The short answer. A radiator usually keeps leaking from one of five places: the valve gland or spindle, a loose bleed valve, a pipe joint, or pinhole corrosion in the body. Tightening or re-packing fixes most valve leaks. If the leak returns or the pressure keeps dropping, the real fault is often hidden pipework, not the radiator.
A radiator that drips, weeps or leaves a rusty stain on the carpet is one of the most common heating worries we hear about in Devon homes. The good news is that most radiator leaks come from a small number of predictable places, and many of them are simple to put right. The trickier part is knowing whether you are looking at a quick fix, a part that needs replacing, or a sign that the water is escaping somewhere you cannot see. This guide walks through each cause, what a sensible repair looks like, and when it is worth getting the system checked properly.
First, find where the water is coming from
Before you reach for tools, dry the radiator and the pipes completely with kitchen roll. Then run the heating and watch closely. Water tends to track downward and pool at the lowest point, so the puddle on the floor rarely sits directly below the actual leak. Wrapping a dry tissue around each joint and valve for a few minutes will usually show you the first spot that gets damp. That single step saves a lot of guesswork and stops people replacing a perfectly good radiator when the problem is a 20p washer.
The five places radiators usually leak
1. The valve gland and spindle
The spindle is the small stem that the control head or thermostatic head sits on. Around it is a gland nut packed with sealing material. Over years of turning the valve on and off, that packing wears and water can seep out, often only when the valve is part-open. This is one of the most common radiator leaks and usually one of the easiest to cure. Tightening the gland nut a small amount with a spanner often stops it. If it keeps weeping, the gland can be re-packed with PTFE tape wound into the recess, which restores the seal.
2. The bleed valve
If the leak is at the top corner of the radiator, the bleed valve is the likely culprit. This is the small square-headed valve you open with a radiator key to release trapped air. The threaded plug can work loose, or the seal behind it can perish, leaving a slow weep. Make sure it is closed fully and seated squarely. If it still leaks once closed, the bleed valve itself is inexpensive to replace and is a tidy permanent fix.
3. A pipe joint or compression nut
Where the valve tails meet the copper pipework there are compression joints sealed by an olive and a tightened nut. These can weep if the nut has loosened slightly, if the joint was never quite right, or if the radiator has been knocked. A careful quarter-turn on the nut sometimes seals it. If it does not, the joint may need remaking with fresh jointing compound or PTFE on the olive. Over-tightening can split the olive, so this is a job to do gently or hand to a heating engineer.
4. Pinhole corrosion in the radiator body
If the leak appears from the face or the seam of the radiator rather than a fitting, it is usually corrosion. Inside an untreated system, sludge and rust build up over time and eat through the steel from the inside out, eventually creating tiny pinholes. Surface rust or staining on the radiator is a strong hint that more is happening within. A pinhole caused by corrosion cannot be sealed reliably for good. A radiator that has reached this stage is generally at the end of its life and is best replaced, then the system protected with a corrosion inhibitor to slow it happening to the others.
5. The hidden pipework feeding it
This is the one people miss. Sometimes the radiator is dry and the real leak sits in the pipes running under the floor or buried in a wall on their way to it. The classic clue is a sealed combi system where the boiler pressure keeps dropping and you find yourself topping it up regularly, yet no radiator looks wet. Damp patches on flooring near a radiator, warm spots on a floor that should be cold, or a faint musty smell can all point the same way. Hidden leaks like this are why a radiator can seem to keep leaking even after the visible parts have been fixed.
Temporary fixes versus a proper repair
It helps to be honest about what each fix really is. Tightening a gland nut or a bleed valve, or remaking a compression joint, is a genuine repair that can last for years. By contrast, the leak-sealer additives and external putties sold for radiators are stopgaps. A liquid sealant added to the system may stem a small weep for a while, and an epoxy putty can buy you a few days over a pinhole, but neither addresses why the metal failed. Sealants can also clog narrow passages and valves if overused. Treat them as a way to get through to the weekend, not a cure.
A quick rule of thumb: if the leak is at a valve or joint, it is usually fixable. If it is from the body of the radiator, plan to replace it. If you cannot see any leak yet the pressure keeps falling, the water is escaping somewhere hidden and worth tracing properly.
When the radiator keeps leaking no matter what you do
If you have tightened everything, replaced the bleed valve, even swapped the radiator, and you are still losing water or pressure, the radiator was probably never the source. At that point chasing the radiator only wastes time. This is exactly the kind of problem non-invasive leak detection is built for. Using thermal imaging, acoustic listening equipment and tracer gas, the leak can be pinpointed under a floor or inside a wall without lifting every board or hacking off plaster. You can read more about how we approach this on our central heating leak detection page, and see the wider area we cover across Devon. If your boiler pressure is the thing falling, our guide to why a boiler keeps losing pressure is a useful next read.
A few sensible precautions
Whatever the cause, lay an old towel under the leak and put a container beneath it to protect your floor while you sort out a repair. If the leak is heavy, turn off both radiator valves to isolate it. Keep an eye on the boiler pressure gauge so you can tell us how often it drops, as that detail genuinely helps narrow down a hidden leak. And if you are at all unsure, it is always safer to have a heating engineer or leak specialist take a look before a small weep turns into damaged flooring or a failed boiler.
Frequently Asked Questions
A leak that returns in the same place usually means the underlying part has not been fully fixed. A worn valve gland will keep weeping until it is properly re-packed or replaced, and a corroded pinhole will keep seeping because the metal has failed. If it keeps coming back after a repair, the part likely needs replacing rather than tightening again.
Often, yes. A weeping gland nut can usually be tightened gently with a spanner, and a loose compression joint may seal with a careful quarter-turn. Re-packing a gland with PTFE tape is also within reach for a confident DIYer. If tightening does not work or you are unsure, it is safer to call a heating engineer than to risk splitting an olive or stripping a thread.
Sealant additives and external putties are short-term measures. They may slow a small weep for a while, but they do not repair the part that failed and can clog valves or narrow passages if overused. Treat them as a way to buy time until a proper repair or replacement, not as a permanent solution.
That pattern often points to a hidden leak in the pipework feeding the radiators rather than the radiators themselves. Water can escape under floors or inside walls while every visible part stays dry. If you are topping the boiler up regularly with no visible cause, it is worth having the system traced with specialist leak detection.
If the leak comes from the body or seam of the radiator and is caused by internal corrosion, replacement is the sensible choice, as pinholes cannot be sealed reliably. Leaks at valves, joints or the bleed valve are usually repairable. Replacing a corroded radiator and adding a corrosion inhibitor also helps protect the rest of the system.
Worried About a Leak in Devon?
If a radiator keeps leaking or your boiler pressure keeps dropping, we can trace it and put your mind at rest.