Radiators cold at the top: why air is the usual explanation

Things worth knowing about a radiator cold at the top

• Trapped air rises to the highest point inside the radiator body
• Bleeding is a homeowner-safe job with a standard radiator key
• Turn the key a quarter turn and stop the moment water appears
• One radiator needing frequent bleeding suggests an ongoing air source
• A whole-system pattern points at pump or automatic air vent faults

A radiator cold at the top explained in plain terms

Air enters a sealed system in small amounts whenever it is topped up, drained down for work, or simply through normal micro-bubble release from the water over time, and it naturally migrates to the highest points, which on many radiators is the top corner opposite the bleed valve. Bleeding lets that trapped air escape and allows water to fill the space it occupied. If bleeding does not resolve it, or the same radiator needs bleeding every few weeks, the cause shifts from ordinary air release to either a system generating air continuously, such as a failing automatic air vent, or a pump drawing in air somewhere it should not.

Where we deal with this

• Twerton (BA2) · 7 mi
• Norton St Philip (BA2) · 6 mi
• Stratton-on-the-Fosse (BA3) · 4 mi
• Doulting (BA4) · 6 mi
• Glastonbury (BA6) · 13 mi

Questions we get asked

How do I bleed a radiator safely?

With the heating on and radiators hot, use a radiator key on the bleed valve at the top corner, turn it slowly a quarter turn, and close it firmly the moment steady water (not air) comes out.

Should the heating be on or off when I bleed a radiator?

On, so you can feel and hear the difference clearly, but be aware the radiator body and any escaping water will be hot.

Why does the same radiator need bleeding every month?

That pattern suggests air is being generated continuously somewhere in the system rather than a one-off pocket, and is worth mentioning at your next service visit.