Common causes
- Trapped air in the system — often shows as radiators that are cold at the top but warm at the bottom. Bleeding the affected radiators can resolve this (see safe checks below).
- A faulty thermostat or timer — if the boiler isn't being told to call for heat, it simply won't fire for the heating circuit.
- A failing pump — if hot water isn't circulating around the system, radiators stay cold even though the boiler itself is working.
- Low system pressure — see our losing pressure page, since low pressure can prevent heating from working even when hot water is fine.
- A stuck diverter valve — the same component covered on our no hot water page, just affecting the opposite circuit.
Safe checks
Bleeding a radiator that's cold at the top is a straightforward, safe job with a radiator key — turn the boiler off first, hold a cloth under the valve, and open it slowly until water (not just air) comes out steadily, then close it. If more than one or two radiators need bleeding repeatedly, or pressure drops noticeably each time, that points to a system fault worth having looked at properly rather than a one-off.
Pricing
Diagnosis and first hour: £99.