Why Is My Radiator Cold at the Top?

Trapped air is the usual cause of a radiator that is hot at the bottom and cold at the top. Here is how to bleed it properly.

Last updated: 2026-10-19

A radiator that is hot at the bottom and cold at the top is the classic signature of trapped air. Air rises to the top of the radiator and stops hot water filling that section.

Why air gets in

• The system was recently drained and refilled, and not fully bled.
• Air is being drawn in through a faulty component or a pump drawing air.
• Corrosion inside the system is producing gas.
• A recently replaced radiator or valve introduced air.

A one-off airlock after work is normal. Air that keeps coming back points to an underlying issue that bleeding will not solve.

The fix: bleed the radiator

Bleeding releases the trapped air. You need a radiator key (or a flat-head screwdriver for some valves) and a cloth.

1. Turn the heating off and let the radiator cool — bleeding a hot system risks scalding.
2. Find the bleed valve at the top corner of the radiator.
3. Hold the cloth under it and open the valve slowly, about a quarter turn.
4. You will hear a hiss as air escapes. When water starts to trickle steadily, close the valve.
5. Check the boiler pressure afterwards — bleeding lowers it. Top up if needed (see our pressure guide).

Our step-by-step bleeding guide covers the details, including what to do if the valve will not turn.

If it keeps happening

Repeated airlocks usually mean air is entering somewhere. Common causes include a failing pump, a blocked cold feed on older systems, or corrosion producing gas. That needs diagnosis, not another bleed.

Not the same as cold at the bottom

A radiator cold at the bottom is a different problem — usually sludge. See why a radiator is cold at the bottom.

When to call

Book a radiator or heating visit if the radiator stays cold after bleeding, the bleed valve is seized or leaking, the problem keeps returning, or several radiators are affected at once.