A combi is sized by the tap, not by the house
Almost every boiler calculator asks how many bedrooms and radiators you have, then hands back a kW figure. For a combi that is the wrong input entirely, and the reason is structural rather than a matter of opinion.
A combi has no cylinder. There is no store of hot water to draw from, so it has to add the heat as the water passes through the heat exchanger, in real time. That ties its output directly to two things and nothing else: how many litres a minute you want at the tap, and how far you want that water raised in temperature.
The equation is one line. Litres a minute, times 4.18, times the temperature rise, divided by 60. Thirteen litres a minute at a 40 degree rise is 36 kW. Ten litres a minute at the same rise is 28 kW, which is how the market's default combi sizes ended up clustered where they are.
Meanwhile the mean measured heat load of a UK dwelling on a cold day is around 6 to 8 kW. So the combi in a typical house is four to five times bigger than the building needs for heating, not because anybody made a mistake, but because it has a second job with a completely different sizing basis and can only be built to one number.
So the number that decides everything is the minimum, not the maximum
A gas valve does not switch between off and full. It modulates, and modern premix valves turn down by roughly 1:6, or 1:10 on newer appliances. That sounds like plenty of range until you do the arithmetic on it.
A 36 kW combi at 1:6 bottoms out at 6 kW. If your house loses 6 kW on the coldest day of the year, the boiler at its very lowest setting is already matching your worst case, and for the rest of the winter it is putting in more heat than the building can absorb at any setting available to it.
So it fires, overshoots the flow temperature, shuts down, waits, and does it again. The field data on real UK homes found a median of 53 cycles a day on boilers rated 25 to 31 kW. In one 209-dwelling dataset, a fifth of boilers averaged under two and a half minutes per firing, and a case study found a quarter of combi firings lasted under two minutes.
Plant Size Ratio, and what it costs
The industry term for the mismatch is Plant Size Ratio: the boiler's rated output divided by the building's design heat loss. Above about 3, simulation against that field data shows an efficiency penalty of 6 to 9 percentage points.
Worth being careful with that number though. On a combi, a high ratio is not automatically a mistake, because the rated output was set by the hot water and had to be. The fix is a boiler that modulates further down, not a smaller one that cannot fill a bath. Those are different products and only one of them solves it.
What the regulations say, which is not what people think
Approved Document L, Volume 1, is the operative guidance in England and it is more precise about this than the industry habit is.
- Paragraph 5.8 requires the space heating system to be sized from a real heat loss calculation, and says systems "should not be significantly oversized". This applies to a boiler replacement in an existing home just as much as to new build.
- Paragraph 5.9 is the one nobody quotes: where a combi is used, "the boiler type should be selected to modulate down to the typical heating load of the dwelling". Note what that tests. It is a rule about the minimum output, not the rated output.
- Paragraph 5.11 requires the hot water side to be sized separately again, and also says it should not be significantly oversized.
- Paragraph 5.18 requires a minimum flow of water through the system specifically to avoid short cycling, which is the standard mitigation when the minimum output is above what the building can take.
There is a wider tell here too. The government's own sizing method, CE54, states plainly that it is for boilers up to 25 kW and "should not be used for combination boilers". It excluded them because there is no honest way to size an appliance with no cylinder from the building it sits in.
And since April 2018 in England, fitting a new gas combi triggers an extra mandatory efficiency measure on top of the 92% ErP standard: flue gas heat recovery, weather compensation, load compensation or qualifying smart controls. Two of those attack cycling directly, by getting the boiler to run cooler for longer rather than flat out in bursts.
Your mains flow rate is a hard ceiling and nobody measures it
This is the cheapest check on this page and the one most often skipped. A combi cannot deliver more hot water than the main brings in cold. If your incoming main runs at 11 litres a minute, a 40 kW boiler gives you precisely the same shower as a 30 kW one, and you have paid for capacity that physically cannot reach the tap.
Measuring it takes a minute. Run the cold kitchen tap fully open into a measuring jug for six seconds and multiply what you catch by ten. That is your litres a minute. Do it before you order anything, because it changes the answer more often than the boiler brand does.
Bear in mind too that a flow rate quoted at a 35 degree rise needs more kW at a 45 degree one. Winter mains water is colder than the catalogue assumes, so a boiler that measured well in July can disappoint in January at exactly the same litres a minute.
System and regular boilers work the other way round
Everything above is about combis. A system or regular boiler heats water in advance into a cylinder, so the flow rate at the shower is set by the cylinder and the pipework rather than by the appliance's instantaneous output.
Those are sized from the heat loss, with a cylinder reheat allowance on top that the installer works out against the actual cylinder. Applying combi-style hot water logic to them oversizes them for no benefit at all, and running the opposite error, sizing a combi purely from heat loss, gives you an appliance that cannot run a decent shower.
Why the rule on paper and the boilers in the field differ so much
A like-for-like boiler swap is normally self-certified by a single Gas Safe registered installer under a competent person scheme. Nobody independently inspects the heat loss calculation, and in practice nobody asks to see it.
So the requirement exists, is clearly written, and is largely unexamined, which is how field surveys keep finding Plant Size Ratios well above 3. The useful move is not to argue with your installer about the rated output, which they are probably sizing correctly for the hot water. It is to ask what the boiler's minimum output is, and to compare that one number against the heat loss. That is the question this tool exists to put in your hand.
Common questions
What size combi boiler do I need for a 3 bed house?
The number of bedrooms does not size a combi, though every other calculator answers it that way. A combi has no cylinder, so it heats water as it flows through and its kW is set entirely by the flow rate you want at the tap and the temperature rise you want. Two bathrooms running at once needs about 13 litres a minute at a 40 degree rise, which is 36 kW of instantaneous heating whether the house has two bedrooms or five.
Why is my new boiler so much bigger than my heat loss?
Because it was sized for the hot water, and it had to be. A typical UK home loses 6 to 8 kW on a cold day, but no 8 kW combi can fill a bath at an acceptable rate. The problem is not that the rated output is large; it is that the minimum output usually is too. A 36 kW combi at a common 1:6 turn-down bottoms out at 6 kW, which is already at or above what a well-insulated house needs.
What is short cycling and why does it matter?
If the boiler cannot turn down below what the house needs, it fires, overshoots, shuts off and starts again rather than running steadily at low output. Field data on real UK homes found a median of 53 cycles a day on boilers rated 25 to 31 kW, with a fifth averaging under two and a half minutes per firing. It costs 6 to 9 percentage points of efficiency once the Plant Size Ratio goes above 3, and the constant starts are what wear out ignition systems, pumps and heat exchangers.
Do the building regulations actually say a combi must match the heat loss?
No. Most people get this detail backwards. Approved Document L paragraph 5.9 says a combi "should be selected to modulate down to the typical heating load of the dwelling". That is a test on the minimum output, not the rated output. Paragraph 5.8 separately requires a heat loss calculation, and 5.11 requires the hot water side to be sized separately again. Two calculations, not one.
Why does my mains flow rate matter more than the boiler?
A combi cannot deliver more hot water than the main brings in cold. If your incoming main only does 11 litres a minute, a 40 kW combi gives you exactly the same shower as a 30 kW one, because the water is not there. Measure it before you order: run the cold kitchen tap full into a jug for six seconds and multiply by ten.
Does the same logic apply to a system or regular boiler?
No, and applying combi logic to them oversizes them for nothing. A system or regular boiler heats water in advance into a cylinder, so the flow rate at the shower is limited by the cylinder and the pipework rather than by the appliance. Those are sized from the heat loss, with a cylinder reheat allowance on top that the installer works out against the cylinder itself.
Will this tool calculate my heat loss?
No, deliberately. A real heat loss needs fabric U-values room by room, and a calculator that guesses them produces an authoritative-looking number that is wrong. Your installer is required to do one when replacing a boiler. What this tool does is take that figure and run the check nobody runs: whether the boiler you are being sold can turn down to it.