A puddle under a radiator has usually run down from one of three points: the valve gland, the union nut where the valve meets the radiator, or the bleed valve at the top. Dry everything and wipe each point with tissue. If every part stays dry and the floor is still wet, the leak is in the pipe below.
You noticed a dark patch on the carpet by the radiator and pressed it with your foot. Wet. Now you are staring at a lump of painted steel with two valves and a bleed screw, wondering which bit is guilty. Radiators only leak from a handful of places, the puddle is nearly always slow, and the first hour of detective work needs nothing more than a towel and some kitchen roll. The one trap is assuming the water came from the lowest wet point. Water runs downhill, so the source is above the puddle, and sometimes it is below the floor entirely.
Before you trace anything, stop it getting worse
- Get something under it. A folded towel or a shallow tray protects the floor and, left overnight, shows you how fast the water is coming.
- Close both valves gently. Turn the control valve at one end clockwise, then lift the cap off the lockshield at the other end and close that with a spanner. Together they isolate the radiator. If either valve is seized, leave it. Forcing a spindle turns a weep into a flood.
- Note the boiler pressure. On a sealed system, read the gauge now and write it down. A radiator leak drains pressure slowly, and the rate of fall is useful evidence later.
- Photograph the damage before you dry anything, in case you end up claiming.
Trace the drip upward, not downward
This is the whole method, and it takes ten minutes. Towel-dry the radiator, both valves, every nut and the visible pipework until nothing is wet. A wet radiator tells you nothing, because a drop from the bleed valve at the top runs down the panel, along the bottom edge and off the lowest corner, well away from where it started.
Then tear off some dry tissue and press it firmly against each point in turn, starting from the top and working down. Watch which piece of tissue darkens first. That is your leak, and everything below it is just the route the water took.
- The bleed valve in the top corner. A perished seal or a valve not fully closed after bleeding dribbles from here and wets the whole panel below.
- The top union nut on the side of the radiator, where the tail enters the panel.
- The valve gland, which is the collar the spindle passes through under the valve head or thermostatic cap. This is the most common leaking radiator valve fault of all, and it often only weeps when the valve is turned or when the heating cycles.
- The bottom union nut, where the valve joins the radiator tail. Inside is a small brass ring called an olive that forms the seal.
- The pipe tail below the valve, and the joint where it meets the pipe coming up through the floor.
- The radiator body, usually along the bottom seam or a low corner, where internal corrosion has eaten through.
Timing helps too. A gland leak shows when the valve is operated or the heating comes on. A body or pipe leak is steady.
The five places, side by side
| Where the tissue wets | What has gone wrong | Safe to try yourself? |
|---|---|---|
| Valve gland or spindle | The packing around the spindle has worn | Yes. Nip the gland nut up a quarter turn, or repack with PTFE tape |
| Union nut (top or bottom) | The nut has worked loose or the olive has failed | Yes for a gentle tighten. No if it keeps weeping, because the radiator must be drained to re-seal it |
| Bleed valve | Not fully closed, or the seal has perished | Yes. Close it fully. Replacement needs the radiator isolated and drained |
| Radiator body | Corrosion from the inside, usually low down | No. There is no lasting home fix. The radiator needs replacing |
| Pipe tail or floor joint | A weeping joint or a split in the pipe below | No. This is a plumber’s job, and if the pipe runs under the floor, a detection job first |
Tightening a gland without making it worse
A weeping gland is the radiator fault most people can fix in minutes, and the one most often turned into a bigger problem by enthusiasm.
Nip, do not crank
Pull the head or cap off the valve so you can see the gland nut, the small hexagonal nut the spindle passes through. Put a spanner on it and turn clockwise a quarter of a turn. Wipe, wait, check with tissue. Another quarter turn if it still weeps. Stop as soon as it is dry. Over-tightening crushes the packing and can shear the spindle, and at that point the radiator has to be drained and the valve replaced.
Repack if tightening does not hold
With the valve closed, undo the gland nut, wind a few turns of PTFE tape around the spindle, push it down into the gland with a small flat screwdriver and refit the nut. It is a five-minute job and it usually buys years.
Hold the valve body when you touch a union nut
One spanner steadies the valve, the other nips the nut. Small movements only. Turning the nut without bracing the valve twists the whole assembly and can loosen the joint on the other side.
If you have drained down at any point, top the corrosion inhibitor back up afterwards. Refilling dilutes it, and running without it is how radiators end up with the low seam corrosion in the table above.
When the puddle is the floor, not the radiator
Sometimes every point stays bone dry under the tissue, yet the towel is wet again by morning. That is your answer: the leak is below the floor. The pipe feeding the radiator may have a weeping joint under the boards, a pinhole where it rubs on a joist, or, in the solid floors common in Cornish cottages and 1960s bungalows, a corroded section buried in the screed. The water surfaces at the radiator because that is the nearest gap.
Two checks confirm it. On a sealed system, turn the heating off for a day and watch the gauge. If pressure falls with nothing running, water is leaving the circuit, and our article on why a boiler keeps losing pressure explains what the rate of loss tells you. Then walk the room in socks. A warm patch on the floor away from the radiator is a heating pipe leaking under it.
Stop topping the boiler up at this point. Each refill brings in fresh oxygen and speeds the corrosion, and it does nothing for the leak. If the radiators look fine but the system still loses water, our guide to central heating losing water with no visible leak covers what to check next.
How we find the pipe under the floor
Lifting boards on a hunch rarely lands on the right spot, because water tracks along a joist or through screed a long way from where it surfaces. Our central heating leak detection survey finds the point first. Thermal imaging picks up the warm trail a heating leak leaves under a floor. Acoustic equipment listens for water escaping under pressure. Where the pipe is plastic or buried in concrete, tracer gas fed into the drained circuit rises through the floor at the exact escape point. The result is one small, confirmed access hole rather than a room full of lifted boards, and a written report describing where the leak was and why.
That report matters if you claim. Most buildings policies include trace and access cover, which pays to find the leak and make good the access damage, though not the pipe repair itself. We provide leak detection across Cornwall, and a photo of the radiator sent ahead of a call often tells us whether you need us or a plumber.
A note on holiday lets and empty cottages
Radiators that sit switched off from October to Easter are the ones that weep when the heating goes back on, because gland packing dries and shrinks over a long idle period. If you manage a let in St Ives or Padstow, run the heating for an hour before the first winter guests arrive and check under each radiator with tissue. Nipping a gland in an empty house beats explaining a wet carpet on changeover day.
Frequently asked questions
Why is my radiator valve leaking?
Most valve leaks come from the gland, the packed collar the spindle turns through. The packing wears with age and use and lets water weep out, often only when the valve is turned or the heating comes on. The other common point is the union nut where the valve joins the radiator, where the olive inside has stopped sealing.
How do I know which part of the radiator is leaking?
Dry the radiator, both valves, every nut and the visible pipe with a towel, then press dry tissue against each point in turn, starting at the top. Whichever tissue darkens first is the source. Water runs down the panel and along the bottom edge, so the lowest wet spot is rarely where the leak is.
Can I tighten a leaking radiator valve myself?
Usually, yes, if it is the gland. Remove the valve head, turn the gland nut clockwise a quarter turn at a time until the weep stops, and repack the spindle with PTFE tape if tightening alone does not hold. Do not force a seized valve, and leave a leaking radiator body or a buried pipe to a professional.
Why is the floor wet but the radiator dry?
The leak is below the floor. A weeping joint or a corroded section on the pipe feeding the radiator lets water out under the boards or in the screed, and it surfaces at the radiator because that is the nearest gap. A falling boiler gauge with the heating off confirms the circuit is losing water.
Radiator dry, floor still wet?
If the tissue test has cleared the radiator and its valves, the leak is under the floor and we can find it without lifting the room apart. You get the exact spot and a report ready for your insurer.
