How to Bleed a Radiator (and Repressurise the System)

To bleed a radiator: turn the heating off and let it cool, hold a cloth under the bleed valve at the top corner of the radiator, turn a bleed key half a turn anticlockwise, let the air hiss out, and close the valve as soon as water spits cleanly. On a sealed combi or system boiler, check the pressure gauge afterwards and top up at the filling loop to 1.0-1.5 bar cold.

When to bleed

• The radiator is hot at the bottom and cold at the top.
• The radiator makes gurgling, popping or banging noises when the heating's running.
• Heating performance has dropped overall — rooms slow to warm up.
• You've just refilled or worked on the system and air is to be expected.

What you need

• A radiator bleed key (a few pounds at any DIY shop) or, on modern radiators, a flat-blade screwdriver.
• An old towel or cloth to catch spits.
• A small container — half-pint mug is plenty.
• Knowledge of where your filling loop is, if you're on a sealed system.

The procedure

1. 1

   Turn the heating off and let it cool

   Switch the boiler off at the programmer. Wait 30 minutes to an hour so radiators are cool to touch. This stops you scalding yourself and stops air being pushed back into the radiator by an active pump.

2. 2

   Start with the radiator nearest the boiler on the ground floor

   Work outwards and upwards — downstairs first, then upstairs, finishing with the radiator furthest from the boiler. Air rises so the highest, furthest radiator usually holds the most.

3. 3

   Locate the bleed valve

   It's a small square or hex-shaped nipple at the top corner of the radiator, usually opposite the thermostatic valve.

4. 4

   Hold a cloth under the valve

   Position the cloth and container directly under, ready to catch water.

5. 5

   Open the valve a half turn anticlockwise

   Don't unscrew it fully — you only need to crack it open. You should hear a hiss as air escapes.

6. 6

   Wait for water to spit out cleanly

   Once the hiss stops and a clean spray of water emerges (no more bubbles), close the valve gently. Don't over-tighten — a snug close is enough.

7. 7

   Move to the next radiator

   Repeat through the house in order.

8. 8

   Check system pressure

   On a sealed system, the gauge will have dropped. See repressurising below.

9. 9

   Turn heating back on and re-test

   Let the system come up to temperature and check that all radiators are now hot top to bottom.

Repressurising a sealed system

A combi or system boiler runs as a sealed pressurised circuit, typically at 1.0-1.5 bar cold rising to about 2 bar when hot. Every time you bleed water out, the gauge will drop. If it falls below 0.5-1.0 bar the boiler will lockout.

1. 1

   Find the filling loop

   Usually under or beside the boiler — a flexible braided silver hose with a lever or small handles at each end.

2. 2

   Check both isolation valves are visible

   There should be a valve at each end of the loop. Both should be closed (handles across the pipe) when you start.

3. 3

   Open slowly

   Open one valve, then the other slowly, while watching the pressure gauge on the boiler. You'll hear water flowing.

4. 4

   Stop at 1.0-1.5 bar cold

   Close both valves the moment the needle reaches the lower target — pressure will continue to creep up slightly. Aim for the lower end of the green band.

5. 5

   Close the filling loop fully

   Both valves shut, handles across the pipe. A loop left open is a slow-flooding risk and is also against the water regs.

If something doesn't go to plan

• Pressure won't come up — filling loop valves are seized or there's a closed isolation valve. Don't force them.
• Pressure shoots up past 2 bar — you've left the loop open. Close immediately and let the PRV vent the excess.
• Same radiators need bleeding every few weeks — likely a failed expansion vessel or a slow leak. See our expansion vessel guide.
• Radiator stays cold at the top after bleeding — air keeps re-collecting, or the radiator is sludged. Time for a system clean.
• You spot dripping or weeping from a valve afterwards — radiator valves don't always seat cleanly after movement. See our radiator leak repair page.

How often should you bleed?

A healthy heating system shouldn't need bleeding more than once a year — typically in the autumn when you first fire it up. Anything more frequent than that means there's a real fault drawing or generating air, and you should stop bleeding and start diagnosing. Repeated top-ups also dilute the inhibitor in the system, which then lets rust and sludge accelerate — which then needs a power flush. The chain is real, so don't ignore it.

Frequently Asked Questions

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