Combi not heating the water hot enough?

The shower is warm rather than hot, and the boiler is firing. Work through these in order - the first two are free.

1. The hot water dial

There is a separate temperature control for hot water on almost every combi. Set it to about 60C. People knock it down and never notice.

2. Flow rate

A combi heats water as it passes through. The faster the water goes, the less time it has to get hot. If your shower has a high-flow head, the boiler may simply be unable to keep up.

Test it: run the hot tap slowly and see if it gets properly hot. If a trickle is scalding but a full flow is lukewarm, the boiler is undersized for the demand, not broken.

3. Limescale on the plate heat exchanger

In hard water areas this is the classic. The plate exchanger scales up, its ability to transfer heat drops, and the hot water goes lukewarm months before anything else fails.

It can sometimes be chemically descaled. Sometimes the plate exchanger needs replacing, which is a routine job and far cheaper than the main heat exchanger it is often confused with.

4. A failing diverter valve

A partly stuck valve sends only some of the heat to the water. Lukewarm rather than cold, which makes it harder to spot than a full failure.

What it means about the boiler

Scale and diverter faults are age faults. If you are seeing them, and the boiler is over ten, this is the beginning of the conversation, not the end of it.