How to Bleed a Radiator (Step by Step, No Tools Drama)

If a radiator is cold at the top but warm at the bottom, there's air trapped in it — and letting it out takes five minutes. Here's exactly how, including the two mistakes that catch people out.

What you need

• A radiator bleed key (about £1 from any hardware shop — some radiators take a flat screwdriver)
• A cloth or small tub to catch drips

Step by step

1. Turn the heating off and let the radiators cool. Bleeding with the pump running pulls more air in, and the water inside can be scalding. This is mistake number one.
2. Find the bleed valve — a small square pin inside a round nut at one top corner of the radiator.
3. Hold the cloth under the valve and turn the key anti-clockwise about a quarter to half a turn. You'll hear a hiss — that's the air escaping. Don't unscrew it fully.
4. Wait for the hiss to stop. When water starts to dribble out steadily, the air is gone. Close the valve snugly — firm, not gorilla-tight.
5. Repeat for any other airy radiators, starting with the lowest floor and working up.
6. Check the boiler pressure. This is mistake number two — bleeding lowers system pressure. If the gauge now reads under 1 bar, top it up to about 1.2 using the filling loop, or the boiler may lock out. Our pressure guide covers it.
7. Heating back on and check the radiator heats evenly top to bottom.

When bleeding won't fix it

• Cold at the bottom instead? That's sludge, not air — bleeding does nothing. See our cold radiator guide.
• Same radiator needs bleeding every week? Air is getting in somewhere — often a small leak or a corroding system making gas. Worth investigating before it damages the boiler.
• Whole floor cold? More likely a balancing or pump issue than air.

Rather we just sorted it?

No shame in it — and if radiators keep going cold there's usually an underlying cause worth finding. We cover Preston and the surrounding areas: 07949 075721, or the form below. More on our central heating page.