No hot water but the heating works

If you have a combi

A combi makes hot water on demand by diverting the flow away from the radiators. When the heating works and the taps do not, the usual suspects are the diverter valve sticking on the heating side, the flow sensor failing to notice a tap has been opened, or the plate heat exchanger furring up with limescale.

Hard water across Warwickshire makes scaled plate heat exchangers a common find, particularly where there is no softener or scale reducer on the incoming main.

If you have a hot water cylinder

On a cylinder system the hot water is a separate zone with its own controls. That means a separate set of things to fail: the motorised valve for the hot water zone, the cylinder thermostat, the programmer channel for hot water, or on an electric backup the immersion heater and its thermostat.

A cylinder that heats slowly or never gets properly hot can also mean a scaled or failing coil inside the cylinder.

Worth checking before you call

Two minutes of checks often narrows it down.

• Programmer set to hot water on, not just heating.
• Cylinder thermostat turned up to around 60C.
• Boiler pressure roughly 1 to 1.5 bar cold.
• Any fault code on the boiler display, note it down.
• Whether hot water fails at every tap or just one, a single tap points at the tap or a blocked cartridge.

Questions we get asked

Heating works but there is no hot water, what is it?

On a combi it is usually the diverter valve, the flow sensor or a scaled plate heat exchanger. On a cylinder system it is more often the motorised valve, the cylinder thermostat or the immersion.

Hot water only lukewarm, is that the same fault?

Often a partially stuck diverter valve or a scaled plate heat exchanger on a combi. On a cylinder, a failing coil or a thermostat set too low.

Can I fix it myself?

Check the programmer, the cylinder thermostat setting and the boiler pressure. Beyond that, anything inside a gas boiler is Gas Safe work, and unvented cylinders need G3 qualification.