Heating but no hot water: what causes it & how to fix it

Got heating but no hot water? This fault is much more common in heating systems with a a hot water cylinder. If you have a combi boiler, it’s usually due to a faulty diverter valve. This valve controls whether the boiler heats water for the taps or water for the radiators.

Your boiler may show a fault code on its little LCD display which may confirm if the diverter valve is indeed the problem. If it is, you’ll have to get an engineer to replace it.

Before we begin, check the obvious. Are the settings correct on your thermostat or programmer?

Heating but no hot water – I have a hot water cylinder

In this case, the most common cause is a faulty motorised valve. This valve controls whether hot water from the boiler goes to the radiators or to heat the water in the hot water cylinder.

Motorised valves are usually (but not always) found in your airing cupboard. The valve connects to two or three pipes, and features a metal or plastic box on top, connected to an electrical wire. We call this little compartment the actuator. Inside it is a little motor which actually operates the valve.

Depending on your heating system, you will probably have either:

  • One, three-port valve (Y plan system) or
  • Two, two-port valves (S plan system)

Important: Motorised valves are connected to mains voltage electricity. Never attempt to work on them if you can’t work with electricity competently and safely. Before carrying out any electrical work, always isolate the system completely – do not simply turn things off at the programmer. Even with the hot water off at the programmer, the valve will still be carrying live current. If you are in any doubt, contact a qualified tradesman.

If you have a three-port valve

The valve has probably jammed in the central heating position or less commonly, the hot water cylinder thermostat has failed. Even with no power supply, the valve’s default position is hot water only. The call for the boiler to heat the hot water cylinder comes directly from the cylinder thermostat and does not require the motorised valve to be energised.

A classic symptom of this problem is no hot water, and hot radiators even though the central heating is off.

Check if hot water is getting to the cylinder

  • Turn the central heating off, let it cool down, and select hot water only via the programmer.
  • With the boiler running, go to the motorised valve.
  • The pipe connected to Port AB should be hot – this is hot water coming from the boiler.
  • Port B should be hot – this port goes to the hot water cylinder.
  • Port A should be cold – this port goes to the radiators.

If Port B is cold and Port A is hot, then the valve is almost certainly the problem.

How to check if your three-port valve is jammed:

  1. Shut off the power supply to your central heating system. Don’t simply turn things off at the programmer, as the valve will still carry live current and the test won’t be effective. With no electricity to the valve, it should return to its default, spring-loaded position, which is hot water only.
  2. Move the lever on the side of the valve without locking it in the manual override slot. You should feel resistance from the spring as you do this.
  3. Let go of the lever. It should return to its original position.
  4. If you don’t feel any resistance from the lever when you push it over, or if it doesn’t return to the default position when you let it go, the valve has probably jammed.

Repeatedly moving the lever from side to side may free the valve temporarily, but this probably won’t be a long-term solution. If the valve has stuck once then it’s likely to stick again. If it’s failed then it’ll need replacing. On some models, the faulty electrical components can be replaced without having to drain the system down and replace the entire valve assembly.

If you have a two-port valve

If you have two or more two-port valves, you’re on an S-Plan or S Plus Plan. Two-port valves are controlled independently and have only two possible positions: open or closed.

You may have already noticed that the boiler comes on for the central heating, but it doesn’t even come on when you want hot water. That’s because the boiler is actually operated via a microswitch inside the motorised valve. If the motorised valve is faulty, the boiler can’t even come on in the first place.

Heating but no hot water – how to test a two-port valve

  1. Establish which motorised valve controls the supply of hot water from the boiler to the hot water cylinder. The pipe will enter the cylinder at around half-way up the side.
  2. Push the lever on the side of the valve without locking it into the manual override slot. You should feel resistance as you do this. Let go of the lever, and the spring should return the valve back to its default closed position.
  3. Call for hot water via the programmer. You may need an extra person to help you if the programmer isn’t located near the valve. If the valve is working, you’ll hear a whirring sound as it opens. The opening of the valve then activates the boiler.
  4. Push the lever on the side of the valve. If the valve is open, the lever should be loose. You should be able to move it from side to side with no resistance.
  5. If there is still resistance from the valve but the programmer is calling for hot water, the motor has probably burned out and will need replacing.
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