Radiators come on with the hot water – why?

If your radiators come on when you open a hot tap, you probably have a faulty diverter valve or motorised valve. These valves control whether your boiler heats the radiators or water for the hot taps. Which one you have depends on what type of central heating system you have. In this article, we’ll help you work out which system you have and explain how the problem can be fixed

If you have a hot water cylinder

If you have a hot water cylinder, you’ll have a motorised valve, usually located in the airing cupboard near the cylinder. In most installations this is usually a three-port valve because it has three pipes connecting to it in the shape of a T. On top of the valve is a metal compartment which houses an electric motor. On the end of the motor’s spindle is a rubber ball which seals and opens the valve’s different ports.

The position of the valve determines how hot water from the boiler flows through the system. There are three possible routes:

  • Through the radiators
  • Through the cylinder coil (heating water in the cylinder for the hot taps)
  • Both the radiators and coil simultaneously

If your central heating comes on with the hot water

The valve is not sealing off the central heating port correctly. Hot water flows through your radiators when it should flow through the cylinder coil instead.

If your radiators are always on but you have no hot water

The valve has almost certainly stuck in the heating position. The cylinder thermostat is calling for hot water from the boiler, but the stuck valve means it can only go through the radiators. If you can only get hot water by boosting the cylinder with the immersion heater and the cylinder thermostat is working properly, then a stuck or jammed motorised valve is the most likely suspect.

How to fix

While it’s possible to replace just the motor assembly and leave the valve in place, this will only solve the problem if the motor is the defective part. If the ball has worn and isn’t forming a watertight seal, the entire central heating system will have to be drained down to replace it. If the spindle has seized up due to corrosion, the best thing to do is to replace the entire valve. The system must then be refilled and an inhibitor solution re-added.

Motorised valves run on mains voltage electricity. Never attempt to repair or replace one if you cannot work competently with electricity. Always consult a qualified tradesman if you are ever in doubt.

If you have a combi boiler and your radiators come on when you open the hot taps

In this case, there’s most likely a fault with the diverter valve. This valve controls whether the boiler heats hot water for the taps or the radiators. Other indications of a faulty diverter valve may include:

  • No hot water when you open a hot tap, but the central heating works normally.
  • Radiators getting hot when you open hot taps.
  • Having to turn on the central heating in order to get hot water from hot taps.

For combi boilers, calling a professional is by far the better option. A replacement diverter valve typically costs approximately £200-£250, including parts and labour. If your boiler is under an extended warranty, contact the manufacturer.