Central Heating Not Working But Hot Water Is Fine — What's Wrong?
Quick Answer
Why is my hot water working but not my central heating?
When hot water works but central heating doesn't, the boiler itself is usually fine. The most likely cause is a stuck or failed diverter valve (in combis), a faulty zone valve (in system boilers) or a thermostat or timer issue.
If your boiler is producing hot water when you turn on the taps but your radiators remain cold, the problem is almost never the boiler itself. The boiler is clearly working — it's firing up and heating water. Something is preventing that heat from being directed to your radiators.
The Diverter Valve (Combi Boilers)
In a combi boiler, a diverter valve switches water flow between the central heating circuit and the hot water circuit. When you turn on a tap, it diverts away from heating to give you hot water. When you switch the tap off, it should switch back.
If the diverter valve sticks in the 'hot water' position — which happens as the valve wears, particularly in older boilers — the boiler keeps serving hot water but cannot send heat to the radiators. The fix is valve replacement, which typically costs £150–£300 including parts and labour.
Zone Valve Faults (System and Regular Boilers)
In system boilers with separate hot water cylinders, motorised zone valves control which part of the system is heated. A stuck or failed zone valve can cut heating to your radiators while leaving hot water unaffected.
Zone valves can be tested by a Gas Safe engineer using a multimeter. Replacement parts are readily available for all common UK heating systems and the repair is usually straightforward.
Thermostat and Timer Issues
Sometimes the cause is simpler than a valve fault. Check: Is the room thermostat set above the current room temperature? Is the programmer set to 'heating on' (not just 'hot water only')? Has the clocks-change season confused your programmer?
Modern programmers and smart thermostats occasionally need a reset. If your Hive, Nest or tado° app shows heating is 'on' but the radiators are cold, check the boiler is actually responding by listening for it to fire when heating is demanded.
- Check the room thermostat is set above the current room temperature
- Check the programmer has heating enabled (not just hot water)
- Try a manual override on your programmer or thermostat app
- Listen to hear whether the boiler fires when heating is demanded
- If the boiler fires but radiators stay cold, you likely have a circulation issue — call an engineer
Frequently Asked Questions
How much does a diverter valve replacement cost?
A diverter valve replacement typically costs £150–£300 including parts and labour. The exact cost depends on your boiler make and model.
Can I fix a stuck diverter valve myself?
No — diverter valves are internal boiler components. Any work inside the boiler casing must be carried out by a Gas Safe registered engineer.
How long does it take to fix a diverter valve?
An experienced engineer can typically replace a diverter valve in 1–2 hours, depending on the boiler model and parts availability.
Why did my heating stop working overnight?
Overnight failures often point to a programmer or thermostat issue — particularly after a power cut resets the clock. Check your programmer's time and heating schedule first.
Will my heating be working the same day you visit?
In most cases, yes. We carry common parts on our vans and can usually complete the repair in a single visit across Manchester, Salford and Greater Manchester.
Need emergency boiler help in Manchester or Salford? Call: 07305 687 331