A boiler that gives hot taps but cold radiators is a common UK fault, especially on a chilly morning. Here are the likely causes, what you can safely check, and when to call an engineer.
Why you can have hot water but no heating
In a combi boiler a diverter valve sends hot water either to the taps or to the radiators. If it sticks on the hot-water setting, you get hot taps but no heating. Other causes affect only the heating circuit, leaving the hot water working.
Common causes
Room thermostat or programmer: a flat battery, wrong setting, or a failed thermostat stops the heating from calling for heat. Low pressure: most combis need around 1 to 1.5 bar; below that the heating can cut out. Airlock or seized pump: air or a stuck circulation pump stops hot water reaching the radiators. Diverter valve: a worn valve stuck on hot water is a frequent culprit.
What you can safely check
Confirm the programmer is set to heating and the time is correct; replace the thermostat batteries and turn it well above room temperature; check the pressure gauge and top up via the filling loop if it is low (follow your boiler manual); and make sure radiator valves are open. Bleeding radiators can clear trapped air.
When to call a heating engineer
If the thermostat, programmer and pressure are all fine, the fault is likely the diverter valve or pump, which are internal boiler repairs for a Gas Safe registered engineer. Do not attempt internal boiler work yourself.
FAQs
Why is my hot water working but not the heating? Most often a stuck diverter valve, a thermostat or programmer fault, low pressure, or an airlock.
What pressure should my boiler be? Usually about 1 to 1.5 bar cold; check your boiler manual and top up if low.
Can I fix a diverter valve myself? No, it is an internal boiler repair for a Gas Safe registered engineer.
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