Boiler size calculator what kW you need

How the calculator picks a size

It reads your home against two boiler makers' published size tables, takes the higher of the two answers, and then checks your hot water use to decide between a combi and a boiler with a cylinder. It does not calculate heat loss. No calculator that only asks about bedrooms can.

The radiator count sets one answer, from Ideal Heating's boiler size guide. The bedroom count sets the other, from Vaillant's guide. When they differ, the calculator shows the bigger band, so neither the heating nor the hot water is left short. The home type you pick does not change the answer, because neither maker's table uses it.

Here it is for a three-bedroom semi with one bathroom, one shower, 12 radiators, and hot water used by one person at a time.

• Radiators: 12 is in Ideal's 10 to 15 band, which is 28 to 34 kW for a combi.
• Bedrooms: 3 is in Vaillant's 3 to 4 bedroom row, also 28 to 34 kW for a combi.
• Hot water: one bathroom, one shower, one outlet at a time. That fits Worcester Bosch's description of a combi home, so the result is a combi.
• Result: a 28 to 34 kW combi. In BTU per hour that is 95,536 to 116,008 (1 kW is 3,412 BTU/h).
• For comparison, the same home with a cylinder: 18 to 26 kW for a system or regular boiler on Vaillant's table, 18 to 24 kW on Ideal's. A boiler that heats a cylinder can be smaller, because it is not heating tap water on demand.
• And a reminder that this is a starting point: Ideal's own example for an average family in a three-bedroom semi, with one bath and one shower, recommends a 24 kW system boiler. More than one answer can be right for the same house.

Combi, or a boiler with a cylinder

A combi suits a home where hot water is used by one person at a time. Once two showers run together, or there are three or more bathrooms, the makers point to a system or regular boiler with a hot water cylinder.

Worcester Bosch puts it plainly: combis suit smaller homes with one bathroom, system boilers suit larger homes with more than one bathroom, and regular boilers suit large homes where a lot of hot water is needed at the same time. Ideal says a combi is best for smaller households, because only one hot tap can be drawn at a time. Viessmann says that with more than four bedrooms and three bathrooms a combi likely won't be the best option.

The calculator turns that into three results:

• Combi: one bathroom, no more than one shower, and hot water used one at a time.
• Either, ask the engineer: two bathrooms, two showers, or two outlets sometimes on at once. Ideal's and Vaillant's tables still list a 28 to 34 kW combi for homes like this, while Worcester steers anything with more than one bathroom towards a system boiler. Your mains supply decides it.
• A boiler with a cylinder: three or more bathrooms, three or more showers, hot water often used in two places at once, five or more bedrooms, or more than 20 radiators.

Why the kW alone does not decide the shower

A combi's kW sets how many litres a minute it can heat, but the water coming into the house sets how many litres a minute it gets. If the mains supply is weak, a bigger combi does not give a better shower.

Boiler makers quote hot water as litres per minute at a stated temperature rise. Worcester Bosch lists its Greenstar 4000 25kW at 9 litres a minute at a 40°C rise. Viessmann's example is 12 litres a minute at a 35°C rise, and it says a good shower from a combi wants at least 12 to 15 litres a minute. Check that two boilers are quoted at the same rise before you compare them.

Then the catch. Viessmann's own advice on shower pressure says you are restricted in flow rate by the mains pressure delivered to the property. A boiler that could heat 15 litres a minute, fed with 10, gives you 10. That is why the engineer checks your mains supply before recommending a combi, and why the final choice of boiler is the installer's, not a calculator's.

The makers' kW tables, side by side

Ideal, Vaillant and Viessmann all publish rough size bands, and they mostly agree for small and medium homes. They part company for larger homes and for boilers with a cylinder. Vaillant calls its own figures "just general guidelines", and that is the right way to read all of them.

The calculator uses Ideal's radiator bands and Vaillant's bedroom bands for combis, and Vaillant's table for system and regular boilers, with Ideal's figure shown alongside. Here is what each maker says.

When you need a heat loss survey instead

Any time the boiler has to be right rather than roughly right, the size should come from a heat loss calculation, not a bedroom count. Viessmann says the same: a much better way to choose a boiler size is to have an expert calculate the home's heat loss, from its floor area, windows, insulation and the coldest day of the year.

Get one when the house has been extended, insulated or opened up since the old boiler went in, when the calculator says "either", when the home is bigger than the tables cover, or when the old boiler was struggling. For a heat pump it is required: MCS installers must follow BS EN 12831, as the BTU calculator page explains.

Viessmann puts the heat load of an average home at around 10 to 20 kW. A combi of 28 kW or more is that big because of the hot water, not the radiators. If you want a feel for the heating side, run each room through the BTU calculator, and treat the total as a rough check, not a design.

Only a Gas Safe engineer can fit it

By law, anyone employed to work on gas appliances in a home must be on the Gas Safe Register. That is the Health and Safety Executive's reading of the Gas Safety (Installation and Use) Regulations 1998. Check the engineer's ID card, including the back, which lists the gas work they are qualified to do, and look them up on the Gas Safe Register before they start.

Ideal's guide ends in the same place: these estimates are a starting point, and a Gas Safe registered engineer will make the precise recommendation, taking in insulation, ceiling height and local water pressure.

If you fit boilers, this page is useful before the survey. A customer who has already seen the kW range for their home, and read why a combi might not suit three bathrooms, is easier to talk to about a cylinder. Put your recommendation and the reasons on the quote.

Common questions