Why is my radiator cold at the bottom?
Cold at the top and cold at the bottom are opposite problems with opposite fixes. Bleeding the wrong one wastes an afternoon.
By Dan, Gas Safe registered engineer5 min read
This is the one people most often get wrong, and it costs them. Cold at the top and cold at the bottom look like the same complaint but they are opposite faults with opposite fixes. Bleeding a radiator that is cold at the bottom does nothing at all except empty a bit of water onto your carpet.
Cold at the top means air
Air rises, so it collects at the highest point in the radiator. The bottom stays warm because water is still circulating through it, and the top goes cold because there is air sitting where water should be. This is the easy one, and you can usually fix it yourself with a radiator key and an old towel.
If you are bleeding the same radiator every few weeks, that is a different problem. Air that keeps coming back is usually being drawn in through a small leak, or is a sign of corrosion inside the system producing gas. Worth getting looked at rather than bleeding it forever.
Cold at the bottom means sludge
This is the expensive one. Over years, corrosion inside the system produces a black iron oxide sludge. It is heavier than water so it settles in the bottom of the radiators, where it sits as a solid layer that water cannot flow through. The top stays warm because water still passes across it, and the bottom goes stone cold.
Bleeding will not touch it. There is no air in there, just muck.
The quick test
With the heating on for twenty minutes, run your hand from the top of the radiator down to the bottom. Warm going to cold means sludge. Cold going to warm means air. That one movement tells you which job you are looking at.
What sludge actually costs you
- Rooms that never get properly warm, so you turn the thermostat up and pay for heat you do not feel.
- A boiler working harder against poor circulation, which shortens its life.
- Pump and valve failures, because the sludge is being pushed through them constantly.
- In bad cases a radiator that has corroded from the inside and starts to weep.
The fixes, cheapest first
Balancing the system
Sometimes what looks like sludge is an unbalanced system: radiators nearest the boiler take most of the flow and the far ones get whatever is left. Balancing costs nothing in parts and is the first thing worth checking.
Dropping and flushing one radiator
If it is one radiator and the rest are fine, that radiator can be taken off the wall and flushed out. Straightforward, and much cheaper than treating the whole system.
A powerflush
If several radiators are cold at the bottom, the system as a whole needs cleaning. A powerflush pushes water and chemicals round at pressure to shift the sludge. Worth doing when the system genuinely needs it, and I will tell you honestly when it does not.
Stopping it coming back
Two things prevent it. A corrosion inhibitor in the system, which needs topping up after any work that drains it, and a magnetic filter on the return pipe which catches the iron oxide before it settles. Neither is expensive, and both are far cheaper than flushing the system again in five years.
I sort cold radiators across Preston, Penwortham, Chorley, Leyland, Garstang and Kirkham. If you have one room that never warms up, ring me and describe it and I will tell you which of the above it is likely to be before I come out.
Need this doing?
I am Dan, and I cover Preston and the towns around it.