Radiator Not Heating Up?
A radiator that will not heat up tells you a lot by where it is cold. Cold at the top usually means air, cold at the bottom usually means sludge, and a radiator that stays cold all over usually means a stuck valve or a system that is not balanced. Here is how to read the pattern, what you can fix yourself, and when the whole system needs attention.
Read the cold spot first
| What you feel | Likely cause | Fix |
|---|---|---|
| Cold at the top, warm at the bottom | Air trapped in the radiator | Bleed the radiator |
| Cold at the bottom, warm at the top | Sludge (magnetite) settled in the base | Flush the radiator or the system |
| Cold all over, others fine | Stuck thermostatic valve pin, or lockshield closed | Free the pin; check the lockshield |
| Far radiators or upstairs cold | System not balanced, or pump/valve fault | Balance the system; check pump and zone valves |
| Several cold, water bleeds out black | Sludge through the whole system | Powerflush, filter and inhibitor |
Cold at the top: bleed it
Air collects at the top of a radiator and stops hot water filling it. With the heating off and the radiators cool, put a radiator key on the bleed valve at the top corner, hold a cloth underneath and turn it a quarter turn anticlockwise. You will hear air hiss out; close it as soon as water appears. If your boiler is a combi or system boiler, check the pressure gauge afterwards, because bleeding lets water out and the pressure may need topping up. If the same radiator needs bleeding every few weeks, air is getting in somewhere, which on an open-vented system can mean the pump is pulling air in, and on a sealed system can point to a small leak.
Cold at the bottom: that's sludge
Steel radiators corrode slowly from the inside, and the black iron oxide that forms, magnetite, settles where the water moves slowest: along the bottom of the radiator. Bleeding will not shift it, because bleeding only lets out air from the top. A single radiator can be taken off, flushed through outside and refitted. If several radiators are cold at the bottom, or the water from the bleed valve is black, the whole system is sludged, and a powerflush followed by a magnetic filter and inhibitor is the lasting fix. Our guide to what a power flush is explains the process.
Cold all over: the valves
If one radiator stays stone cold while the rest are fine, look at its valves. A thermostatic radiator valve has a small pin under the head; after a summer off it can stick down, closing the valve. Take the head off (most unscrew or unclip), and if the pin is stuck down, a gentle tap or wiggle usually frees it. At the other end of the radiator is the lockshield valve, under a plain plastic cap; if someone has closed it, the radiator gets no flow. Opening it a little at a time sorts that, but the setting matters for the rest of the house, which is where balancing comes in.
Some rooms warm, others not: balancing
Water takes the easiest route, so the radiators nearest the boiler heat first and the far ones get what is left. Balancing means setting each lockshield valve so every radiator gets its share, measured by the temperature difference across flow and return rather than by feel. It also helps the bill, because a condensing boiler only condenses when the water coming back to it is cool enough. Our heating engineers balance systems as a routine job.
When the whole house is cold
If no radiator heats, the problem is upstream: the boiler, the pump, a zone valve, or the controls. Check the thermostat and programmer are calling for heat and the boiler pressure is normal. If the boiler is running and the hot water works but the heating does not, the fault is often a diverter or motorised valve; see no hot water but the heating works for the reverse case. If the boiler shows a fault code, our boiler repair page explains what the common codes mean.
Common questions
Why is my radiator cold at the bottom but hot at the top?
Almost always sludge: magnetite settled in the base of the radiator. Bleeding will not fix it. One radiator can be flushed out on its own; several cold at the bottom means the system needs cleaning, with a filter and inhibitor afterwards.
Why is my radiator cold at the top?
Air trapped at the top. Bleed it with the heating off, then check the boiler pressure, which may need topping up on a combi or system boiler. If it keeps coming back, air is getting into the system and the cause needs finding.
Why is one radiator cold when all the others are hot?
Usually a thermostatic valve pin stuck down, or the lockshield valve closed. Free the pin under the valve head, and check the lockshield is open. If the radiator is at the end of a long run, the system may need balancing.
Do I need a powerflush if my radiators are cold?
Only if the cause is sludge. Cold at the top is air, which bleeding fixes. Cold at the bottom on several radiators, black water when you bleed, or a kettling boiler point to sludge, which a powerflush clears.
Updated 15 October 2026
