How to Bleed a Radiator
Bleeding a radiator solves exactly one problem: air trapped at the top of it. It is a five minute job, it needs a key that costs very little, and it is worth doing once a year at the start of the heating season.
It is also worth knowing what it does not fix, because a lot of people bleed a radiator repeatedly for a problem that air was never causing.
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Does yours actually need bleeding?
Cold at the top, hot at the bottom.
Yes. Air rises, collects at the top, and stops that section filling with water. Bleed it.
Cold at the bottom, hot at the top.
No. That is sludge settled in the bottom of the radiator, and bleeding will do nothing at all. It needs the system cleaning. See our power flushing page.
Cold all over while other radiators are hot.
No. That is usually a valve, most often a thermostatic valve with a seized pin, or a lockshield closed too far.
Cold in the middle.
Also sludge, further along.
That one distinction, top versus bottom, saves a lot of wasted effort.
What you need
A radiator bleed key, available from any hardware shop for very little. Some modern radiators take a flat blade screwdriver instead. And a cloth and a small container, because what comes out is dirty and it stains.
How to do it
- 1
Turn the heating off
and let the system cool. Bleeding a hot system risks scalding, and the water will be under more pressure.
- 2
Find the bleed valve.
A small square or slotted fitting at the top of the radiator, at one end.
- 3
Hold the container underneath
and put the cloth around the valve.
- 4
Turn the key anticlockwise, slowly
no more than a quarter to half a turn. You should hear a hiss as air escapes. Do not open it fully, and do not remove the valve.
- 5
Wait for water.
When the hissing stops and water dribbles out steadily rather than spitting, the air is out.
- 6
Close it
snugly rather than forced. Overtightening damages the seat.
- 7
Work through the house.
Downstairs first, then upstairs, and within each floor start furthest from the boiler.
- 8
Check the boiler pressure.
This is the step people skip. Letting air out lowers system pressure, and if it has dropped below roughly 1 bar it needs topping up with the filling loop before the boiler will run properly.
What it means if the same radiator keeps needing it
Once a year is normal. Every few weeks is not, and it is telling you something.
Air is getting in from somewhere. The usual causes:
- A leak on the system. Where water escapes, air is drawn in. This is the most common cause and it usually shows as pressure that also keeps dropping. See our guide on boiler pressure.
- Repeated topping up. Fresh water carries dissolved air, so a system being refilled regularly is being given a fresh supply.
- A failing automatic air vent which should be releasing air by itself.
- Corrosion producing hydrogen which happens inside a system with poor inhibitor levels and is a sign the system water needs attention.
If you are bleeding the same radiator monthly, the bleeding is not the job
Finding out why is.
While you are there
- Check whether the radiator in the room with your room thermostat has a thermostatic valve on it. It should not. If it does, the two controls fight each other, and it is a common installation error worth correcting.
- Open every thermostatic valve fully at the end of the heating season and leave them open through summer. They seize in whatever position they sat in, and that is the single most common cause of one permanently cold radiator.
Frequently asked questions
How often should I bleed my radiators?
Once a year, at the start of the heating season, is plenty for a healthy system.
Should the heating be on or off?
Off, and cooled. Bleeding a hot system risks scalding.
Nothing came out when I opened the valve.
Then there is no air in that radiator, and the problem is something else. Cold at the bottom means sludge, and cold all over usually means a valve.
The water that came out was black.
That is magnetite, the corrosion product that builds up in heating systems. A little is normal. Very dark water suggests the system water is worth testing.
My boiler pressure dropped after bleeding.
Expected. Letting air out lowers pressure. Top it up with the filling loop to the correct figure for your boiler.
The same radiator needs bleeding every few weeks.
Air is getting in, usually because water is getting out. Worth finding the cause rather than repeating the bleed.
What to do about it
If bleeding did not fix it
Call 07514 312117 or send a WhatsApp, and tell us where the radiator is cold: top, bottom, middle or all over. That single detail identifies which of three completely different problems you have. Gas Safe registered 981362, CIPHE member, fully insured.