Combi Boilers: How They Work and Who They Suit
A combi heats water on demand from the mains, so there is no cylinder and no cold water tank in the loft.
A combi heats water on demand from the mains, so there is no cylinder and no cold water tank in the loft. That makes it the simplest system to live with in a flat or a smaller house, and the one most sensitive to how good your incoming mains supply is.
What the appliance actually does
One heat exchanger serves the radiators, a second (or a plate exchanger) transfers heat to the tap water as it flows through. Nothing is stored, so output is limited by flow rate rather than by tank size.
Where the limits show up
Two showers at once, or a bath filling while someone washes up, will drop the temperature. If your mains flow is weak the combi cannot invent pressure it never received.
What replacing one involves
Gas supply sizing, flue route, condensate run, a system flush and a magnetic filter. We check the incoming flow rate before recommending an output, not afterwards.
Not sure a combi suits your home?
Tell us how many bathrooms you have and how they are used and we will say honestly whether a combi or a cylinder system is the better answer.
Questions we get asked
Can a combi run two showers at once?
Rarely to a comfortable standard. If two showers are used together we would normally look at a system boiler with a cylinder instead.
Do combis need an expansion vessel?
Yes, and it is internal. A vessel that has lost its charge is one of the most common causes of pressure creeping up and dropping again.
Is finance available on a combi installation?
Finance is available on boiler and air conditioning installations, subject to status and the compliance terms shown on our finance page.