How the two demands are worked out
Heating demand is the design heat loss of the house, which comes from a heat loss calculation rather than from floor area.
Hot water demand is the energy needed to raise the water as it flows: litres per minute multiplied by 4.18 kilojoules per kilogram per kelvin, multiplied by the temperature rise, divided by 60. Twelve litres a minute through a 35 °C rise is 29.3 kW.
A combi serves both, but not at the same time. The output has to cover the larger of the two, which is almost always hot water.
Worked example
A house with an 8 kW heat loss and a 12 litre a minute shower through a 35 °C rise needs 29.3 kW for hot water and 8 kW for heating. The boiler must be at least 29.3 kW, so the next common size is 30 kW.
Reduce the flow rate to 9 litres a minute and the hot water demand falls to 22 kW, which a 24 kW boiler covers.
Why oversizing costs money
A combi modulates down, but only to a minimum. A boiler that is far too large spends more of the year cycling on and off at its minimum output, which is less efficient and wears the components.
Oversizing also costs more to buy. The right size is the one that meets the hot water demand at the flow rate you actually use, not the largest number on the price list.
A stored hot water cylinder changes the calculation entirely, because the boiler can heat the cylinder over time rather than instantaneously.
Limits of this calculator
It sizes a combi by output only. It does not check the gas supply, the flue route, the condensate drain or the radiator sizes, all of which can rule out a boiler regardless of its rating.
It assumes a single hot water outlet at a time. Two showers running together need the combined flow rate.
It does not choose a model, and it does not account for the difference between a boiler's rated output and its output at your flow temperature.
For US boiler sizing in BTU per hour, use the US boiler size calculator.
Inputs for the boiler size calculator
House heat loss
Design heat loss in kW from a heat loss calculation, not an estimate from floor area.
Hot water flow rate
Litres a minute through a shower or tap. A good shower is 10 to 12.
Temperature rise
Cold inlet to hot outlet. Mains at 10 °C heated to 45 °C is a 35 °C rise.
CALCULATOR QUESTIONS
UK boiler size calculator questions
What size combi do I need for a 3 bedroom house?
Usually 24 to 30 kW, set by the hot water flow rate rather than the number of bedrooms.
Does the boiler size follow the house size?
Not directly. Hot water demand sets the size in most homes with a combi, and it depends on flow rate.
Can I run two showers from a combi?
Only if the flow rate covers both at once. Add the two flow rates and size to the total.
Is a bigger boiler better?
No. It costs more and cycles more. Size to the demand you actually have.