Radiators Cold at the Top? How to Bleed Them Safely

You've turned the heating on, but the radiators are only warm at the bottom and stone cold at the top. This is a classic sign of trapped air in your central heating system.

What's Causing the Issue?

Over time, air naturally accumulates in a central heating system. Because air is lighter than water, it rises to the top of your radiators, preventing the hot water from circulating fully and heating the metal.

What You Can Do Right Now

You can easily fix this yourself by bleeding the radiators:

• 1. Turn off your heating and let the radiators cool down.
• 2. Place a cloth under the bleed valve (top corner of the radiator).
• 3. Insert a radiator key and turn slowly anti-clockwise until you hear a hissing sound.
• 4. Once water starts dribbling out, quickly close the valve.
• 5. Check your boiler pressure afterwards and top up if it's dropped below 1 bar.

When It Becomes Serious

If you have to bleed your radiators every few weeks, air is actively entering the system. This usually points to a leak, a failing pump, or a degraded expansion vessel. Leaving it will cause internal rust (sludge) and destroy your boiler.

When to Call an Engineer

If the radiator is cold at the *bottom* but hot at the top, bleeding won't help. That means the radiator is full of thick sludge, and you need a professional powerflush to clear it out.

Frequently Asked Questions