No heating but the hot water works: what is wrong?

Hot water fine, radiators stone cold. It is one of the most common calls we get in Greater Manchester once the weather turns, and the good news is that it points at a short list of parts rather than a dead boiler.

If the boiler is producing hot water, the boiler is firing. That rules out most of the frightening possibilities straight away.

Start with the controls

More of these calls than you would think end here, so check before you book anyone.

• Is the programmer set to heating on, and is the clock right? Programmers lose time after a power cut, and the hour change catches people out twice a year.
• Is the room thermostat calling for heat? Set it well above room temperature and listen for a click.
• If the thermostat is wireless, has it lost pairing or run its batteries flat? A flat battery in a wireless stat produces exactly this fault.
• Are the thermostatic valves on the radiators open rather than sitting on frost setting?

The diverter valve

On a combi boiler this is the usual culprit.

A combi has one heat exchanger and a valve that decides whether the heat goes to your taps or your radiators. That valve defaults to hot water, because hot water demand always takes priority. When it sticks or the motor fails, it stays in the hot water position, and you get exactly what you are describing: perfect hot water, cold radiators.

It is a known wear part. Depending on the boiler it can be a new cartridge, a new motor or a full valve assembly, and any Gas Safe engineer who works on combis will have seen it many times.

The motorised valve

On a system or regular boiler with a hot water cylinder, the equivalent part is the motorised zone valve, usually sitting in the airing cupboard on the pipework.

Most homes run a two zone setup with separate valves for heating and hot water. If the heating valve fails to open, the boiler never gets the signal to run the heating circuit. You can often hear a healthy one click and hum as it drives open.

The pump

If the pump has seized or is running weakly, hot water still works on a combi because that circuit does not always depend on it in the same way, but the radiators get nothing moving through them.

A pump that is hot to the touch but silent, or humming without circulating, is a strong sign.

Air or sludge in the system

If some radiators are working and others are not, this is a circulation problem rather than a boiler fault.

Cold at the top means air, so bleed them. Cold at the bottom while the top is hot means sludge sitting in the base of the radiator, which usually needs a power flush rather than bleeding.

What to check before you ring

Have these three answers ready and you will save time on the visit:

1. Are all the radiators cold, or only some?
2. Does the boiler fire up and then stop when you call for heating, or does nothing happen at all?
3. Is there a fault code on the display?

Getting it sorted

Almost everything above is a part rather than a new boiler, which is worth remembering if someone tries to sell you a replacement on the first visit. We diagnose the fault, tell you what it is, and price the repair before we do it.

Our boiler repair and central heating repairs pages explain how we work. If you have no heating and no hot water in the middle of winter, that is an emergency plumber job and you should ring rather than fill anything in.

We cover Manchester, Salford, Stockport and the towns around them. Ring 0161 515 9977.