If a radiator is warm at the bottom but cold or lukewarm at the top, it almost certainly has trapped air in it. The water can’t circulate properly through the upper part of the radiator, so you’re running your heating but not getting the full heat output. Bleeding lets the trapped air escape, water fills the space, and the radiator heats evenly again.
It’s one of the easier DIY jobs in a house — it takes about 5 minutes per radiator and the only tool you need is a radiator bleed key.
What You Need
- A radiator bleed key (a small square-ended key that fits the bleed valve). We sell these in store.
- A cloth or small bowl to catch any drips of water.
Step 1: Turn the Heating Off and Let It Cool
Always bleed radiators with the heating turned off and the system cooled down. If the heating is on, the pump is actively circulating water, which makes air harder to release and can pull air in through the valve. Turn the heating off and wait until the radiators are cool — about an hour after the system goes off.
Step 2: Find the Bleed Valve
The bleed valve is a small square-headed screw at the top of one end of the radiator — usually behind a plastic cap. It looks like a small plug. Remove any cap and fit the bleed key over the square head.
Step 3: Open the Valve Slowly
Hold your cloth or bowl beneath the valve. Turn the key anticlockwise — just a quarter to half a turn. You should hear a hissing sound as trapped air escapes. Keep the valve open until the hissing stops and a steady trickle of water appears. Once water is running consistently, the air has cleared.
Close the valve by turning the key clockwise. Don’t overtighten — snug is enough. Mop up any water that dripped onto the radiator or floor.
Step 4: Check the Boiler Pressure
Releasing air from the system reduces the water pressure in your central heating. After bleeding all your radiators, check the boiler pressure gauge — it should read between 1 and 1.5 bar when cold. If it’s dropped below 1 bar, you’ll need to repressurise the boiler using the filling loop (usually a small flexible hose under the boiler with two valves). Your boiler manual will have the steps for this, or it’s usually printed on a label inside the boiler cover.
Step 5: Turn the Heating Back On
Once all radiators are bled and the pressure is correct, turn the heating back on and wait for the system to warm up. Check each radiator you bled — it should now be evenly hot from top to bottom.
When Bleeding Doesn’t Fix It
If a radiator is still cold after bleeding, or if the whole radiator is cold rather than just the top, the problem may be different:
- Sludge build-up: Over time, iron oxide sludge can accumulate in radiators and block circulation. A cold radiator at the bottom is more likely sludge than air. This requires a powerflush — a job for a plumber.
- Thermostatic valve fault: If the TRV (thermostatic radiator valve) on the radiator is stuck shut, the radiator won’t heat regardless. Try turning the valve head to a higher setting and see if the radiator comes on.
- Balancing issue: If some radiators heat well and others don’t, the system may need balancing — adjusting the lockshield valves to distribute flow evenly across all radiators.
The Short Version
Cold at the top, warm at the bottom — that’s trapped air, bleed it. You need a radiator bleed key, five minutes per radiator, and to start from the ground floor up. Check the boiler pressure afterwards and top it up if it’s fallen below 1 bar.
We stock radiator bleed keys in store.
Everything you need to sort your radiators — bleed keys, PTFE tape, washers, and general plumbing supplies. Come and pick one up at 248–250 Lavender Hill, SW11 1LJ. Open 7 days.