“The radiator’s not working” covers about five completely different faults, and the good news is that you can usually tell them apart without any tools at all. You just need to notice where the cold is. Put your hand flat on the radiator when the heating has been running for twenty minutes or so, and work across it — top, middle, bottom, both ends.
Here is what each pattern means.
Hot at the bottom, cold at the top — that is air
Air rises, so trapped air collects along the top of a radiator and pushes the hot water down. This is the most common complaint and the easiest fix: bleed it. Radiator key, cloth underneath, quarter turn until it hisses, close it as soon as water appears.
Afterwards, check your system pressure and top it up if it has dropped. And if the same radiator needs bleeding again a few weeks later, stop treating it as routine maintenance — air is getting into a sealed system from somewhere, and finding out where is a proper job. Common causes are a failing automatic air vent, a small leak drawing air in, or a pump pulling air through at speed.
Hot at the top, cold along the bottom — that is sludge
This is the one people bleed at repeatedly and never fix, because bleeding removes air and this is not air. It is magnetite: black iron oxide sludge, formed as the water inside the system slowly reacts with the steel in your radiators. It is heavy, so it settles along the bottom of each radiator, and the hot water simply flows over the top of it.
A radiator with an inch of sludge in the bottom has lost a good chunk of its surface area, so the room never quite gets warm and your system burns energy trying. Worse, the same sludge is circulating through the rest of the system, wearing pumps and clogging the narrow passages inside a heat exchanger.
The fix is a power flush — a proper clean of the whole system rather than one radiator — followed by a magnetic filter and inhibitor to stop it coming straight back. We covered this in detail in our post on power flushing, including how to tell whether your system genuinely needs one.
Cold at one end, warm at the other
Usually a valve problem. If the cold end is the valve end, the thermostatic valve may be stuck closed — a very common outcome after a summer spent in one position. Take the head off and check whether the pin underneath moves freely; if it is stuck down, gently working it up and down often frees it.
If the cold end is the far end, the lockshield valve at that side may be closed too far, which is a balancing question rather than a fault.
Cold all over, while other radiators are hot
If one radiator is stone cold and the rest of the house is fine, check the obvious first: both valves actually open, and the thermostatic head not sitting at its lowest setting. After that, the likely candidates are a completely seized valve or a radiator so blocked that almost nothing is getting through it.
There is one more possibility worth mentioning because it catches people out after building work: a radiator that was isolated for decorating and never turned back on. It happens more often than you would think.
The far radiators are cold and the near ones are roasting
This is not a fault in any single radiator — it is balancing. Water, like everything else, takes the easy route: the radiators nearest the pump get more flow than the ones at the end of the run, so the box room over the garage never warms up while the hallway is tropical.
Balancing is the process of deliberately restricting the easy circuits, using the lockshield valve on each radiator, so that flow is shared out properly across the house. It is fiddly, it is done with thermometers and patience rather than a single adjustment, and it is one of the most underrated jobs in heating — it costs nothing in parts and frequently fixes the “one cold room” that people have lived with for years.
Everything is lukewarm at once
When the whole house is tepid rather than one radiator, the problem is upstream of the radiators — flow or heat, not distribution. Check the system pressure gauge, and check the pump is actually running rather than humming. Beyond that it needs someone to look at it.
If the heat source itself is a gas appliance, that is a job for a Gas Safe registered engineer, which is a different registration from the plumbing and heating work we do — worth being clear about rather than leaving you to guess.
What we would do
Most of the faults above are an hour’s work and a small part. The ones worth acting on before winter are the sludge and the balancing, because both get blamed on “an old system” and both are fixable — we have taken plenty of houses in Cheltenham and Gloucester from “we just live with that cold room” to a house that heats evenly, without replacing anything major.
If your list from the walk-round is longer than you fancy, we cover radiator work, valve replacements and full central heating across the county. Call 07386 705571 or get a free quote.