No hot water but the heating works — what is it usually?

It blends very hot water with cold so that what leaves the tap is at a safe temperature. If it breaks or drifts out of adjustment, the whole house can get lukewarm water even though the cylinder is hot. It is a common fault and it feels exactly like "the heating is on, but the shower is lukewarm".

If no hot water comes at all and the cylinder is cold on the outside, that is what needs looking at. In an electric cylinder it can be a tripped thermal cut-out or a failed element; on district heating it is more often the heat exchanger or its control.

That the hot water exists but is not arriving quickly. In buildings with hot water circulation it is often a stopped circulation pump, or a failed check valve in the circulation. It shows up most clearly at the draw-off point furthest from the cylinder.

That is the first question to answer. One tap alone usually means a failed mixer or a limescaled aerator in that tap. The whole house means cylinder, heat exchanger, mixing valve or circulation. So try several draw-off points before troubleshooting further.

The cylinder or heat interface unit in full plus the data plate, any display or controller, and the mixing valve if you can find it. Say whether it is lukewarm or completely cold, whether it is one place or all of them, and whether anything has changed recently.

Partly, and the part that is settled is the application key. An ECL Comfort 210 or 310 is an empty shell without one: the key carries the software layout, such as A266 for heating plus domestic hot water. Keys are not locked to a serial number and move freely between controllers of the same type.

Almost always air in the radiator. Hot water comes in and heats the lower part, but air has collected at the top where it cannot escape on its own, so the top stays cold. Bleeding the radiator is the first step and usually fixes it.

There are three common sources, and they can be told apart without tools. The trap and its connections only leak when you run water. The supplies to the tap leak continuously, including overnight, because they are under pressure. The dishwasher hose or connection leaks during and just after a cycle.

Condensation spreads evenly along a cold pipe, often as many small droplets on the underside, and gets worse when it is warm and humid indoors. A leak has a point: the damp is heaviest at a joint, a valve or a weld, and runs from there.

Not without knowing what it sits on. A valve can be the isolation for a single tap, for a whole flat, for a heating circuit, or for a system that cannot be left standing — and from the outside they look much alike. A closed heating or circulation circuit can cause frost damage or stagnant hot water long after you have forgotten you touched it.