A radiator that is warm across the top and cold along the bottom is the most common heating complaint we get, and it almost always means the same thing.
It is sludge
Rust and debris settle out of the water and sink. They collect in the bottom of the radiator, where the water is slowest, and build up until that part of the radiator stops heating at all.
The other signs travel together. Water that comes out black when you bleed. A boiler that bangs or gurgles as it fires. Heating that takes forever to come up. One radiator that never gets warm however you set the valves.
How to check it yourself in two minutes
Put the heating on and let it run for twenty minutes. Then run your hand down each radiator from top to bottom.
Cold along the bottom third, warm across the top, on more than one radiator: that is sludge. Cold across the top with a warm bottom: that is air, and it needs bleeding. Stone cold all over while everything else is hot: check the valve at each end is actually open before you assume the worst.
Why it matters beyond a cold room
Your boiler is working harder to move water through a restricted system. That costs money every month, and it shortens the life of the pump and the heat exchanger. This is a problem that gets more expensive the longer it is left.
It also matters if you are thinking about a new boiler. Fitting one onto a sludged system is how people end up with an expensive appliance that underperforms and a manufacturer warranty that will not pay out. If a new boiler is on your mind, read what a new boiler actually costs first.
But sometimes it is not sludge
Cold at the top and warm at the bottom is air, not sludge, and bleeding it fixes that in a minute.
And if only one radiator is cold while the rest are fine, it is more likely a sticking valve than a sludged system. A power flush would be money you did not need to spend.
That is why we find out which it is first. It is a conversation, not automatically a job.
What a flush actually involves
A pump goes on the system and the water is driven round hard, with a cleaner in it, until what comes out runs clear. Then the system is refilled with an inhibitor to slow it happening again. It is a day’s work on a normal house, and it is done and dusted the same day.
You can read what is involved on our power flushing page, or ring us and describe what your radiators are doing. We will tell you straight whether it needs flushing or bleeding.
Still not sure what yours is doing?
Only if it is cold at the top. Bleeding releases trapped air, and air rises. Cold along the bottom is sludge, which sits at the bottom, and bleeding does nothing for it.
A day on a normal house, and it is done and dusted the same day. The system is refilled with an inhibitor afterwards to slow it happening again.
If the rest of the system is clean, yes. If several radiators are cold at the bottom the problem is the water, not the radiator, and a new one will sludge up like the last.
Over time. The boiler works harder to push water through a restricted system, which shortens the life of the pump and the heat exchanger and costs you money every month it is left.
