Radiator cold at the bottom

1 Why the bottom goes cold

Sludge is heavier than water, so it settles low in radiators and pipework, Baxi explains⁴. Ideal says it blocks the channels inside, which stops hot water reaching the lower part of the radiator³. Any warmth left low down, Ideal says, may just be the metal itself being warmed³.

Baxi adds that in hard water areas limescale can build up as well¹. Ideal notes that aluminium radiators do not suffer iron oxide sludge if the system contains no steel³.

2 How it differs from air

Trapped air collects at the top, so the two patterns are opposites. Worcester Bosch's guide draws the same line, saying a cold top means bleeding and a cold bottom could mean you need the system cleaned². If your radiator is cold higher up, see cold at the top.

A radiator that has only just come on is not a fair test. Ideal's guidance is to judge once the system is up to temperature³.

3 Other signs of sludge

4 What an engineer may do

Vaillant says a power flush uses a dedicated machine and chemicals and that it does not recommend users do it. An engineer may refill with clean water and add inhibitor afterwards⁵. Baxi says a Gas Safe registered installer can flush and clean the system and treat the water⁷.

Ideal's and Vaillant's web pages also describe taking a single radiator off the wall to clean it³⁵. That involves draining and disconnecting pipework, so those steps are not repeated here. Vaillant itself says the job is best done by a professional⁵.

Keeping it from returning is covered under sludge, inhibitor and filters. Baxi warns that repairs caused by sludge are not covered by its warranties¹, so check the terms for your own boiler.