What size boiler do I need?
Count the radiators and the bathrooms

For most homes we allow about 1.5kW of heat for each radiator, and another 3kW if there is a hot water cylinder, then round up to the next size the manufacturers make, so ten radiators and a cylinder come to about 18kW and an 18kW boiler. The hot water decides the type of boiler: with one bathroom and good water coming in, a combi of 24 to 30kW is usually right, and where two showers or baths run at the same time we would look first at a system boiler with an unvented cylinder. These are averages from the jobs we do, and the survey is where the size is confirmed.

Five questions, about a minute

This is a guide from averages. The survey is where we measure the water coming in, look at the radiators and the gas pipe, and confirm the size.

About 1.5kW a radiator, and 3kW for a cylinder

Each radiator is different and each system is different, but on average we allow about 1.5kW for every radiator in the house, and if there is a hot water cylinder, vented or unvented, we add another 3kW to heat it. The total is then rounded up to the next size the manufacturers actually make, so thirteen radiators and a cylinder come to 22.5kW and a 24kW boiler.

Underfloor heating is worked out room by room at the survey, so if you have some, we add it to the figure then.

One bathroom and good water: a combi

With one bathroom and a good flow of cold water coming in, a combi of 24 to 30kW is usually the right boiler, because it heats the water as you use it and there is no cylinder to find room for. A combi gives roughly 0.4 litres of hot water a minute for every kW, at the 35°C rise the manufacturers quote, so a 24kW combi gives about 10 litres a minute and a 30kW combi about 12. When we checked that against the manufacturers' own figures for the 35 combis on our boiler comparison, 32 of them came out between 0.40 and 0.43 litres a minute per kW, and the other three, which use heat from the flue to warm the incoming water, gave a little more.

The one thing to check in a larger home is the heating side, as many combis heat the radiators at less than their headline figure, so a 30kW combi may give only 24kW or 25kW to the heating.

Two showers at once: the order we would look at

If two showers or baths are going to run at the same time, or you would like nobody's shower to go cold when someone opens the kitchen tap, our first choice is a system boiler with an unvented cylinder, which stores the hot water and feeds it at mains pressure to more than one outlet.

Where there is no room or budget for a cylinder, a combi-store is the second option. It holds only about 20 litres of hot water, which is usually enough to stop someone getting cold water in the shower while a kitchen tap is open for a short while, as long as the cold water coming in is good.

A large combi of 35 to 40kW is the last option and not an ideal one. It is often used where there is no other way, and it may or may not work out, because it is still one supply of hot water shared between everything that is open. The bigger the boiler, the more likely the gas pipe from the meter needs upgrading, which we check at the survey.

What size cylinder?

For the cylinder we go by bathrooms: up to 180 litres for one bathroom, up to 250 litres for two and up to 300 litres for three, and above that we size it at the survey from how the house uses its hot water. It does not have to sit next to the boiler, as an airing cupboard, a loft, the eaves if there is enough space, a garage, or an outbuilding as a last resort can all work.

All of it depends on the cold water coming in

Every one of these options depends on a good supply of cold water from the street, which is why the guide asks you to time a bucket. A 10-litre bucket filling in under 30 seconds means more than 20 litres a minute, which is the minimum Heatrae Sadia gives for its Megaflo Eco cylinder alongside a working pressure of 1.5 bar, and a bucket that takes over a minute means less than 10 litres a minute, which would hold back even a smaller combi.

Where the water coming in is weak there are ways round it: the supply from the street can be upgraded, an accumulator can store water at mains pressure for the busy moments, or a booster pump with a break tank can do the work, and we have fitted all of them. We measure the flow and the pressure properly at the survey before recommending any of them.

The short version

Count the radiators, allow about 1.5kW for each and 3kW for a cylinder, and round up to the next size made. One bathroom with good water usually means a combi of 24 to 30kW, and two showers at once means a system boiler with an unvented cylinder first, a combi-store second and a large combi last. Every option depends on the cold water coming in, and the survey is where the size is confirmed.

Quick answers