Radiator Valve Leaking? How to Find the Leak and Fix It

Updated July 2026 · Emergency Plumber Chelsea

Start by drying the whole valve and the pipe below it with kitchen roll, then wait and watch. Water runs downhill and gathers at the lowest point, so a drip hanging off the bottom of the pipe nut may actually be coming from higher up. Wrap a dry tissue around each joint in turn: the one that goes damp first is the leak. Heating water is often dark grey or black because of sludge and inhibitor, so look for dark staining on the carpet or boards, and check whether the leak is worse when the heating is on and the system is hot and under pressure.

A leak from under the head or cap is a spindle gland leak. On a thermostatic radiator valve (TRV) the head unscrews or unclips to reveal a small pin; on a lockshield valve at the other end of the radiator the plastic cap pulls off. Around the base of the pin or spindle there is a small gland nut packed with an O-ring or packing. As these age, water weeps up the spindle. Holding the valve body still with one spanner, turn the gland nut a quarter turn clockwise with a second, small spanner. That is often enough. Do not keep tightening, because over-tightening can seize the spindle so the valve no longer opens or closes. Our guide to a thermostatic radiator valve not working covers heads and pins that stick.

A leak where the valve meets the radiator is a union leak. The large nut between the valve and the radiator tail pulls the two halves together, and the seal is made on the tail threads and the cone inside the union. Nipping the union nut up slightly, while holding the valve body firmly so you do not twist the pipe, sometimes stops a weep. If it does not, the tail needs resealing, which means isolating the radiator, draining it, undoing the union and remaking the joint with fresh PTFE or jointing compound. Radiators in period Chelsea flats are often old cast iron or early steel panels, and tails that have been in place for decades can be corroded enough that they need replacing too.

A leak where the valve meets the pipe is a compression joint leak. The smaller nut at the bottom of the valve squeezes a brass or copper olive onto the pipe. When a radiator is knocked, moved for decorating or the floor has settled, that joint can start to weep. A quarter turn of the nut, again holding the valve body still, usually cures it. If the olive is damaged, or the pipe is plastic without an insert, it needs remaking properly. Our guide on how to fix a leaking pipe joint explains the stopgap and the proper repair.

If water is coming from the valve body itself, through a hairline crack or a corroded patch, there is nothing to tighten. Old brass valves can crack if they were overtightened when fitted, if they have been knocked, or after a frost in an unheated room. A cracked body means the valve has to be replaced. The same applies to a valve that keeps leaking from the gland after tightening, or whose spindle has seized and started weeping as well.

To stop the leak getting worse while you wait for a repair, isolate the radiator. Turn the TRV to its lowest setting, or off if it has one; some TRVs still open slightly on the frost setting, so a manual cap is better if you have one. At the other end, remove the lockshield cap and turn the spindle clockwise with a spanner or adjustable grips until it stops, counting the turns so it can be set back to the same position later. With both valves closed, the radiator is cut off from the system, although the valve you are working on may still be under pressure from the pipe side.

On a sealed system with a combi or system boiler, a leak anywhere on the heating circuit lets pressure drop. Check the gauge on the boiler. If it has fallen below about 1 bar, the boiler may lock out. You can top it up with the filling loop once the leak is contained, but if it keeps dropping, the leak has not been stopped. Our guide to a boiler losing pressure explains how to top up and when constant loss points to a leak elsewhere. On an older open-vented system fed from a header tank in the loft, a leak will not show on a gauge but will keep drawing fresh water in, which brings in oxygen and speeds up corrosion.

Replacing a radiator valve is a proper plumbing job. The system, or at least that section of it, has to be drained down or the pipe frozen with a freeze kit, the old valve removed, the tail changed if needed, and the new valve fitted and tested. The system then has to be refilled, bled and topped up with inhibitor. If you are thinking of draining the radiator yourself, our guide on how to drain a radiator walks through it, but in a flat with wooden floors and neighbours below, a mistake is expensive. Leak sealer added to the system is not a good fix for a valve gland or union: it is designed for tiny weeps elsewhere in the system, and our guide to central heating leak sealer explains its limits.

Do not ignore a slow drip. Water from a valve at floor level soaks straight into floorboards and, in a mansion block or a converted house, into the ceiling of the flat below. Heating water is also dirty, so it stains carpets and plaster. If you can see dark marks spreading from the radiator, or a damp patch appearing below, treat it as urgent. Our guide to why a radiator is leaking covers leaks from the radiator itself, such as pinholes and bleed valves. A drip from the small brass drain-off valve near the bottom of the pipework is a different fault, covered in our guide to the drain-off valve.

If the valve keeps weeping after a gentle tighten, the leak is at the union or the valve body, or you would rather not put a spanner on it, call Emergency Plumber Chelsea on 020 3002 6667. Our central heating repairs team replaces TRVs, lockshield valves and radiator tails across SW3, SW10, Knightsbridge, Sloane Square, South Kensington and Fulham, and there is no call-out fee on jobs over 3 hours.

Quick tips

• Dry the valve completely, then wrap tissue around each joint to see which one gets damp first.
• A leak under the TRV head or lockshield cap is usually the spindle gland: try one quarter turn, no more.
• Hold the valve body with one spanner while you turn a nut with another, so you do not twist the pipe.
• Isolate the radiator by closing the TRV and the lockshield, and count the lockshield turns.
• Watch the boiler pressure gauge; if it keeps dropping, the leak has not been stopped.

FAQs