Radiator cold after taking it off to decorate? Here's how to get it working
Check for leaks first. If water is dripping from either valve joint, put a towel and bowl underneath and close both valves on that radiator (step 2 shows how). A slow drip can do a lot of damage to floors and ceilings. Never open up the boiler.
You took the radiator off the wall to paint or paper behind it, hung it back up, and now it stays cold, or only warms up a little. The other radiators are fine. This happens a lot, and it's usually one of a few simple things. Taking a radiator off means closing its valves, draining it and letting air in, so all three need putting right afterwards.
- Check both joints for leaks.
- Make sure both valves are open again.
- Bleed the air out of the radiator.
- Check the boiler pressure and top it up.
- Check the valve pin isn't stuck.
- Push the hot water through it.
- Call a heating engineer if it's still cold.
Each step has important details below. Tap a step to jump to it.
Why it happens
When a radiator comes off the wall, three things happen:
- Both valves get closed to stop water coming out. If either one isn't opened fully afterwards, hot water can't flow through.
- The radiator is drained, so when it's reopened it fills with water and a lot of air. Air collects at the top and stops it heating.
- Some water is lost from the system, so the pressure can drop. On a combi or system boiler, the boiler may then stop working altogether.
Less often, tipping the radiator up to drain it loosens sludge (black, muddy deposits) inside, which can then block it.
Step by step
Turn the heating off and let the radiators cool before you start, unless a step says otherwise.
1Check for leaks
Run a dry tissue round the nut on each valve where it joins the radiator. If it comes away damp, the joint isn't sealed. Sometimes tightening the nut by about a quarter turn with a spanner fixes it. Hold the valve body still with a second spanner so you don't twist the pipe. If it still drips, close both valves and call a plumber.
2Make sure both valves are open
Most radiators have two valves, one at each end:
- The thermostatic valve (TRV) has numbers on it, usually 0 or ✱ up to 5 or 6. Turn it to the highest number for now.
- The lockshield valve on the other end has a plain plastic cap. Pull or unscrew the cap off. Underneath is a small spindle you turn with a spanner or a lockshield key (sold cheaply at DIY shops).
Whoever took the radiator off probably closed the lockshield by turning it clockwise. Ideally they counted the turns. If you know how many turns it was, open it anticlockwise by the same number. If you don't know, open it about one and a half to two turns. That's enough for most homes, and an engineer can fine-tune it later if the room is slow to warm up.
Some radiators have an older manual valve with a round handle instead of a TRV. Turn it fully anticlockwise.
3Bleed the air out
A refitted radiator is often half full of air. Use a radiator key on the bleed valve, which is the small square nut at the top corner. Hold a cloth under it and turn it slowly anticlockwise, about a quarter turn. Air will hiss out. Close it as soon as water starts to appear.
You may need to do this again once the heating has run for a while, as more air works its way up.
4Check the boiler pressure
If you have a combi or system boiler, look at its pressure gauge. With the heating cold, it should usually read about 1 to 1.5 bar. Draining a radiator and bleeding it can drop it below 1 bar, and many boilers then switch off and show a fault.
If it's low, top it up with the filling loop and reset the boiler. Our guide Boiler stopped working after bleeding the radiators shows how to do this safely.
If you have a tank in the loft (an older "open vented" system), there's no gauge to check. The system tops itself up.
5Check the valve pin isn't stuck
If the pipes going into the radiator get hot but the radiator stays cold, the thermostatic valve may be stuck shut. This often happens when it's been knocked or left turned off.
- Turn the TRV to its highest number. Then unscrew the head, the part with the numbers. Most unscrew by hand, but some have a small locking ring or screw.
- Underneath, you'll see a small metal pin sticking up. Press it gently with your thumb or the back of a spoon. It should spring back up.
- If it's stuck down, gently wiggle it up and down with pliers until it moves freely. Don't pull it out.
- Screw the head back on.
6Push the hot water through it
If the radiator is still slow, give it more of the flow for a while:
- Turn the TRVs on your other radiators down low, or to the frost setting.
- Put the heating on, with the problem radiator fully open.
- After 20–30 minutes, bleed it again, then turn the other radiators back up.
7If it's still cold, call a heating engineer
If you've done all of the above and it's still cold (especially cold at the bottom and warm at the top), it may be blocked with sludge, or a valve may be faulty or have been damaged when it was closed. A heating engineer can flush the radiator or change the valve, usually in one visit. Tell them you took it off to decorate, which valves you opened and by how many turns, and what happened when you bled it.
Questions people ask
Does it matter which way round I put the radiator back on?
Usually not. It goes back on the same brackets and joins to the same valves. Just make sure the bleed valve is at the top. Some older TRVs only work properly when water flows one way, but that rarely matters if the valves weren't moved.
Can I take a radiator off without draining the whole system?
Yes. Closing both valves isolates it, so only the water in that radiator comes out. Count the lockshield turns as you close it so you can put it back the same. Have plenty of towels and a shallow tray ready.
Other radiators have gone cold too since I did this. Why?
That points to the whole system rather than this one radiator: usually low pressure on a combi or system boiler, or air that has moved round. Check the pressure (step 4) and bleed every radiator, downstairs first and then upstairs.