Leaking Radiator? Finding the Source and What to Do Next

A leaking radiator is almost always leaking from one of five places: the valve spindle, the joint between the valve and the radiator, the bleed screw, a pipe joint below it, or a pinhole in the radiator itself. The first three are common and usually repairable. A pinhole in the panel means the radiator needs replacing, and it tells you something about the water in the rest of your system. The job is to find where the water starts, because on a radiator it rarely drips from the same place it escapes.

First, Contain It

Put a towel and a shallow tray under the drip and turn the heating off. If the leak is steady, you can isolate the radiator from the rest of the system by closing the valves at both ends:

• The control end. Turn a thermostatic radiator valve (TRV) to its off or 0 setting, or turn a manual valve fully clockwise.
• The lockshield end. Pull off the plastic cap and turn the spindle clockwise with a spanner or lockshield key, counting the turns so it can be set back to the same position later. Our guide to what a lockshield valve does explains why that count matters.

Isolating stops fresh water feeding the leak, but the radiator is still full. If the leak is at the bottom or at a valve, expect it to keep weeping until the radiator is drained.

Then check the pressure gauge on the boiler. On a sealed system, every drop that escapes lowers the pressure, and once it falls far enough the boiler will lock out. That is often how people discover a slow leak in the first place.

Find Where the Water Actually Starts

Water runs along pipes and down the face of a radiator before it drips, so the wet patch on the floor is a poor guide. Dry every surface thoroughly: the valves, the nuts on each side, the bleed screw and the pipes down to the floor. Then wrap a piece of dry tissue around each part in turn, or hold it underneath, and wait. The first tissue to darken shows you the source. Running the heating for a few minutes can help, because some joints only weep once they are hot and have expanded.

Look at the colour too. Clear water is normal. Black or brown water means sludge and corrosion inside the system, which matters later.

Leaking From the Valve Spindle or a TRV

If the water appears where the spindle enters the valve body, just under the TRV head or the lockshield cap, the seal around the spindle has worn. On older valves there is a small gland nut at that point. Tightening it by a quarter turn often stops a weep, but it is easy to overdo, and on some valves the gland cannot be serviced at all.

A leaking TRV is one of the most common versions of this. The valve body and its seals age, especially where the valve has been left closed all summer. If the gland will not seal, the valve needs replacing, and that means draining the radiator or freezing the pipe. Our TRV installation service covers replacement, and it is usually worth changing both ends of an old radiator at the same time.

Leaking From the Joint Between Valve and Radiator

The valve connects to the radiator with a short threaded tail screwed into the radiator and a union nut that pulls the valve onto it. Water at this nut is common after a radiator has been removed for decorating, or on radiators that heat and cool through many seasons.

• At the union nut. A small nip with a spanner, holding the valve body still with a second spanner so the pipe does not twist, can close it.
• Where the tail enters the radiator. The seal on the thread has failed. Tightening the nut will not help. The radiator has to be drained, the tail removed, the thread cleaned and resealed.

Heavy-handed tightening is how a slow weep becomes a cracked fitting, so stop if a nut does not respond to a small turn.

Leaking From the Bleed Screw

If the drip is at the top corner, the bleed screw is either not fully closed or its seat is worn. Close it gently with the radiator key. If it still weeps, the bleed valve itself can be replaced, which again needs the radiator isolated and partly drained. A bleed screw that leaks straight after bleeding has usually just been left a fraction open. Our guide on bleeding radiators covers the safe way to do it.

Leaking From the Radiator Itself

A pinhole in the panel, usually along the bottom edge or at a welded seam, is a different kind of fault. Steel radiators corrode from the inside when the system water has lost its protection. Sludge, which is mostly iron oxide, settles at the bottom of the panel and the metal thins there first. By the time water breaks through, the panel is past repair and the radiator needs replacing.

Treat this as a warning about the rest of the system. The same water runs through every radiator and the boiler. If one has corroded through, it is worth checking whether the others are cold along the bottom, whether the system is due a power flush, and whether a magnetic filter would stop the new radiator going the same way. We replace radiators through our radiator installation service, and the radiator replacement cost guide explains what drives the price.

Leaking From the Pipework Below

If the valves and radiator are dry and the water is coming from below, look at the compression fittings where the pipes rise through the floor. A joint that has worked loose, or a pipe fretted by floorboards over years of expansion, can leak where you cannot see it. A stain on the ceiling underneath is a common first sign. Finding that kind of leak means lifting a board or two.

Why a Small Leak Is Worth Fixing Quickly

A slow drip does more harm than the damp patch suggests. On a sealed system, each top-up of the pressure brings in fresh mains water, and fresh water carries oxygen that speeds up corrosion inside the radiators and boiler. The inhibitor that protects the system is diluted every time as well. So a leak that has been managed by topping up the pressure every few weeks is quietly shortening the life of everything else. Our articles on why a system won't hold pressure and whether to repressurise it yourself go into this in more detail.

What About Leak Sealer?

Liquid leak sealers added to the system water can close very small weeps, and they have a place as a stopgap. They are not selective, though. The same particles that seal a pinhole can collect in narrow passages elsewhere, including the boiler heat exchanger, and they do nothing for a failed valve seal or a corroded panel. Fixing the actual fault is almost always the better route.

What You Can Do Yourself, and When to Call Us

Closing a bleed screw, nipping up a union nut a small amount and isolating the radiator are reasonable things to try. Anything that needs the radiator drained, a valve replaced or a tail resealed is a job for an engineer, because it involves draining down, refilling, bleeding, restoring the pressure and redosing the inhibitor so the rest of the system is left in better shape than before. If the leak has come from corrosion, we will also look at the water condition before fitting anything new.

If you have a leaking radiator, valve or TRV anywhere in London, get in touch to book a free survey. We will find the source, tell you whether it is a repair or a replacement, and give you a fixed quote before any work starts.