A radiator leaking water is one of the most common heating faults, and most of the time the leak is at a valve, which makes it one of the most fixable. Many valve leaks stop with a careful quarter turn of the right nut. Others need the radiator draining so a seal can be remade. And occasionally, a small drip at a valve is a distraction from a larger leak elsewhere in the system.
The key is working out exactly where the water is coming from, because a radiator valve has several joints close together and each one is fixed differently.
Where exactly is the radiator valve leaking?
With the heating on, wipe the valve and pipework completely dry. Then hold a dry tissue against each joint in turn, working from the top down, and see which one comes away damp. Water runs downwards, so the lowest wet point is often not the source.
| Where it is wet | What is leaking | Usual fix |
|---|---|---|
| Around the spindle, under the cap or head | Gland seal | Tighten the gland nut slightly, or repack or replace the gland |
| At the large nut between the valve and the radiator | Union nut on the radiator tail | Tighten, or drain and remake the joint |
| Where the tail screws into the radiator | Tail thread seal | Drain the radiator and reseal the thread |
| At the nut where the pipe enters the bottom of the valve | Compression joint and olive | Tighten, or drain the system and replace the olive |
| At the top corner of the radiator | Bleed valve | Close it gently, or replace the bleed screw or plug |
| From the valve body itself | Cracked or corroded valve | Replace the valve |
Why is the radiator valve leaking from the spindle?
The spindle is the part that turns when you open or close the valve. Where it enters the valve body there is a seal, called the gland, held in place by a small gland nut. As valves age, or after being turned many times, the seal wears and a slow weep appears, usually leaving a crust under the cap.
On a manual valve or a lockshield valve, the gland nut is the small nut just below the cap or head. A slight clockwise turn, often no more than a quarter turn, can compress the seal enough to stop the weep. Plumbers sometimes repack a worn gland with PTFE tape instead, but that means undoing the gland nut on a valve connected to a pressurised system, which is best left to them.
On a thermostatic radiator valve, the head unscrews to reveal a small pin. A leak around that pin usually means the internal seal has failed. Some valves have replaceable parts, but the practical fix is often a new valve body.
What if the leak is at the union nut or the tail?
The union nut is the larger nut that joins the valve to the radiator tail, the short threaded fitting screwed into the radiator. A leak at the union nut can often be cured by tightening it slightly, while holding the valve body steady with a second spanner so the pipe underneath is not twisted.
If the water is coming from where the tail screws into the radiator, tightening the union will not help. That joint relies on a thread sealant such as PTFE tape or jointing compound, and remaking it means isolating and draining the radiator, removing the valve, unscrewing the tail and resealing it.
What about a leak where the pipe meets the valve?
Below the valve, the pipe usually enters through a compression joint: a nut that squeezes a brass or copper ring, called an olive, onto the pipe. A weep here can sometimes be stopped with a small turn of the nut, again holding the valve body with a second spanner.
If that does not work, the olive may be damaged or the pipe end may be poor. Replacing the olive is a bigger job than it looks, because this joint is on the system side of the valve. Closing the radiator valves does not isolate it, so the heating system normally has to be drained down, at least partly, before it can be remade.
Why is the radiator bleed valve leaking?
A bleed valve that drips after bleeding is often just not closed fully. Close it gently with the radiator key, without forcing it, because the small screw is easily damaged. If it still weeps when closed, the bleed screw or its seal may be worn, and the screw or the whole bleed plug may need replacing.
Changing a bleed plug means isolating the radiator by closing both valves and letting the pressure off first. On a sealed system, check the boiler pressure gauge afterwards, because bleeding lowers the pressure and a top-up may be needed.
What can a homeowner safely tighten?
- Turn the heating off and let the radiator cool, so the pipes and water are not hot.
- Put a towel and a shallow tray under the valve.
- Use two spanners that fit properly: one to hold the valve body still, one to turn the nut. Adjustable spanners can round off soft brass nuts.
- Turn by a small amount, no more than a quarter turn, then dry the joint and check it.
- Stop if it gets worse, if the nut feels as though it is slipping, or if the pipe starts to twist. Overtightening can split an olive or crack an old valve.
- Turn the heating back on and check the joint again once it is hot, because some joints only weep when warm.
Anything beyond a small adjustment, such as undoing a union, removing a valve or working on the boiler itself, is a job for a plumber or heating engineer. Work inside the boiler casing needs a Gas Safe registered engineer.
When does the radiator or system need draining?
A single radiator can usually be isolated by closing both its valves. If you do this, count the turns it takes to close the lockshield valve so it can be reopened to the same position, which keeps the system balanced. The radiator can then be drained into a tray while the joint is remade.
A wider drain down is needed when:
- the leak is on the pipe side of a valve, such as the olive;
- the valves do not close fully, which is common on older valves;
- the valve itself needs replacing and cannot be isolated.
After any drain down, a sealed system needs refilling to the pressure in the boiler manual, bleeding, and dosing with inhibitor to protect against corrosion.
What if the radiator itself is leaking?
Sometimes every joint is dry and the water is coming from the radiator panel. It usually shows as a damp spot or a rust-coloured crust low down on the panel or along the bottom seam, sometimes with a fine spray when the heating is on. This is corrosion working from the inside. Oxygen in the system water and the black sludge it produces eat the steel from within, and by the time a pinhole shows, the metal around it is thin too.
A radiator body leak cannot be tightened or repacked. Turn the thermostatic valve to off, close the lockshield valve at the other end while counting the turns so it can be reset to the same balance later, and put a tray and towel underneath. The radiator then needs replacing. Because a corroding radiator means corrosive water, it is worth having the system water tested and cleaned at the same time, or the next radiator will follow.
When does a slow drip mean a bigger leak?
A single weeping valve loses very little water. If the boiler pressure is dropping faster than the drip could explain, look further. Warning signs include:
- pressure that keeps falling after the valve has been fixed;
- damp, stained or warm patches in floors or ceilings away from the radiator;
- several valves or radiators showing corrosion at once;
- radiators that need bleeding much more often than before.
These point to a leak on hidden pipework or a wider corrosion problem. Our guide to the signs of a central heating leak covers where else to look, and our guide to a boiler that keeps losing pressure explains how to read the gauge.
Chemical leak sealers are sold for heating systems, but they are designed for very small weeps, not visible drips, and not every boiler manufacturer approves them. Our guide to central heating leak sealer covers when one is worth trying.
When should you call a leak detection specialist?
A visibly leaking valve is a plumbing repair, not a detection job. Call a leak detection specialist when the valve has been fixed but the pressure keeps dropping, when water is appearing somewhere away from the radiator, or when there are signs of a leak under a floor or inside a wall. Our central heating leak detection uses thermal imaging, pressure testing and other methods to find hidden heating leaks. Thermal imaging surveys are priced at £150 to £250. Call 020 3912 0200 to discuss it.
Frequently asked questions
Why is my radiator valve leaking?
Most often because a gland seal around the spindle has worn, a union nut or compression nut has loosened, or the seal on the radiator tail has failed. Old valves can also corrode or crack.
Can I tighten a leaking radiator valve myself?
You can try a small tightening of the gland nut, union nut or compression nut, with the heating off and cool, using two spanners and no more than a quarter turn at a time. If that does not stop it, call a plumber.
Do I need to drain the system to fix a leaking radiator valve?
Not always. Many gland and union leaks can be fixed without draining. Resealing a tail usually means draining that radiator, and replacing an olive or a valve that will not isolate usually means draining the system.
Is a leaking radiator valve an emergency?
A slow weep is not usually an emergency, but it should be fixed soon. A steady drip or a spray, water near electrics, or a boiler that keeps losing pressure needs attention straight away.
Why does my radiator valve only leak when the heating is on?
Metal expands as it warms, which can open up a small gap in a worn seal or joint. Check the valve both hot and cold when looking for the source.
