Start with the pattern
How the radiator is cold tells you a lot about why. Put the heating on for 20 to 30 minutes, then feel the radiator and both pipes going into it.
| What you find | Likely cause | Can you fix it? |
|---|---|---|
| Whole radiator and both pipes cold | Stuck TRV pin or closed valve | Often, yes |
| Cold at the top, warm at the bottom | Trapped air | Yes, by bleeding |
| Warm at the top, cold at the bottom | Sludge | Engineer or plumber |
| Lukewarm all over, far from the boiler | Poor balancing | Often, yes |
| One pipe hot, radiator cold | Valve closed or blocked on one side | Check valves, then plumber |
If most or all of your radiators are cold, the fault is more likely with the boiler or its controls. See our guide to heating not working when the hot water is.
A stuck TRV pin
This is the most common cause of a single cold radiator, especially after the summer. A TRV has a plastic head with numbers on it, sitting on a valve body. Under the head is a small metal pin. When the room is warm, the head presses the pin down and closes the valve. If the heating has been off for months, the pin can stick in the down position, so the valve stays shut however high you turn the head.
How to check and free a TRV pin
- Turn the TRV head to its highest setting.
- Unscrew the head by hand. Most unscrew anticlockwise at the collar, and some have a clip or release button. No water comes out, because the head sits outside the sealed valve.
- Look at the pin in the centre of the valve. A working pin sticks up a few millimetres and springs back when you press it gently.
- If it is stuck down, press and release it a few times with your thumb, or tap it lightly with the handle of a screwdriver. Do not pull it with pliers, as a damaged pin means a new valve.
- Once it moves freely, refit the head on its highest setting and let the heating run.
At the end of the heating season, leave all TRVs turned to their highest setting. It keeps the pins open and makes sticking less likely.
A closed valve
A radiator has a valve at each end. One is the TRV or manual control valve. The other is the lockshield valve, usually hidden under a plain plastic cap. Lockshield valves are sometimes closed when a room is decorated or a radiator is taken off the wall, and never reopened.
Pull or unscrew the cap and look at the spindle. If it has been turned fully clockwise, the valve is shut. You can open it anticlockwise with a lockshield key or small adjustable spanner. Opening it a turn or two is usually enough to see whether heat comes through. If water weeps from the spindle as you turn it, stop and call a plumber. Any drip that continues, or a damp patch that appears under a radiator or near the boiler, is covered in our guide to a boiler or heating system leaking water.
Also check the valve on the other end. A manual wheel-head valve (one without numbers) may simply have been turned off. Turn it anticlockwise to open it. If the valve has a TRV head, turn it to its highest setting and check the pin as described above.
Newly fitted or moved radiators
If the radiator has recently been replaced or moved, the valves may have been left partly closed, or air may still be trapped from the refill. It is also possible the flow and return were connected the wrong way round, which some TRVs do not tolerate well, making them noisy or slow to open. A quick call back to the installer is usually the easiest route.
Trapped air
If the radiator is warm at the bottom and cold at the top, air is sitting in the panel. With the heating off and the radiator cool, bleed it using a radiator key until water appears, then check the boiler pressure. Our guide to a radiator cold at the top walks through it step by step. On a combi or system boiler, top up to 1 to 1.5 bar afterwards using our repressurising guide.
Balancing the system
If the radiator heats eventually but stays lukewarm while those near the boiler are hot, the system may be out of balance. Water takes the easiest route, so radiators close to the boiler take most of the flow.
Balancing means partly closing the lockshield valves on the radiators nearest the boiler so more water reaches the ones further away. Many homeowners do this themselves with a lockshield key and a clip-on radiator thermometer, aiming for a drop of roughly 10 to 12 degrees between the flow and return pipes on each radiator. It is slow, fiddly work and can take a few rounds. A heating engineer can do it in an hour or two if you would rather not.
A quick balancing method
- Turn the heating off and let it cool. Open every TRV and lockshield valve fully.
- Turn the heating on and note the order in which the radiators warm up.
- Turn the first radiator's lockshield almost closed, then open it about a quarter turn.
- Work along the list, opening each lockshield a little more than the last, so the furthest radiators are the most open.
- Check again after an hour and fine tune.
When to call a plumber or engineer
Call a plumber or heating engineer if the TRV pin will not free, a valve will not turn or weeps water, or the radiator is warm at the top but cold at the bottom. Typical UK costs for common fixes are below. Prices are higher in London and for designer valves or awkward pipework.
| Job | Typical UK cost |
|---|---|
| Replace one TRV or radiator valve | Typically £60 to £150 |
| Balance the whole system | Typically £60 to £150 |
| Remove and flush one radiator | Typically £80 to £150 |
Replacing a valve often means draining some or all of the system, which is why a single valve can cost more than the part suggests. Asking for several valves at once is usually better value per valve, and many engineers will look at a sticking TRV during an annual service if you mention it when you book. Our guide to boiler service costs explains what a standard service covers.