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.
If it's the middle of winter
A boiler that cuts out entirely in freezing weather — rather than just leaving radiators cold — is often a frozen condensate pipe, which is one of the few boiler faults you can usually fix safely yourself. And if the boiler fires, runs briefly and shuts down over and over, that's a different pattern with its own causes — see our keeps switching off guide.
Pricing
Diagnosis and first hour: £99.
What we test when we arrive
Every diagnosis follows the same discipline as our emergency callouts — our advice article on what we check on every callout covers it in full, but in short: the engineer confirms the boiler fires on demand, then follows the heat: is the thermostat actually calling, does the diverter or motorised valve move, is the pump running and circulating, and are any parts of the system airlocked or blocked? System pressure and the expansion vessel get checked alongside, and electrical and gas-side safety checks are part of every visit as standard.
Likely repairs
The components that most often put heating right are the pump, the diverter valve (or a motorised valve on system boilers), a thermostat or its receiver, or simply removing air and sludge from a system that's stopped circulating properly — our power flush guide explains that side. Parts are quoted before fitting and most repairs carry a 12-month parts warranty. For an example of what sludge does to a heat exchanger when it's left, see the Stourbridge Ariston case in our case studies.
Not certain this page matches your fault? The Boiler Problem Finder will point you to the right guide.
Frequently asked questions
Why is my hot water fine but my radiators cold?
Because on a combi the hot water and heating circuits are separate — hot water working proves the boiler can fire, so the fault sits in what serves the radiators: the diverter valve, the pump, trapped air, low pressure, or the controls not calling for heat.
Some radiators are hot and some are cold — same problem?
Usually a circulation or balancing issue rather than a boiler fault. Radiators cold at the top need bleeding; radiators cold at the bottom or in the middle often have sludge settled in them, which points towards a system clean rather than a boiler part.
Could my thermostat be the problem?
Genuinely, yes — flat batteries in a wireless thermostat or a lost pairing between thermostat and boiler are among the cheapest "no heating" faults there are. Check batteries and settings before assuming the worst.
What will the repair cost?
Diagnosis is £99 including the first hour. If a part such as a pump, diverter valve or motorised valve is needed, it's quoted before fitting, and most repairs carry a 12-month parts warranty.