When your radiators are warm but the taps run cold — or you've got hot water but no heating — it usually points to a specific component rather than a total boiler failure. Here are the most common causes we see, and what's safe to check yourself.
First, the simple checks
- Is the boiler switched on with no fault or error code showing?
- Is the thermostat set high enough and calling for heat?
- Is the hot water/heating timer or programmer set correctly?
- Is the boiler pressure at around 1–1.5 bar?
Hot water but no heating
If your taps are hot but the radiators stay cold, common causes include a faulty room thermostat, a seized motorised (diverter) valve, a timer/programmer fault, or trapped air in the radiators — try bleeding them first.
Heating but no hot water
If the radiators heat up but the taps run cold, the most likely culprit on a combi boiler is the diverter valve, which switches the boiler between heating and hot water. A stuck diverter valve, a faulty flow sensor or a failed thermostat can all cause it.
When to call an engineer
If the basic checks do not fix it, the fault is usually inside the boiler (diverter valve, PCB, sensors) and needs a Gas Safe registered engineer to diagnose and repair safely. Do not attempt internal boiler repairs yourself.