Radiator Cold at Top? How to Bleed It UK Guide 2026

If a radiator is cold at the top but warm at the bottom, trapped air is the most common cause. Air prevents hot water reaching the top of the radiator, reducing heat output and comfort. This UK guide explains how to bleed a radiator safely and what to do if the problem comes back.

What You’ll Need

Radiator bleed key (or screwdriver depending on valve type)
Cloth/towel
Small container to catch water

Step-by-Step: How to Bleed a Radiator

Turn heating off: let radiators cool so you don’t release hot water under pressure.
Place a cloth and container: under the bleed valve.
Open the bleed valve slowly: you should hear a hiss as air escapes.
Close when water flows steadily: once the hissing stops and water runs without spurting.
Check boiler pressure: sealed systems may need topping up afterwards.

If the Radiator Still Has Cold Spots

Cold bottoms often point to sludge rather than air. If only one radiator is affected, it might also be a stuck TRV or balancing issue.

When to Call an Engineer

If you repeatedly need to bleed radiators, pressure keeps dropping, or you hear gurgling/banging in the system, it can indicate air ingress or broader system issues. An engineer can check for leaks and recommend a longer-term fix.

If You’re Replacing the Boiler, Check Output First

Heating problems can be a prompt to upgrade. Use our calculator to estimate the right boiler kW output for your home.

Use the Boiler Size Calculator

Frequently Asked Questions

Why is my radiator cold at the top?

Trapped air is the most common reason. Bleeding releases that air so hot water can fill the radiator fully.

Do I need to turn the heating off to bleed radiators?

It’s recommended so you’re not dealing with hot water and so pressure behaviour is more predictable.

Will bleeding reduce boiler pressure?

It can on sealed systems. Check the boiler gauge afterwards and top up if needed.

What if no air comes out?

The radiator might not be getting flow due to a valve issue, balancing, or sludge. Check TRVs/lockshields and consider a system check.

Why do I need to bleed radiators so often?

Frequent air can indicate a leak, pressure problems, or ongoing corrosion. If it keeps happening, book an engineer.