Radiator cold at the top: how to bleed it
If a radiator heats at the bottom but stays cold at the top, there's air inside blocking the hot water. You remove it by bleeding: a simple, quick job.
What you need
The bleed key (or a screwdriver, depending on the valve), a cloth and a bowl or glass for the water that comes out.
The steps
- Turn off the boiler or pump, and wait for the system to settle.
- Find the small bleed valve: it's at the top, on the side opposite the tap.
- Hold the cloth and bowl underneath, open the valve slowly.
- Hear the air come out (a hiss). When water flows steadily, close it.
- Repeat on every cold radiator, starting with those furthest from the boiler.
After bleeding
Often the system pressure drops: check the boiler gauge (usually it should sit between 1 and 1.5 bar) and if needed top up with the filling tap.
Frequently asked questions
Why is the radiator cold only at the top?
Because there's trapped air at the top stopping the hot water from filling the whole radiator.
How do I bleed a radiator?
Slowly open the small bleed valve at the top, with the boiler off, until water comes out instead of air, then close it.
Do I need to repressurise afterwards?
Often yes: after bleeding check the gauge and top up if below 1 bar.
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