Radiators cold but the boiler is on: causes and safe checks
- Checked September 2026
- 4 min read
- By PROS AI editorial team
Quick answer
The short version, before the detail.
When the boiler runs but the radiators stay cold, the heat is not reaching them. The usual reasons are trapped air, sludge in the bottom of the radiators, a thermostatic valve that has stuck shut over summer, or a control that is only asking for hot water. Bleeding a radiator and checking the controls clears the common cases; a pump or diverter valve needs an engineer.
What is the likely cause?
Most common first, with how to tell.
Trapped air.
How to tell: the radiator is hot at the bottom and cold at the top, and you hear bubbling or gurgling when the heating starts.
Sludge.
How to tell: hot at the top and cold at the bottom, usually worst on ground-floor radiators. London's hard water adds limescale to the mix.
The boiler or controls are not calling for heating.
How to tell: the taps run hot but every radiator is cold. A combi left on hot water only, a timer that is off or a thermostat set low all do this.
A stuck radiator valve.
How to tell: one radiator stays cold with its valve turned up while the others heat, especially after months unused. The pin under the thermostatic head sticks down.
Low system pressure.
How to tell: the gauge is below 1 bar and the boiler cuts out or shows a warning.
The system needs balancing.
How to tell: radiators nearest the boiler get hot and the furthest ones stay cool or arrive last.
A pump or diverter valve fault.
How to tell: every radiator is cold, the boiler fires then stops, the pump is noisy or very hot, or hot water works and the radiators never come on. Valves stick over time; this is engineer work.
What can I safely check myself?
None of these steps opens the boiler, disconnects a pump or forces a valve. If a step needs tools beyond a radiator key and a cloth, leave it.
- Controls. Put the boiler in heating mode, not hot water only. Check the timer is on and set to now. Turn the room thermostat above room temperature and replace the batteries in a wireless one.
- Radiator valves. Turn every thermostatic valve head to its highest number. Do not remove the head or press the pin.
- Bleed a radiator. Turn the heating off and let the radiators cool. Put a cloth under the bleed valve at the top, insert the key and turn it anti-clockwise. You will hear a hiss as the air escapes; when water flows steadily, turn it clockwise to close.
- Top up the pressure afterwards. Bleeding lets water out of the system. Read the gauge: cool, it should sit between 1 and 1.5 bar. If it is lower, open the filling loop until it reaches 1 to 1.5 bar, then close both valves.
- Reset once. If the boiler shows a fault, press reset once and wait two to three minutes.
- Feel the pattern. Once the heating has been on for half an hour, note which radiators are cold at the top, cold at the bottom, or cold all over. That pattern tells the engineer which of the causes above they are looking at.
When do I stop and call an engineer?
- If you smell gas, leave the property, do not switch anything on or off, and call the National Gas emergency line on 0800 111 999 from outside.
- If water is leaking near sockets, the fused spur or the consumer unit, switch off at the main switch only if it is dry and safe to reach.
- Stop if the radiators are still cold after bleeding and the controls are right; if a radiator is cold at the bottom, which means a flush; if the pressure drops again within days; if a valve will not free without force; or if the boiler fires and stops. Flushing a system, freeing a valve and replacing a pump or diverter valve are engineer jobs.
- Anything on the gas side or behind the casing must be done by a Gas Safe registered engineer. Check the seven-digit number on their ID card at gassaferegister.co.uk, or text Gas and the number to 85080.
What does it cost in London?
A weekday call-out with diagnosis in London is £60 to £125, often waived if the repair goes ahead, and labour is £80 to £140 an hour. A circulation pump is £180 to £420 fitted and a diverter valve £150 to £390. If sludge is the cause, a magnetic filter is £150 to £240 fitted and a power flush £450 to £620 for six radiators or £580 to £800 for eight.