One Radiator Not Heating Up? How to Find the Cause Before Winter
If one radiator stays cold while every other one in the house gets hot, the boiler is almost certainly fine — the heat is being made, it just is not reaching that one panel. In nearly every case we attend it comes down to one of three things: trapped air at the top, a thermostatic valve whose pin has seized shut, or sludge sitting in the bottom of the radiator.
The quickest way to tell them apart costs nothing. Put the heating on, wait twenty minutes, and feel the cold radiator all over with the back of your hand. Where the cold sits tells you which of the three you have, and two of them you can often fix yourself. Here is how our engineers work through it — and why late summer is the time to sort it, rather than on the first freezing morning in November.
Why is my radiator not heating up when the others are?
A heating system is a loop. The pump pushes hot water out of the boiler, round every radiator in turn, and back again. Each radiator takes a share of that flow through the valve at one end and gives it back through the valve at the other.
That means a single cold radiator is a local problem — something in or immediately around that one panel is stopping water moving through it. If every radiator is cold, or the ones upstairs are hot and the ones downstairs are not, you are looking at something central instead: the pump, a motorised valve, or low pressure.
So before anything else, walk the house and count. One cold radiator is a twenty-minute job. Half the house cold is a different conversation, and worth ringing us on 01304 381999 about rather than guessing.
Feel the radiator before you touch anything
With the heating running properly, there are three patterns worth knowing:
- Hot at the bottom, cold across the top — trapped air. Air rises to the highest point in the panel and sits there, pushing the water out of the way.
- Hot at the top, cold along the bottom edge — sludge. Magnetite settles under gravity and silts up the bottom of the panel where the water is slowest.
- Stone cold all over, top and bottom — no flow at all. Usually a valve: either a thermostatic valve with a seized pin, or a lockshield that has been wound shut.
That one check saves you bleeding a radiator with no air in it — the most common wasted half hour we see.
Cold at the top: trapped air
This is the easy one. Turn the heating off and let the system cool for half an hour — bleeding a radiator while the pump is running just draws more air in. Then put a cloth and a small container under the bleed valve at the top corner, open it a quarter turn with a radiator key, and let the air hiss out until water dribbles steadily. Close it before it starts running properly.
The bit people miss is what happens afterwards. On a sealed system — which is nearly every combi and system boiler installed in the last twenty-five years — letting air out drops the pressure. Check the gauge on the boiler once you have finished. If it has fallen below about 1 bar, top it back up to between 1 and 1.5 bar using the filling loop, following the instructions in the boiler’s own manual.
If you are bleeding the same radiator every few weeks, stop topping it up and give us a ring. Air that keeps coming back is being made, not left over from filling — usually by corrosion inside the system, occasionally by a small leak drawing air in. Both need finding rather than repeating.
Radiator not heating up after bleeding? Check the valve pin
If nothing but water came out and the radiator is still cold, the problem is at the valve end. On the thermostatic radiator valve — the one with the numbered dial — there is a small metal pin under the head that opens and closes the flow. After a summer sitting in the closed position, that pin can stick, and the radiator stays shut no matter what the dial says.
You can check it in a couple of minutes. Turn the dial to its highest setting, then unscrew the plastic head — most undo by hand. Underneath, the pin stands proud of the valve body. Press it down gently with your thumb; it should spring straight back. If it is solid, a careful tap on the side with a screwdriver handle, then a gentle pull with pliers, will usually free it. Refit the head once it moves freely.
While you are down there, look at the valve at the other end of the radiator — the one with a plastic cap rather than a dial. That is the lockshield, and it sets how much of the system’s flow this radiator is allowed. If someone has closed it, or if the system was never balanced properly after a boiler swap, the radiator furthest from the pump can end up starved while the nearest one runs scalding. Balancing is a methodical job with a thermometer on every pipe, but the result is immediate.
Radiator cold at the bottom: sludge
If the top half is hot and the bottom edge stays cold, the radiator has silted up. What is in there is magnetite — black iron oxide, formed as the steel inside the radiators slowly corrodes. It is heavier than water, so it drops out wherever the flow slows down, and the bottom of a radiator is the slowest place in the whole loop.
We see a lot of it across East Kent, and hard water is part of the reason: scale and sludge together give the system more to deposit than a soft-water area would. Older properties that have had radiators changed piecemeal over the years are worst affected, because fresh oxygenated water has been let in several times over.
A single sludged radiator can sometimes be taken off, carried outside and flushed through with a hose. If several are cold at the bottom, or the water that comes out looks like ink, the whole system needs doing properly — that is a power flush, and it is the only one of these three faults that genuinely needs us out with equipment.
Sludge is also what quietly kills boilers: it wears pumps, blocks the narrow waterways in a heat exchanger and drives up your gas use for the same warmth. Left long enough, a dirty system takes the boiler with it.
When it needs an engineer
Bleeding and freeing a valve pin are fair game for anyone. Anything involving the boiler itself, the gas supply, or draining and refilling the system should be done by a Gas Safe registered engineer — we are Gas Safe registered, number 2836.
Repairs like this are charged by time rather than at a fixed price: £80+VAT for the first hour, then £40+VAT per half hour after that. The first hour is always billable, and in most single-radiator jobs it is all that is needed. If it turns out to be the pump or a motorised valve instead, that falls under boiler repairs, and if pipework is involved it is a plumbing job — either way we will tell you what we have found before we carry on.
Sorting it now rather than in December
Every year the same thing happens: people turn the heating on for the first time in October, find half the radiators cold, and everyone rings in the same fortnight. A radiator that was fine in March will not necessarily be fine in November — a summer of sitting still is exactly what seizes a valve pin and lets sludge settle.
Run the heating for twenty minutes on a mild day, walk round and feel every radiator, and deal with what you find while there is no urgency. An annual boiler service is the other half of that: it is when we check system pressure, inhibitor levels and whether a magnetic filter is doing its job, all of which decide how much sludge you face next year.
Members on a BOYLA HomeCare plan get their annual service included and priority booking through the busy months, plus emergency response within 24 hours Monday to Friday.
Cold radiator you cannot shift, or half the house not heating up? Ring us on 01304 381999 — a real person answers — and we will get an engineer to you at the earliest working-hours slot we have.