Boiler heats water but radiators stay cold (Worcester Bosch & Baxi)
⚠️ This needs a qualified professional
This fault involves gas or mains electrical work. Don't attempt a DIY repair — UK law requires gas work to be carried out by a Gas Safe registered engineer. Below is what to check and describe to them, not step-by-step repair instructions.
When hot taps work fine but the radiators stay cold (or vice versa — heating works but no hot water), the fault sits in how the boiler switches between the two, not the boiler’s ability to heat water at all. That’s useful: it usually rules out the most expensive parts.
What’s safe to check yourself first
- Check your room thermostat is set above the current room temperature and isn’t on a schedule that’s switched heating off.
- Check the boiler’s own timer/programmer has heating scheduled “on” for the current time, and hasn’t reverted to a default schedule after a power cut.
- Bleed each radiator individually (a common, safe DIY task) — trapped air is one of the most common reasons radiators stay cold even when the boiler is firing, though this usually affects individual radiators rather than the whole system.
- Check the boiler display isn’t showing a fault code — note it down before calling anyone.
If the thermostat, timer, and radiator bleeding don’t fix it, the likely cause is the diverter valve (which routes hot water to either the taps or the heating circuit) or a faulty motorised valve/actuator. These sit inside the boiler casing or heating system pipework and are gas/heating engineer work — do not open the boiler casing yourself.