How to bleed a radiator: the UK and German steps, and what to check afterwards

Turn the heating off, fit a radiator key into the small valve at the top corner, turn it anticlockwise until air hisses out, close it clockwise as soon as water comes, then check your boiler pressure.

By Abu Bakar SaadatPublished 27 October 20263 min readPublisher: Saadat Rafiq

Illustration

At a glance

Open the valve: Anticlockwise
Close the valve: Clockwise, as soon as water comes out
You need: Radiator key, cloth, small cup
UK pressure after (Worcester Bosch): 1 to 1.5 bar on the gauge
Advice from: Worcester Bosch and Viessmann, read 27 October 2026
Checked: October 2026

Does it need bleeding?

Cold at the top, warm at the bottom: air is trapped. Bleed it (Worcester Bosch).
Gurgling or flowing noises, or radiators that stay partly cold: bleed it (Viessmann).
Warm at the top, cold at the bottom: this is more likely sludge, not air. Bleeding won't fix it; Worcester Bosch says the system may need cleaning.

The UK and German routines side by side

	UK (Worcester Bosch)	Germany (Viessmann)
Before	Heating on to find the cold radiators, then off and let them cool	Circulation pump off (renters: close the flat's own heating circuit if it has a shut-off valve), all thermostats fully open, wait 30 to 60 minutes
Which first	Any radiator that needs it	The highest radiator, where most air collects
Open	Key anticlockwise	Square key, slowly anticlockwise
Close	Clockwise once water flows steadily	Quickly, as soon as water comes out
After	Heating on, check radiators are warm all over, top up boiler pressure	Pump back on, check the pressure gauge, refill if needed; after an hour, recheck the radiator farthest from the boiler

Step 1: find the cold radiators

Illustration of a woman resting her hand on a radiator that is cold (blue) at the top and warm (yellow) below — Illustration

Put the heating on, wait for the radiators to warm up, and feel each one. Note the ones that are cold at the top (Worcester Bosch).

Step 2: turn the heating off and open the thermostats

Illustration of a woman turning a knob on a wall boiler with one hand and the radiator thermostat valve with the other — Illustration

Switch the heating off and let the radiators cool (Worcester Bosch). Viessmann also says to turn all radiator thermostats fully open and wait 30 to 60 minutes so the air collects at the top. Have a cloth and a small cup ready.

Step 3: turn the key anticlockwise until air hisses out

Illustration of a hand turning a radiator key anticlockwise in the bleed valve at the top corner, with air hissing out — Illustration

The bleed valve is the small square-centred screw at the top corner, on the opposite side from the thermostat (Viessmann). Push the key in and turn it slowly anticlockwise. You will hear a hiss as the air escapes. The water can be hot, so keep your hand clear of the opening.

Step 4: catch the drips

Illustration of a cloth held under the bleed valve catching drops of water — Illustration

Hold the cloth or cup right under the valve, with another cloth on the floor below (Viessmann).

Step 5: close it clockwise as soon as water comes out

Illustration of a hand turning the radiator key clockwise to close the bleed valve, with a cloth underneath — Illustration

Water instead of air means the radiator is clear. Turn the key clockwise to close the valve. Viessmann says to close it quickly so you don't lose too much heating water. Repeat for each radiator that needs it.

Step 6: heating back on, then check the pressure

Illustration of a finger pointing at a round boiler pressure gauge with the needle inside the marked band — Illustration

Turn the heating (or pump) back on and check the radiators are warm all over. Bleeding lets out water, so the boiler pressure can drop. Worcester Bosch's homeowner guide says to top it up until the gauge reads between 1 and 1.5 bar, using your boiler's filling link, key or external hose. If you go too far, bleed a radiator to bring it down. Other boilers may use a different range: check your boiler's manual.

If this doesn't work

Still cold after bleeding: Viessmann says a stuck thermostat valve, missing hydraulic balancing or the heating controls can be the cause. Ask a heating engineer.
You rent: you are allowed to bleed your radiators, but other repairs are the landlord's job. If heat still doesn't spread properly, tell your landlord first (Viessmann, for Germany).
You have to bleed again and again: Viessmann says this can point to a deeper problem; it lists leaks and a porous expansion vessel among the ways air gets in. Get the system checked.
Pressure keeps dropping after top-ups: call the boiler maker or a heating engineer.

Questions readers ask

Which way do I turn a radiator key?

Anticlockwise to open the bleed valve and let the air out, clockwise to close it. Worcester Bosch and Viessmann both say this.

How long does it take to bleed a radiator?

Usually not long. Viessmann says to keep it open until no more air comes out and water appears, then close it quickly.

How often should I bleed my radiators?

Viessmann recommends once a year, ideally in autumn before the heating season, and whenever radiators are only partly warm or gurgle.

What if I lose my radiator key?

Viessmann says a pointed pair of pliers or a flat screwdriver can work on some valves, but warns you can round off the valve so it no longer closes. A new key is cheaper than that repair.