One Radiator Not Heating Up
Usually a small fix, not a whole-system job.
One radiator not heating up while the rest of the house is warm is one of the most common heating complaints, and one of the cheapest to put right. It is almost always that radiator’s own valves or trapped air, not sludge across the whole system. Here is how to work out which, and what to check before you call anyone.
One radiator not heating up, when every other radiator is fine, is rarely a whole-system problem. The usual causes are trapped air at the top of that radiator, a thermostatic valve with a stuck pin, a lockshield valve closed too far, or a valve that has seized. All four are small jobs. A full power flush is almost never the right answer for a single cold radiator, and we would tell you so rather than quote for one.
Why one radiator stays cold when the others work
Your heating system is a loop. Hot water leaves the boiler, passes through each radiator in turn and returns. If the boiler is firing and every other radiator is hot, then the water is circulating perfectly well. The problem is confined to the one radiator that is not getting its share.
That is a useful piece of information, because it rules out most of the expensive explanations straight away. Sludge across the system, a failing pump, a stuck motorised valve or a boiler fault would all affect several radiators, not one. When just one is cold, the cause is nearly always at that radiator: its valves, or air trapped inside it.
Engineer noteIf someone quotes you for a power flush on the strength of one cold radiator, ask them what they think is wrong with the other radiators. A flush cleans the whole system, so it is only worth paying for when the whole system needs it.
The four usual causes
Trapped air
Air collects at the highest point inside a radiator, so the top stays cold while the bottom is warm. It is the easiest cause to identify and the easiest to fix. Bleeding lets the air out and the water in. If the same radiator needs bleeding repeatedly, something is drawing air into the system and that is worth investigating.
A stuck thermostatic valve pin
The valve with the numbered dial on it, usually at one end of the radiator, works by pressing a small pin down to close the flow. If the radiator has been turned off all summer, that pin can stick in the closed position. The radiator then stays cold no matter what the dial says. It is an extremely common fault the first time the heating goes on in autumn.
Take the head off the valve and look at the pin underneath. If it is stuck down rather than springing up when pressed, that is your answer. Gently working it free with a pair of pliers usually releases it, and a drop of lubricant helps.
A lockshield valve closed too far
At the other end of the radiator is a valve with a plastic cap rather than a dial. That is the lockshield, and it controls how much flow the radiator takes. If it has been closed off, or turned down too far during balancing, the radiator will heat slowly or not at all. Opening it up is a five-minute job, though doing it properly means checking it has not simply been robbed of flow by another radiator.
A seized or failed valve
Valves that have not been moved in years can seize entirely. Sometimes the head has failed rather than the valve body. Either way it is a part replacement, not a system clean.
How to check it yourself, in order
Run the heating for an hour
Then feel the radiator at the top, the middle and the bottom.
Check both valves are open
Turn the dial to maximum and take the plastic cap off the other end.
Bleed it
If the top is cold and the bottom warm, air is the likely cause.
Check the valve pin
Head off, look underneath. A stuck pin will not spring back when pressed.
Those four checks solve most single cold radiators without anyone needing to visit. If the radiator is still cold after all of them, or if you would rather not take valve heads off, that is when it is worth a call.
When it is genuinely the whole system
Occasionally a single cold radiator is the first sign of something wider. Here is how to tell the difference.
Almost certainly just that radiator
- Every other radiator gets properly hot.
- The water runs clear when you bleed it.
- It went cold suddenly, often after the summer.
- The boiler is quiet and the house warms up otherwise.
Worth checking the whole system
- Other radiators are cold along the bottom too.
- The water runs black or rusty when bled.
- More radiators have gone cold over the last few winters.
- The heating is slow to warm up or the boiler is noisy.
If you are in the right-hand column, the system may benefit from a clean, and you can read what that involves on our central heating cleaning page or try the do I need a power flush? checker. If you are in the left-hand column, you almost certainly do not need one.
What it should cost to put right
A single cold radiator is a small job. Bleeding costs nothing and you can do it yourself. Freeing a stuck valve pin or opening a lockshield is usually a short visit. Replacing a thermostatic valve head is a part and a few minutes; replacing the valve body means draining down that section of the system, so it takes longer.
What it should not cost is a full system clean. If you have been quoted for a power flush on the strength of one cold radiator and nothing else, it is worth getting a second opinion. See what actually affects the price of a flush, or read when you do not need a power flush.
Related guides
One radiator not heating up, answered
Why is one radiator not heating up when the rest are fine?
Do I need a power flush for one cold radiator?
Why is my radiator cold at the top but warm at the bottom?
How do I know if a radiator valve is stuck?
Why has one radiator gone cold since I turned the heating back on?
When is one cold radiator a sign of a bigger problem?
Still cold after those checks?
Tell us which radiator it is, what the top and bottom feel like and what the water looks like when you bleed it. We will tell you honestly whether it is a valve, air or something wider, with no pressure to flush if it is not the right fix.
