Radiator Cold at the Top? Here's Why & How to Fix

Quick Answer: A radiator cold at the top usually has air trapped inside. Fix it by "bleeding" the radiator - releasing the air through the bleed valve. Takes 5 minutes and only needs a radiator key.

Why Is the Top Cold?

Air naturally rises in your heating system. When air gets trapped in a radiator:

• It collects at the top
• Prevents hot water filling that space
• Results in cold top, hot bottom

How to Bleed a Radiator

You'll Need:

• Radiator key (or flat screwdriver)
• Small container or cloth
• 5 minutes

Step-by-Step:

1. Turn heating off and let radiators cool
2. Find the bleed valve - small square or slot, usually top corner
3. Hold container below valve - some water will come out
4. Insert key and turn anti-clockwise - you'll hear hissing (air escaping)
5. Wait for water to appear - steady stream means air is out
6. Close valve - turn clockwise until snug (don't overtighten)
7. Check boiler pressure - may need topping up after bleeding

Which Radiators to Bleed?

Start with the radiator furthest from boiler, work back toward it.

When Bleeding Doesn't Fix It

If still cold at top after bleeding:

Check for blockages:

• Sludge buildup in radiator
• May need removing and flushing
• Or full powerflush (£350-£500)

Check the valve:

• Thermostatic valve stuck
• Try removing and cleaning
• Or replace valve (£80-£150)

Check circulation:

• Pump not pushing water effectively
• Balance radiators by adjusting lockshield
• May need pump replacement

When to Call a Professional

• Multiple radiators affected
• Pressure keeps dropping after bleeding
• Boiler making noise
• Bleeding doesn't release any air

Glasgow heating engineer: +44 141 468 9930