Radiators cold at the bottom? What it means and when a power flush helps
A radiator that is warm at the top and cold at the bottom is telling you something. Here is what it usually means, what you can check yourself and when a power flush is the answer.
2 min read
Put your hand on a radiator when the heating has been on for a while. It should be warm all over. If it is hot at the top and cold at the bottom, or cold in the middle, that is worth looking into before winter.
Cold at the top: air
A radiator that is cold at the top and warm at the bottom usually has air trapped in it. Bleeding it often fixes that. With the heating off and the radiator cool, open the bleed valve at the top corner a quarter turn with a radiator key, holding a cloth underneath. When the hissing stops and water appears, close it. If you have a combi boiler, check the pressure gauge afterwards and top it up if it has dropped.
Cold at the bottom: sludge
A radiator that is cold along the bottom is a different problem. Over the years, the water in a heating system picks up rust from the inside of radiators and pipes. It settles as a black sludge at the lowest point, which is the bottom of the radiator. The hot water cannot get through it, so that part stays cold.
Other signs of sludge include:
- dark or black water when you bleed a radiator
- radiators that need bleeding again and again
- a boiler or pump that is noisy, or that bangs and gurgles
- a system that takes a long time to warm the house
What a power flush does
A power flush connects a pumping machine into the heating system and pushes water through it at speed, usually with a cleaning chemical, to lift the sludge and carry it out. Each radiator is worked on in turn until the water runs clean. The system is then refilled with fresh water and an inhibitor, which helps stop the rust coming back.
It normally takes most of a day, depending on how many radiators you have and how bad the sludge is. Your heating is off while the work goes on.
When a power flush is not the answer
Not every cold radiator needs one. A single cold radiator could be a stuck valve, a radiator that needs balancing, or simply one at the far end of the system that gets the least flow. Very old or badly corroded radiators are sometimes better replaced than flushed. That is why it is worth having someone look at the whole system before deciding.
If it does turn out to be sludge, a flush can make a real difference to how quickly and evenly the house warms up, and takes some strain off the boiler and pump. Find out more on our power flushing page.
Radiators not warming up?
Tell us what your heating is doing and we will tell you whether a power flush is the answer.
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