How to Bleed a Radiator

Bleeding a radiator is one of the few heating maintenance tasks a homeowner can safely carry out themselves. It takes around 10–15 minutes, requires only a bleed key (available from any hardware shop for under £2), and can make a noticeable difference to heating efficiency. Here is how to do it correctly — and what the symptoms mean for your wider system.

When Should You Bleed Your Radiators?

Bleed your radiators if you notice any of the following:

• The top of a radiator is cold or cool while the bottom is warm
• The whole radiator heats up slowly or unevenly
• You can hear gurgling or trickling sounds when the heating is on
• Your boiler pressure drops regularly, suggesting air is entering the system

These are all signs of trapped air in the radiator circuit. Bleeding releases this air and allows hot water to fill the space fully.

What You Will Need

• A radiator bleed key — a small flat-ended tool, usually sold for £1–£2 at hardware shops
• An old cloth or towel to catch drips
• A small bowl or container to hold under the valve

Step-by-Step: How to Bleed a Radiator

1. 1

   Turn your heating on and identify which radiators need bleeding

   Run the central heating for 15 minutes. Feel each radiator carefully — if the top half is noticeably cooler than the bottom, there is trapped air inside. Note which radiators need attention before turning the heating off.

2. 2

   Turn the heating off and allow the system to cool

   Turn the heating off and wait at least 30 minutes before proceeding. The radiators need to be cool to the touch before you open any bleed valve.

3. 3

   Locate the bleed valve on the radiator

   The bleed valve is a small, square-headed screw set into the body of the radiator near the top, usually at one end. It may have a small plastic cap that can be removed by hand.

4. 4

   Open the bleed valve slowly — a quarter turn anti-clockwise

   Hold your cloth beneath the valve and insert the bleed key. Turn it anti-clockwise by a quarter-turn only. You will hear hissing as air escapes. Do not fully remove the valve — a quarter-turn is sufficient and keeps the valve under control.

5. 5

   Close the valve when water appears

   When a steady trickle of water replaces the hissing air, the radiator is bled. Close the valve by turning the key clockwise until snug. Do not overtighten — the valve seats are brass and can be damaged by excessive force.

6. 6

   Check and restore your boiler pressure

   Releasing air from radiators reduces the pressure in your sealed system. Check your boiler pressure gauge immediately after bleeding. Most systems should read between 1.0 and 1.5 bar when cold. If it has dropped below 1.0 bar, top it up using the filling loop until the pressure is restored.

When Bleeding Is Not Enough

If your radiators need bleeding more than once a year, there is an underlying problem. Repeated air ingress usually means one of three things:

• A failing automatic air vent — allowing air to enter the system continuously
• A pinhole leak in the pipework — which draws air in as water escapes
• Significant internal corrosion — releasing hydrogen gas as metal deteriorates, which accumulates at radiator tops

The third cause — hydrogen gas from corrosion — is the most serious. It indicates that the system water is contaminated with sludge and should be power flushed and fitted with a magnetic filter. Continuing to bleed without addressing this damages your boiler and shortens the life of the entire system.

If bleeding does not resolve the problem, or if the same radiators need bleeding repeatedly, call a Gas Safe registered engineer for a diagnostic assessment. We carry out these assessments across Blackheath, Greenwich, Charlton, Lewisham, Eltham, and Plumstead — usually same-day or next-day.