition and gas valve. A boiler matched to the heat loss can run for longer at a lower output, which is where modern condensing boilers are most efficient, especially with a lower boiler flow temperature.

When a combi is the wrong answer altogether

If two showers regularly run at the same time, or a bath needs to fill quickly while someone else uses hot water, no sensible combi size will fix it. The better answer is usually a system or regular boiler with a hot water cylinder, sized for your peak demand. An unvented cylinder can give strong mains-pressure hot water to several outlets at once, if the incoming supply is good enough.

Our comparison of a combi or system boiler covers the trade-off in more detail, and the guide to what size hot water cylinder you need explains how storage is sized around baths, showers and recovery time.

Do not just copy the old boiler's size

When a boiler fails, the easy answer is to fit a new one with the same output. That often repeats an old mistake. Many older boilers were oversized, and the home may have been improved since with loft insulation, new windows or an extension, all of which change the heat loss.

Size also affects the installation. A larger combi burns more gas at full output, and the gas pipe from the meter has to be big enough to supply it. If it is not, the pipe needs upgrading as part of the job. A good boiler replacement quote will say how the size was chosen and whether the gas supply was checked. Our guide to comparing boiler quotes lists what else a quote should show.

What to have ready for a survey

You do not need to work out the size yourself. It helps, though, to have the basic facts ready, so the engineer can check the right things on the day.

• Number of bathrooms, en-suites and showers, and whether any are used at the same time
• Whether you have a bath that you fill often
• The make, model and age of the current boiler, and any cylinder
• Any rooms that never get warm, or radiators that were added later
• Recent changes to the home, such as an extension, a loft conversion or new windows
• Any existing problems with water pressure or flow

Get the size right before you buy

Loyal Plumbers replaces and installs combi, system and regular boilers across London and inside the M25. Gas work is carried out by Gas Safe registered engineers. On a survey we measure the mains flow, look at the gas supply, ask how the household uses hot water and size the boiler to the home, not to the one being taken out.

Call us or send an enquiry with the number of bedrooms and bathrooms, your current boiler and any hot water problems you have now. If you are also weighing up the timing, our guide to how long does a boiler replacement take explains what the day involves.