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How to bleed a radiator
If a radiator is cold at the top but hot at the bottom, trapped air is probably stopping hot water filling it. Bleeding it takes a few minutes and a cheap radiator key.
Signs a radiator needs bleeding
- Cold patch at the top while the bottom is hot
- Gurgling or tapping noises
- Rooms taking a long time to warm up, while the boiler seems to be working
If the whole radiator, or the bottom, is cold, trapped air is not the cause. That points to sludge in the system or a stuck valve, which is covered at the end of this page.
What you need
- A radiator key (cheap from DIY shops; some radiators have a bleed screw you can turn with a flat screwdriver)
- A thick cloth or towel
- A small container or old cup
Step by step
- Turn the heating on and let the radiators warm. Check which are cold at the top.
- Turn the heating off and wait until the radiators cool. This stops hot water spraying out and lets the pump stop pushing water around.
- Find the bleed valve at the top corner of the radiator. Place the cloth under it.
- Turn the key slowly anticlockwise about a quarter to half a turn. You should hear air hissing. Do not unscrew it fully.
- Close the valve when water starts to appear. Turn it clockwise, snug but not very tight.
- Wipe up and repeat on other radiators if needed. Start with the radiator nearest the boiler and work outward, ground floor first, then upstairs.
Check your boiler pressure
Letting air out lowers the pressure in a sealed heating system. Look at the pressure gauge on your boiler. The correct pressure depends on your boiler, so check the manual, which will give the range for a cold system; many gauges also mark a recommended zone. If it has dropped, top it up using the filling loop (a flexible hose or a lever under the boiler) according to the manual, and close it fully afterwards. If you do not have a manual, search the boiler's make and model online for it.
Do not open or alter the boiler itself. Anything beyond the gauge and filling loop is Gas Safe territory; see our legal DIY page for how to check an engineer.
If it keeps happening
Repeatedly needing to bleed radiators suggests a leak or air getting into the system. Pressure that keeps falling is a sign of a leak somewhere. Cold patches at the bottom or the middle of the radiator usually mean sludge and rust; a heating engineer can power flush the system or fit a magnetic filter. A stuck thermostatic radiator valve (TRV) may be cured by tapping the pin gently, but if not, replacing the valve head is simple.
Safety
The water can be hot and may be dirty and black. Keep the cloth ready. If water keeps coming from the valve after closing it, or the valve spins freely, stop and ask a plumber. Never bleed a radiator when the heating is running.
Quick checks before you call an engineer
Make sure the radiator valves are open: the lockshield valve on one end and the thermostatic head on the other. Check the thermostat or timer is calling for heat and that other radiators are warming. If every radiator is cold, look at the boiler for an error code and the pressure gauge before anything else. The boiler manual explains what each code means. If a code points to a fault, report it to the manufacturer or a Gas Safe engineer rather than trying to fix it yourself.
For other quick jobs, see the basic toolkit and fixing a dripping tap.
Frequently asked questions
How often should I bleed radiators?
Only when they need it, and typically once a year at the start of the heating season is enough. If you need to do it often, ask an engineer to check for leaks or air getting in.
Why is my radiator cold at the bottom?
This is usually sludge or a blocked valve, not air. A heating engineer can flush the system.
What pressure should my boiler be?
It depends on the model. Check the boiler manual for the cold-system range.