Radiator cold at the top

1 Why the top goes cold

Air rises above the water and collects at the top of a radiator, where it stops hot water filling the space. Ideal's explanation is that the trapped air is not being heated, while hot water keeps moving through the lower part³. Viessmann describes the same pattern and adds that the air may build up while heating is off over the summer⁵.

2 How to test it

A radiator that has only just come on can feel hotter near the inlet at the bottom, which Ideal says is normal. Its test is to wait 15 to 20 minutes, put your palm low on the middle of the radiator and slide it upwards³. A noticeable drop towards the top is the sign to look for.

Take care, as radiators can get very hot. Baxi group's Main Eco Compact manual says they may reach 85 °C depending on settings and warns against touching them for long⁶.

3 What to do

Worcester Bosch and Baxi both treat a cold top as a job for a radiator key¹². The steps, and where makers differ on them, are on the bleeding page. Afterwards, check the gauge, because Worcester Bosch notes that boiler pressure can be lost when radiators are bled¹.

4 If it keeps coming back

Ideal says it is almost impossible for air to get into a pressurised system from outside. Its likeliest cause is corrosion inside pipes and radiators, which forms sludge at the bottom and gas at the top, and it recommends an inhibitor against this³. See sludge and inhibitor for what that means.

Worcester Bosch's Greenstar 4000 manual is firmer. If pressure is repeatedly rising and you keep needing to vent, it calls for urgent action from a heating engineer⁴. Vaillant also says repeated bleeding should be raised with your installer at the service, as there may be an underlying problem⁷.

5 Other patterns