Why Are My Radiators Cold at the Top (or the Bottom)?

It is worth learning this one, because it saves a call-out about half the time. Where a radiator is cold tells you what is wrong with it, and the two most common patterns have entirely different causes and entirely different fixes.

Cold at the top: that is air

Hot water rises through a radiator, so if there is air trapped inside it collects at the top and pushes the hot water down. The radiator is warm at the bottom and cool or cold across the top few inches. The fix is bleeding it: turn the heating off, let it cool a little, open the bleed valve with a key until the hissing stops and water appears, close it, and top the system pressure back up if it has dropped.

Cold at the bottom: that is sludge

This one is not a radiator fault, it is a system symptom. Magnetite — iron oxide, the black sludge that forms as steel radiators corrode from the inside — is heavier than water. It settles in the bottom of the panel and blocks the flow across it, so the top of the radiator gets hot and the bottom stays stone cold. No amount of bleeding will touch it, because there is no air in there.

Why air keeps coming back

Bleeding a radiator once a season is normal. Bleeding the same one every fortnight is not, and it means air is getting into the system from somewhere: a slow leak drawing air in, a pump on the wrong side of the system, an automatic air vent that has failed, or corrosion actively generating hydrogen inside the radiators. That is worth investigating rather than living with.

What to do about sludge

A chemical flush, or a powerflush on a badly affected system, followed by an inhibitor and a magnetic filter to catch what forms afterwards. On a very heavily silted system, particularly one on microbore pipework, individual radiators sometimes have to come off and be flushed out separately. It is also the reason we clean a system before hanging a new boiler on it, because that sludge will otherwise find the new heat exchanger.

Sludge is telling you something about the whole system

A radiator cold at the bottom is not really a radiator problem — it is the visible end of a system that has been corroding quietly for years, probably because the inhibitor ran out or was never replaced after the system was last drained. Left alone it silts up the pipework as well, blocks the pump and eventually finds the boiler heat exchanger, which is by far the most expensive component it can reach. That is why we clean a system and fit a filter before hanging a new boiler on it rather than selling it as an upgrade afterwards.

When it is neither

If a whole radiator is cold, look at the valves first — a TRV head that has seized with the pin stuck down is extremely common, especially on valves that have not moved since the last cold spell. If several radiators at the far end of the house are cold while the ones nearest the boiler are roasting, the system needs balancing rather than cleaning. Our central heating and radiators page covers both.