How to bleed a radiator

Inspired by a helpful YouTube guide. This walk-through is based on the popular "How to Bleed a Radiator" video from the British Gas channel, which shows a real engineer going through each step. Worth watching before you start — the part about checking the pressure gauge afterwards catches a lot of people out.

1. Check whether bleeding is actually needed

Run the heating and, once the radiators are warm, carefully feel the top and bottom of each one. Cold at the top but warm at the bottom almost always means trapped air. Uniformly cold radiators or radiators that take a very long time to heat up may point to a different problem — sludge, a pump issue, or low pressure.

If every radiator in the house is cold, check the boiler first. It may have lost pressure, locked out on a fault, or the timer may simply not be set correctly.

2. Turn the central heating off

Switch the heating off and let the system cool down before you bleed anything. Hot water under pressure will spray out of an open bleed valve, which is both dangerous and messy.

Ten to fifteen minutes is usually enough time for the radiators to lose the worst of their heat. They can still feel warm — that is fine — but they should not be scalding.

3. Lay down a cloth and have a container ready

The bleed valve sits at the top corner of the radiator, usually behind a small square spindle. Place an old cloth or some folded kitchen roll directly below it, and hold a small bowl or cup underneath. Water will trickle out when the air clears.

Mind you, the water can be discoloured — dark brown or black from sludge in the system. That is not unusual, but if you see heavy sludge every time you bleed, it may be worth getting the system power-flushed.

4. Open the bleed valve slowly

Insert the radiator bleed key into the square spindle and turn it anti-clockwise by about a quarter turn. You should hear a hissing sound as air escapes. Keep the cloth in place and do not open the valve too far — a quarter turn is enough.

Some older radiators can be bled with a flat-head screwdriver instead of a key. If you are unsure, have a look at the valve before you start. Bleed keys cost very little from any DIY shop and it is worth keeping one in a kitchen drawer.

5. Close the valve as soon as water trickles out

Once the hissing stops and a steady trickle of water appears, the air has cleared. Turn the key clockwise to close the valve immediately. You only need it snug — do not overtighten it or you risk damaging the valve seat.

Wipe any drips from the radiator body and the floor, then move on to the next radiator that needs attention. Work through the house systematically, starting with the ground floor and finishing upstairs.

6. Check the boiler pressure

Bleeding releases a small amount of water from the system, which can drop the boiler pressure. Check the pressure gauge on the boiler — it should read between 1 and 1.5 bar when the system is cold. If it has dropped below 1 bar, you will need to repressurise the boiler using the filling loop.

The filling loop is a flexible hose usually found underneath the boiler. Open both valves slowly until the pressure reads around 1.2 bar, then close them again. Your boiler manual will show you where everything is, and most modern boilers have a diagram on the inside of the front panel.

7. Turn the heating back on and check the results

Switch the heating on again and feel the radiators as they warm up. The ones you bled should now heat evenly from top to bottom. If a radiator is still cold or partially cold, it may need bleeding a second time, or there could be a circulation issue worth investigating.

To be fair, most homes only need their radiators bled once a year — usually at the start of the heating season in autumn. Making it a regular habit keeps the system running efficiently and can reduce heating bills a little.

When to call a handyman

Call Richard if the bleed valve will not open, if it leaks after you close it, if the pressure keeps dropping, or if multiple radiators refuse to heat properly after bleeding. The Sandwich Handyman can help with small heating maintenance tasks, property checks, and general home repairs across Sandwich and nearby East Kent villages.