Combi boilers explained
One appliance for heating and hot water, with no cylinder and no tanks. The most common boiler in London flats, and the wrong choice for some houses.
Hot water
On demand, unlimited
Space needed
One cupboard
Best for
One-bathroom homes
Depends on
Strong mains pressure
The short answer
A combination boiler does two jobs in one case. It feeds the radiators like any other boiler, and when a hot tap opens it diverts all of its power into heating mains water as it flows through. Nothing is stored, so there is no cylinder, no loft tank and no reheat wait — but the flow rate you get at the tap is limited by the boiler and by the incoming mains supply.
How it works
A tap opens
A flow sensor detects water moving and fires the burner within a second or two.
The diverter valve switches
Hot water takes priority: the valve closes off the radiator circuit and sends all the heat to the hot water side.
A plate exchanger transfers the heat
Mains water passes through a stack of thin plates against the hot primary water and leaves at tap temperature.
Flue gases condense
Waste heat is recovered from the flue gases before they leave, and the resulting condensate drains away through a plastic pipe.
What it needs in the property
A gas supply sized for the appliance. Older properties are often fed by a pipe intended for a smaller boiler.
A strong incoming cold main. Below roughly 12 litres a minute, showers feel weak whichever boiler is fitted.
A flue position on an external wall or roof, at the distances the manufacturer sets from windows and boundaries.
A condensate drain to a waste pipe or an external soakaway, insulated if it runs outside.
A filling loop, an expansion vessel and a magnetic filter on the return.
Advantages and trade-offs
Advantages
No cylinder and no loft tanks, so a cupboard or an airing cupboard is freed up.
Hot water never runs out and needs no reheat time.
Cheapest gas boiler to install where one is already fitted.
Nothing stored means no standing heat loss and no legionella regime.
Fewer external components: pump, valves and expansion vessel are all inside the case.
Trade-offs
Only one outlet gets full flow. Two showers at once is a poor experience.
Performance is capped by the incoming main, which you cannot change.
No hot water at all if the boiler fails — there is no immersion heater to fall back on.
Hard London water scales the plate heat exchanger unless the system is treated.
Large homes with three or more bathrooms usually need a cylinder instead.
Is it right for your home?
Suits
Flats, maisonettes and houses with one bathroom and one main shower, where the incoming mains supply is good and cupboard space is worth reclaiming.
Less suitable
Homes with two or more bathrooms in simultaneous use, properties on a weak or shared main, and houses where a power shower or bath filling speed matters.
Sizing it properly
Combi output is quoted twice: a heating output for the radiators and a higher domestic hot water output for the taps. The hot water figure is the one that decides how the shower feels, and it is chosen from the number of bathrooms rather than from the size of the property.
24–27 kW
One bathroom, up to about ten radiators
28–32 kW
One bathroom plus a cloakroom, or a fast bath fill
33–38 kW
Two bathrooms with occasional overlap
We size from a heat loss calculation and a measured mains flow rate, not from the output of the old boiler.
Book a surveyWhat goes wrong
Pressure dropping
Usually a leak on the system, a failed expansion vessel or a weeping pressure relief valve. Repeated topping up is a symptom, not a fix.
Heating fine, hot water lukewarm
Typically a stuck diverter valve or a scaled plate heat exchanger. Both are repairable.
Fires up then cuts out
Often a blocked condensate pipe in cold weather, a faulty fan or a dirty flame sensor.
Hot water surging hot and cold
Scale or debris in the plate exchanger, or an incoming main that fluctuates with neighbouring demand.
Combi boiler questions
Get a fixed price
Send the property details and a photo of the existing boiler and we will quote the whole job in writing, itemised.