No hot water from your combi? Work through this

How a combi decides to make hot water

Open a hot tap and a little turbine called the flow sensor notices water moving. It tells the board to fire the burner and swing the diverter valve, which switches the boiler’s attention from the radiators to a plate heat exchanger that heats mains water on its way to your tap. Heating and hot water share one machine but travel different paths through it.

Each part fails with its own signature. A stuck diverter valve gives you heating but cold taps, or hot taps but cold radiators. A lazy flow sensor gives hot water that comes and goes, or only arrives with the tap fully open. A scaled plate heat exchanger, the classic in hard-water areas, gives water that is warm rather than hot and cools as it runs.

The tree assumes the basics: boiler switched on, the clock or smart control not quietly set to off, and nobody mid-shower elsewhere. A combi serves one hot outlet well; two at once splits the flow, and the temperature goes with it.

The ten-minute pre-call checks

Engineers charge fairly for diagnosis, which makes free information valuable. Everything below happens outside the casing, takes a couple of minutes, and rules out the causes that would make a call-out embarrassing.

Warm, then cold, then warm again

Hard water kills more plate heat exchangers than old age does. Scale builds on the hot side, the transfer surface shrinks, and the boiler overshoots then backs off, producing that maddening warm-cool-warm cycle at the tap. An engineer can descale or replace the plate; asking them about a scale reducer at the same time buys the new one a longer life.

What the engineer will do

Diverter valves, flow sensors and plate heat exchangers all live inside the casing, so testing and replacing them is engineer work by law and by sense. The consolation: these are among the most routine combi repairs there are, and most land in the common-repair band below rather than anywhere frightening.

Intermittent faults deserve their own note when you book: hot at low flow but not high, fine in the evening but cold at breakfast. Patterns like these point at specific parts, so describe them. If the shower is the only unhappy outlet, the shower guide separates boiler faults from mixer and head problems first.

If your home heats water in a cylinder instead, the suspects live outside the boiler entirely: see hot water cylinder problems. And for the full map of boiler misbehaviour, from pressure to noises to lockouts, keep the complete boiler guide to hand.