How do I bleed a whole heating system properly?

Heating off, cold radiators, ground floor first — and topping the pressure back up afterwards.

Difficulty: beginner
Time: 1 hr
Read: 2 min
Safety: caution

Short answer

Turn the heating off and let everything cool, then bleed from the ground floor upwards, lowest radiator first. Air rises, so working upwards moves it in the direction it wants to go.

Bleeding one cold radiator is a familiar job. Doing the whole system in order, once a year before the heating season, is what actually removes the air rather than moving it around.

Safety

Turn the heating off and let the radiators cool before bleeding — hot water under pressure causes scalds. Have a cloth and a container ready. Check and restore the boiler pressure afterwards; bleeding always reduces it, and running a boiler at low pressure can damage it.

What you'll need

Radiator bleed key
Cloth
Small container
Torch (optional)

Step by step

Turn the heating off and let it cool.Completely — at least an hour. Bleeding a hot system sprays scalding water, and the air distribution is different when the pump is running.
Check which radiators need it first.Feel each one after a period of heating: cold at the top and warm at the bottom means air. Note them, then turn the heating off.
Start with the lowest radiator on the ground floor.Air rises, so working from the bottom up pushes it towards the highest points rather than trapping it. Order genuinely matters here.
Open the valve slowly.Bleed key on the square valve at the top corner, a quarter to half turn anticlockwise. You should hear hissing. Cloth underneath, container ready.
Close it as soon as water appears.A steady dribble with no hissing means the air is out. Close it firmly but do not overtighten — the valves are soft brass and strip easily.
Work upwards through the house.Ground floor first, then upstairs, then any radiator in a loft conversion. Finish with the highest one in the house.
Check and restore the boiler pressure.It will have dropped. Top up via the filling loop to 1-1.5 bar cold. Skipping this is the most common mistake and it can cause a lockout.
Turn the heating back on and re-check.Let it run, then feel every radiator again. Anything still cold at the top needs a second bleed; cold at the bottom is sludge rather than air.

Tips

Do this every autumn before the heating season, along with balancing. It takes an hour and it is the difference between a system that works and one that half works.
If radiators need bleeding every few months, air is entering the system from somewhere. That is worth investigating rather than repeating.
Cold at the bottom is not air — it is sludge, and bleeding will not help. That needs a flush.

Questions people ask

What order should I bleed radiators in?

Ground floor first, lowest radiator first, then work upwards through the house. Air rises, so this pushes it towards the highest points.

Do I need to top up the pressure after bleeding?

Yes — bleeding always reduces it. Check the gauge and top up to 1-1.5 bar cold via the filling loop. Skipping this can cause a lockout.