Radiators Hot When Only the Hot Water Is On: Causes and Fixes

It is a confusing fault: the heating is switched off, only the hot water is on, and yet the radiators are warm. It usually shows up in summer, when most homes run the hot water alone, and it is rarely dangerous. It does waste gas, though, and it is a sign that a part controlling where the heat goes has stopped doing its job.

What causes it depends on the type of system you have, so start by working out which one is on your wall.

Which system do you have?

• Combi boiler: no hot water cylinder in the house. Hot water comes straight from the boiler when you open a tap.
• System or conventional boiler: a hot water cylinder in an airing cupboard or loft. Conventional (regular) boilers also have cold water tanks in the loft; system boilers do not.

On a cylinder system: a motorised valve stuck open

Homes with a cylinder use one or more motorised valves to send hot water from the boiler to either the cylinder, the radiators or both. There are two common layouts:

• One three-port valve (often called a mid-position valve), with one motor head serving both the heating and the hot water.
• Two two-port valves (zone valves), one for the heating and one for the hot water.

When the programmer asks for hot water only, the valve should close the route to the radiators. If the motor head has failed, the valve has stuck, or its wiring is faulty, the heating route stays open and the radiators warm up with the cylinder.

Signs it is the valve: the radiators warm up at the same times the hot water is on, and cool down when it switches off.

Gravity circulation on older systems

Some older conventional systems heat the cylinder by gravity rather than with a pump, and on these a little heat can drift into radiators near the cylinder pipework. If one or two radiators close to the airing cupboard are slightly warm, this may be how the system was built rather than a fault. An engineer can tell the difference.

On a combi: the diverter valve

A combi boiler has a diverter valve inside it that switches the heat between the radiators and the hot taps. When it is worn or sticking, it can let heat pass into the heating circuit while you run a hot tap, or leave the radiators warm afterwards.

Signs it is the diverter: radiators warm up while a hot tap is running, or the hot water is lukewarm because some of the heat is going to the radiators instead. The opposite fault, heating that works but no hot water, is covered in the guide to no hot water but heating works.

Checks you can safely do yourself

1. Check the controls first. Make sure the programmer really is set to hot water only, and that the heating has not been switched to "on" or "constant" by mistake. Check the room thermostat too.
2. Check which radiators are warm. One towel rail may be plumbed on the hot water circuit on purpose. Several radiators around the house warming up points to a fault.
3. Note the timing. Do the radiators warm up whenever the hot water is on, only while a tap is running, or all the time? That pattern is the most useful thing to tell an engineer.
4. Leave the valves alone. Motorised valves are wired into the controls at mains voltage, and diverter valves are inside the boiler casing, which should only be opened by a Gas Safe registered engineer.

What an engineer will do

The engineer works out which part is letting the heat through, then explains the repair and quotes it before starting. Common fixes are:

• replacing the motor head on a three-port or two-port valve, which can often be done without draining the system
• replacing a valve body that has seized, which means draining down and refilling the system
• correcting wiring at the programmer, thermostat or wiring centre
• repairing or replacing the diverter valve on a combi

Getting it fixed in South London

InstaGasWorks repairs heating controls, motorised valves and diverter valves across South West London, Croydon and Sutton. The boiler repair page has the current breakdown visit rates and explains how repairs are quoted, and a visit can be booked online.