Why is my radiator cold after putting it back on the wall?
Four things stop a radiator heating after decorating, and they happen in a predictable order. Working through them takes ten minutes and needs no tools beyond a bleed key.
- Difficulty
- beginner
- Time
- 25 min
- Read
- 5 min
- Safety
- caution
Short answer
Almost always one of: a valve left closed, the lockshield not reset to where it was, air trapped in the panel, or the system pressure dropped when it was drained. Open both valves fully, bleed the radiator, check the boiler pressure, and reset the lockshield to the number of turns you counted on removal.
Do this first
Taking a radiator off to decorate is one of the more satisfying small jobs, right up to the point where it goes back on and stays stubbornly cold. The good news is that the list of causes is short, they are all reversible, and none of them require anything to be taken apart again. Work through them in order and it is nearly always solved within ten minutes.
Safety
What you'll need
- A radiator bleed key
- An adjustable spanner or lockshield key
- Old towels
Step by step
- Open the control valve fully.The TRV or wheelhead. On a TRV, turn it to its maximum, not to a number. If a decorator's cap or a TRV head was removed and refitted, check the head is properly seated and screwed down — a head sitting loose on its body does not push the pin.
- Reopen the lockshield.This is the usual culprit. It was closed to isolate the radiator, and if it has not been reopened by exactly the number of turns it was closed by, the radiator is throttled or shut. If you counted the turns, reset to that number. If you did not, open it fully, get the radiator hot, then close it gradually until the radiator's return pipe is comfortably warm rather than hot.
- Check the TRV pin is not stuck down.With the head off, the pin in the top of the valve body should stand proud and spring back if pressed. A pin that has stuck down after being left compressed for days keeps the valve shut. Free it by working it gently with pliers, or a light tap and a squirt of penetrating oil.
- Bleed the radiator.Almost inevitable — the panel was full of air when it went back on. Open the bleed valve with a key until air stops hissing and water appears, then close it. Have a towel underneath: the first water out is usually black.
- Check the boiler pressure.On a sealed system, draining a radiator drops the system pressure, and a boiler below its minimum pressure will not fire at all. Look at the gauge and top up through the filling loop into the cold band if needed. This catches out almost everyone the first time.
- Run the heating and check again after twenty minutes.A radiator that was drained takes a while to warm through, particularly a big one. Judging it after five minutes is premature.
- Feel the pipes at both valves.Flow pipe hot and return cold means water is not passing through — a closed valve or a blockage. Both cold means nothing is reaching it at all, which points at a valve further back or at the system pressure. Both hot with a cold panel means air.
- Check every joint for weeping.Dry the valve tails, the bleed valve and both unions, and check them with tissue after the system is hot, then again the next day. Disturbed olives and tails are exactly where a slow leak starts, and a slow leak on a sealed system shows up as pressure dropping over weeks.
Tips
- Next time, count the lockshield turns as you close it and write the number on a piece of tape stuck to the valve. It turns the refit from guesswork into a two-minute job.
- Refit with the radiator slightly loose on its brackets, get both unions started by hand, then tighten. Cross-threading a tail is the one mistake that turns this into a real job.
- New fibre or rubber washers on the valve tails cost pennies and are worth using rather than reusing the old ones.
- Do the whole check twenty-four hours later as well. The leak that is going to appear appears then, not on the day.
If it doesn't work
Radiator completely cold, pipes cold too
Cause: Both valves closed, or the boiler is not firing because the pressure is too low — Fix: Open both valves and check the pressure gauge. A sealed system below its minimum will not run at all, which people misread as a radiator fault.
Cold at the top, warm at the bottom
Cause: Air trapped in the panel — Fix: Bleed it. If it fills with air again within days, the system is drawing air in somewhere and that is a separate problem worth chasing.
Warm only at the very top of the panel
Cause: Barely any flow — the lockshield is almost closed — Fix: Open the lockshield further, a quarter turn at a time, allowing a few minutes between adjustments.
This radiator is fine but others have gone cold
Cause: The lockshield has been opened much further than it was, taking flow from the rest of the system — Fix: Close it back gradually until the balance is restored. If several radiators are now wrong, the system needs balancing properly.
TRV head fitted but radiator stays hot
Cause: The pin is stuck up, or the head is not screwed fully down onto the body — Fix: Take the head off, check the pin moves freely, and refit the head hand-tight against its stop.
Boiler pressure keeps dropping since the job
Cause: A disturbed joint is weeping slowly — Fix: Dry and tissue-test every joint on that radiator, including the bleed valve and the blanking plug. A slow weep can take a week to show as a stain.
Questions people ask
Do I need to bleed the whole system or just that radiator?
Usually just that one. If several radiators are now cold at the top, air has moved around the system and a full bleed downstairs-first is worth doing.
Should I top up the pressure before or after bleeding?
Bleed first, then top up, because bleeding releases pressure. Topping up first and then bleeding just means doing it twice.
Why is the water black?
That is magnetite — normal in an older system but a signal worth acting on. Black bleed water that a magnet attracts means the system is corroding and would benefit from cleaning and inhibitor.