Blog>Questions>Why do my radiators heat up when the hot water is on?
Last updated: 22 August 2023
Why do my radiators heat up when the hot water is on?
Find out why your radiators are heating up when the hot water is on.
If your radiators heat up when you open the hot tap, or vice versa, the likely fault is with the three-port (diverter) valve. The diverter valve is vital to the proper functioning of a boiler because it controls whether your boiler heats the hot water taps or the radiators.
Basically, a diverter valve does what it says on the tin - it controls the direction of hot water through your central heating. With your boiler settings, you can adjust the water to be directed to the hot tap, radiators or both.
The three-port valve opens and closes for heating or hot water. Select heating, and the diverter valve should close to allow hot water to divert to your radiators rather than taps. The situation is reversed if you want hot water.
It's common for combi boilers over 10 years old to develop a faulty diverter valve. Because the valve is constantly used, it gets worn out, damaged, blocked and often gets stuck in the on-off or mid-position.
What boiler system do I have?
Before diagnosing your central heating fault, always determine the type of central heating system you have, as this will affect the diagnostics and repair.
Most combi central heating and hot water systems send the water to the radiators, the hot water cylinder, or both. In this instance, a faulty diverter valve is common.
On the other hand, the valve could be working fine, but there could be a problem with the control wires between the valve and its commands, which affects the switches and thermostats.
Luckily, repairing a faulty valve is usually less expensive than other, more severe faults. Find out how much boiler repairs could cost.
Boiler repair
To save your boiler from further complications and yourself a headache, hire a Gas Safe boiler engineer to diagnose and repair the fault correctly. If you don't, your radiators will continue to get hot even if the thermostat says the heating is turned off. Your hot water tap won't get as hot as it should or will only work when the heating is on.
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