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Bleeding a radiator

Checked against its sources: Updated

To bleed a radiator, turn the heating off, let it cool, open the bleed valve with a radiator key until hissing stops and water appears, close it, then check the boiler pressure.12 Makers differ on the details, so each step names its source.

Covers
Water-filled radiators on boiler central heating, UK
You
Bleed with the heating off and cool, unless your boiler's manual leaves it to a competent person. Top up pressure only as that manual says.
A professional
Repeated bleeding, repeatedly rising pressure, leaks, sludge, and anything inside the boiler.

1 Steps from the makers' guides

These steps come from web guides published by Worcester Bosch and Vaillant. No single user manual read gives this full sequence, and a radiator that is cold at the bottom needs a different fix.

  1. With the heating running, feel each radiator and note those colder at the top (Worcester Bosch)1
  2. Turn the heating off and let the radiators cool (Worcester Bosch)1. Vaillant says to leave thermostatic valves fully open2
  3. Fit the key to the bleed screw at the top and hold a cloth underneath for drips (Worcester Bosch)1
  4. Turn the key slowly anticlockwise until you hear hissing, and open it no further (Vaillant)2
  5. When the hiss stops and water comes out, turn the key clockwise to close, without overtightening (Vaillant)2
  6. Read the pressure gauge before switching the heating back on and top up if your manual allows (Vaillant, Worcester Bosch)12
  7. Turn the heating back on and check the bled radiators are warm all over (Worcester Bosch)1

Water from a radiator can scald. Keep your face and hands clear of the valve, and wait until the radiator has cooled before you start.

2 Where the makers disagree

Radiator order and timing vary, and none of these is an official rule. Follow your own boiler manual where it gives a method.

Which radiator first, by source
SourceStart with
Vaillant web guideGround floor, then upstairs2
Vaillant ecoTEC plus manual, 2023Nearest the boiler, then further away5
Baxi FAQClosest to the boiler3
Viessmann web guideFurthest from the boiler4
Main Eco Compact manualLowest in the property6

The Main Eco Compact manual also differs on temperature. It bleeds with the radiators warm and the boiler switched off, and warns that the water will still be hot6. Worcester Bosch, Vaillant, Viessmann and Baxi's own web guide wait for the system to cool1234, and that cooler method is the one given above.

3 Check your manual first

Permission differs between documents. Vaillant's 2023 manual lets the user purge radiators, while its 2017 ecoTEC pro issue leaves purging to a competent person57. Glow-worm's Energy7 manual also says to have a competent person do it8.

Target pressure differs as well, so no single figure is given here. Vaillant's guide says 1 to 1.5 bar cold2, and Baxi's says around 1.5 bar3. See boiler pressure for the figures by model.

4 When bleeding is not the answer

Worcester Bosch's Greenstar 4000 manual warns against repeatedly venting radiators. If pressure keeps rising and you keep having to vent, it says to contact a heating engineer urgently, as this can indicate corrosion9. Baxi links more frequent bleeding, or signs of rust or water, with a possible leak for a Gas Safe registered engineer to fix3.

From the Baxi channel on YouTube. Baxi shows bleeding a radiator with a vent key and cloth, boiler off and starting at the lowest radiator, then checking the pressure. Other makers' first-radiator advice differs. Nothing loads from YouTube until you press play.

Sources

  1. How to bleed a radiator, Worcester Bosch FAQ read 3 November 2026.
  2. Bleeding in 7 steps, Vaillant read 3 November 2026.
  3. Baxi: bleed in 8 steps and Baxi: why radiators are cold read 3 November 2026.
  4. Viessmann UK bleeding guide read 3 November 2026.
  5. ecoTEC plus operating instructions, 0020308119_03, 28.11.2023 p15, read 3 November 2026.
  6. Main Eco Compact Combi guide, 7716571-04 p19, read 3 November 2026.
  7. ecoTEC pro operating instructions, 0020244996_01, 2017 p12, read 3 November 2026.
  8. Energy7 operating instructions, 0020300878_00, 2020 p14, read 3 November 2026.
  9. Greenstar 4000 instructions, 6720891170 (2020/09) p17, read 3 November 2026.