Why radiators go cold at the top, how to bleed them safely step by step, checking boiler pressure afterwards, and what to do if a radiator is still cold.
If a radiator is warm at the bottom but cold at the top, air has probably collected inside it. Bleeding the radiator lets that air out so hot water can fill it again, making your heating more effective. It is one of the simplest home maintenance jobs and needs only a radiator key and a cloth.
Signs a radiator needs bleeding
- Cold at the top, warm at the bottom.
- Gurgling or clanking noises when the heating comes on.
- Rooms taking longer to warm up than usual.
A radiator that is cold at the bottom but warm at the top usually has a different problem: a build-up of sludge, which needs flushing rather than bleeding.
What you need?
- a radiator bleed key (cheap from hardware shops) or a flat-head screwdriver for some modern valves;
- a cloth or old towel, and a small container to catch drips.
Step by step
- Turn the heating on so all radiators warm up, then check which ones are cold at the top.
- Turn the heating off and let the radiators cool. The water inside can be very hot, and bleeding a hot radiator risks scalds. Turning the pump off also stops more air being drawn in.
- Find the bleed valve, usually at one of the top corners of the radiator: a small square spindle inside a round fitting.
- Hold a cloth underneath and fit the key onto the valve.
- Turn the key anticlockwise slowly, about a quarter to half a turn. You should hear a hiss as air escapes.
- When water starts to come out in a steady stream rather than spluttering, close the valve by turning clockwise. Do not overtighten.
- Repeat for other radiators. Many people start with the radiator furthest from the boiler on the ground floor, then work upstairs.
Check the boiler pressure
Bleeding radiators releases water from a sealed system (common with combi and system boilers), which lowers its pressure. Check the pressure gauge on the boiler. Many manufacturers recommend around 1 to 1.5 bar when the system is cold, but always check your boiler's manual. If it is too low, top it up using the filling loop as the manual describes. See our guide to boiler pressure.
If you have an open-vented system with a feed-and-expansion tank in the loft (common with older regular boilers), it refills automatically.
If the radiator is still cold
- Check the valves at both ends of the radiator are open. One is usually a thermostatic radiator valve (TRV): turn it to its highest setting. The other, the lockshield valve, usually has a plastic cap and should not need adjusting unless the system has been balanced.
- Stuck TRV pin: if a TRV stays shut, the pin under the head may be stuck. Removing the head and gently freeing the pin often fixes it.
- Sludge: cold patches at the bottom or the middle suggest sludge (black iron oxide). A heating engineer can power-flush the system or fit a magnetic filter.
- Balancing: if radiators far from the boiler stay cooler, the system may need balancing by adjusting the lockshield valves.
How often to bleed radiators?
There is no fixed schedule. Check them at the start of the heating season and whenever you notice cold patches. Needing to bleed radiators very often may point to a leak or another fault in the system, so get it checked by a heating engineer.
Safety
Bleeding radiators is safe DIY. Any work on the boiler itself, the gas supply or flue must be done by a Gas Safe registered engineer.
Frequently asked questions
Brown or black water came out. Is that normal?
Discoloured water suggests corrosion or sludge in the system. An inhibitor and a filter can help; ask a heating engineer.
Sources and further reading
Facts checked on 20 October 2026. Spotted an error? Tell us and we will correct it.