What Size Boiler Do I Need?

A combi heats mains water as it flows through, so its size sets how much hot water you get at once. Each kW heats about 0.4 litres a minute by 35°C, which is roughly what it takes to turn winter mains water into a hot shower. That's the rise makers quote their flow rates at, and our figures match the flow rates in their manuals to within half a litre.

The number in a combi's name is how fast it heats hot water. Its heating output is often lower, and often the same across sizes. Ideal's Logic MAX C24, C30 and C35 all heat at 24.2 kW, and Baxi's 30 and 36 both heat at 25 kW. The smallest combis heat at 18 to 24.2 kW.

That's still far more than most homes need. The government's own modelling puts the average house at about 6 kW of heat at around minus 2°C, and it notes that most combis sold are 24 kW or more. So if a combi suits your hot water, it will almost always cope with the heating.

A system or regular boiler heats a hot water cylinder, so its size comes down to the heating. That depends on how much heat your home loses on a cold day. BRE has worked this out for typical homes of each type and age, using the government's housing survey:

A combi can't give you more hot water than your mains supplies. Baxi's example: if the mains only gives 12 litres a minute, anything above about 29 kW is wasted. Worcester Bosch makes the same point: a bigger kW won't always mean more flow.

To check yours, turn off every other tap and time the kitchen cold tap, turned fully on, filling a 4.5 litre bucket. The water regulator's yardstick is 9 litres a minute at about 1 bar, which fills that bucket in 30 seconds. The legal minimum is lower, at 0.7 bar. If the pressure drops below that for an hour, twice in 28 days, your water company owes you £50 in England (up to £250 a year) or £25 in Wales, unless it was down to essential work or drought.

No. England's building regulations guidance says heating systems should not be significantly oversized, and that a combi should be able to turn down to the home's typical heating load. Most of the year your home needs a fraction of its cold-day figure, and a boiler that can't turn down that far switches on and off instead. The government says that cycling cuts efficiency and adds wear.

Bigger combis tend to have higher minimums. The smallest combis we checked turn down to between 2.5 and 5.2 kW, and the biggest to between 3.7 and 7.2 kW. Baxi's 24 goes down to 4.8 kW and its 36 only to 7.2 kW. If you don't need the extra hot water, the smaller boiler is cheaper to buy and better at gentle heating.