Leaking Radiator or Radiator Valve: How to Find the Source and Fix It

A leaking radiator almost always comes from one of four places: the valve's spindle seal (the gland), the union nut that connects the valve to the radiator, a compression fitting at the pipe joint below it, or a pinhole in the radiator's own metal panel caused by internal corrosion. The first three can usually be reset or resealed once the radiator is isolated. The fourth is different: a corroded pinhole in the panel has no lasting repair, and the radiator eventually needs replacing. Working out which of these you're actually looking at, before you pick up a spanner, is what decides whether this is a short job or one to hand to a plumber.

• Dry the radiator completely, then trace the leak with tissue or paper towel to find the exact point it's coming from before doing anything else.
• Turn off the heating, let the radiator cool, and close both valves to isolate it before you touch any joint.
• A gland leak, a union leak and a pipe-joint leak can usually be resealed with PTFE tape once the radiator is isolated.
• A pinhole in the radiator panel is corrosion working from the inside out, and no sealant makes it permanent; the radiator needs replacing.
• A cracked valve body, or a leak that keeps coming back after resealing, is a job to hand to a plumber.

Where a Leaking Radiator Is Actually Coming From

Before reaching for a spanner, dry the radiator and the pipework beneath it completely, including the floor, so you're not chasing an old stain. Then run a piece of dry tissue or kitchen paper around each likely point in turn: the bleed valve and cap at the top, the plastic cap and body of the thermostatic or manual valve, the union nut where the valve meets the radiator, and the compression fitting on the pipe coming up from the floor. Wherever the tissue comes away damp tells you which fix applies, because a leaking radiator bleed valve and cap, which can weep if they were never fully tightened, needs a very different fix from a leak lower down at the pipe joint. Skipping this step and simply retightening everything in sight tends to waste PTFE tape on joints that were never the problem.

Safety Steps Before You Touch Any Valve

Once you know where the drip is coming from, turn the heating off so the pump stops circulating hot water, and give the radiator time to cool fully before you handle it; working on a hot valve risks a scald as much as it risks a mess. With the system cool, close both the valve that controls flow into the radiator and the lockshield valve on the other side, counting how many turns it takes to close the lockshield so you can wind it back to exactly the same setting afterwards and keep the radiator balanced with the rest of the system. Keep a bucket, old towels and a cloth within reach, and if the system is pressurised, open the bleed valve briefly first to release trapped pressure before you loosen anything. None of this takes long, but doing it in order is what keeps a small drip from turning into standing water on the floor while you're working.

Fixing a Drip From the Valve Spindle (Gland Leak)

A bead of water forming under the plastic cap, right where the spindle enters the valve body, points to a worn gland seal, and it's usually the easiest of the four leaks to deal with. Start small: with the cap off, give the gland nut a slight clockwise turn with a spanner and check again after each adjustment rather than winding it down hard in one go, since overtightening can crack the nut or seize the spindle. If that doesn't stop the drip, undo the gland nut fully, wind PTFE tape around the spindle, and ease the spindle back into the valve body with a small flat-head screwdriver so the tape beds in evenly. A light touch of silicone grease on the spindle before you retighten the gland nut and refit the cap helps it turn smoothly afterwards (source: HomeServe).

Tools for this fix

• An adjustable spanner sized for the gland nut on your particular valve
• PTFE plumber's tape
• A small flat-head screwdriver
• A little silicone grease
• A cloth and a small container to catch drips while you work

Fixing a Leak at the Valve Tail or Union Nut

If the drip is coming from lower down, at the joint where the valve screws into the radiator itself rather than from the spindle above, the seal at the valve tail has likely failed. This one needs the radiator properly isolated first: close both valves as described above, then drain a little water out below the level of the leak so you're not undoing a union nut under pressure. Once it's loose, wrap PTFE tape densely around the male threads of the tailpiece, roughly ten to twelve wraps, before retightening the union nut by hand and then a final quarter-turn with a spanner. Reopen both valves afterwards, restoring the lockshield to the turn count you noted earlier so the radiator heats at the same rate as before.

Fixing a Leaking Pipe Joint (Compression Fitting)

A leak a little further down, at the compression fitting where the pipe rises from the floor into the valve, is worth a small tightening attempt first: hold the fitting steady with one spanner and give the compression nut a modest turn with another, keeping the bleed valve closed while you do it. If it still weeps afterwards, isolate the radiator and drain it below the joint, slacken the compression nut, and re-wrap or renew the olive's seal with PTFE tape before retightening (source: PlumberParts). This is the point where it's worth being clear about what you're actually looking at: a leak here is a joint problem specific to this radiator's connection, not the same thing as a pinhole forming along a run of copper supply pipe, which comes from a different cause and needs a different response.

When the Leak Is From the Radiator Body Itself

A weep from the flat metal panel of the radiator, rather than from any joint or valve, is a different problem entirely. It's almost always sludge, iron oxide built up inside an older or under-treated system, slowly eating through the metal from the inside until it breaks through as a pinhole (source: BestHeating). No amount of retightening helps here, because there's nothing to tighten; the metal itself has been thinned from within. A plastic resin leak sealant can buy a little time, but only in an open-vented system that has a feed-and-expansion tank, and it must never be used in a sealed, pressurised central heating system, where the pressure it's meant to hold back works against it. Once corrosion has broken through as a pinhole, the radiator needs replacing regardless of any temporary sealant, and flushing the system and adding a corrosion inhibitor afterwards gives the new radiator a better chance of avoiding the same fate.

When to Call a Plumber Instead

Some situations are worth handing over rather than pushing further on your own. A crack in the valve body itself, as opposed to a worn gland or a loose union, means the whole valve needs replacing rather than resealing (source: Engineer Fix), and that's a job better suited to someone who fits valves regularly. The same goes for a leak that keeps coming back after you've retightened and re-taped it, or one that shows up at more than one point at once, since that usually points to a system issue beyond a single worn seal. And if isolating or draining the radiator safely isn't something you're confident doing, calling a plumber before you start is the more sensible order of operations. If a slow leak has already gone on long enough to leave a damp patch or staining nearby, it's worth checking how quickly mould can take hold in the window after a leak and, where a wall or floor has already taken on water, how to dry out the affected area properly before it becomes a bigger repair.

Catching a Slow Radiator Leak Before It Becomes a Flooring Repair

Once the visible drip is sorted, it's worth thinking about the leaks that aren't visible at all: a slow weep at a valve or joint tucked behind a sofa or in a spare room can go unnoticed for weeks, showing up only as a damp patch, a warped floorboard or an unexplained jump in the water bill. A whole-home leak detector installed on the main line watches overall water flow rather than any single fixture, so when flow stays abnormal for longer than a normal top-up, bleed or tap use would explain, it can flag the household or shut off the supply automatically. It won't tell you which radiator is dripping the way tracing the leak by hand does, but it catches the pattern a small, steady drip leaves in the home's overall water use, which is exactly the kind of leak that's easy to miss until it's already caused damage.

The Smart Whole House Water Leak Detector with Auto Shut-Off Valve sits in this category, and the broader whole-house leak detection collection covers the range of devices built around the same idea. For a closer look at how this kind of detector and shut-off valve actually work together, this explainer on smart leak detectors and auto shut-off valves goes through the mechanism in more detail.