Why Is My Radiator Leaking? Causes and Fixes

Updated July 2026 · Emergency Plumber Chelsea

The valve is the first place to check, because valve and gland leaks account for most radiator drips. Every radiator has a valve at each end: a wheelhead or thermostatic valve you adjust, and a lockshield that balances the flow. Water most often escapes from the gland nut, the small nut around the spindle where you turn the valve, especially on older brass valves that have been opened and closed for years. It can also weep from the union nut that joins the valve body to the tail screwed into the radiator. A gland leak usually shows as dampness or a bead of water around the top of the valve rather than a steady run, and it often appears or worsens just after someone has turned the radiator up or down.

The spindle is the moving part inside the valve, and a leak there is closely related to the gland. Behind the gland nut sits a packing or an O ring that seals against the spindle as it turns. When that packing dries out or the O ring perishes, water tracks up alongside the spindle and out under the head. A gentle quarter turn to nip up the gland nut sometimes stops a minor weep, but over tightening cracks old brass and makes things worse, so it is a light touch only. If nipping the nut does not stop it, the gland needs repacking with new PTFE or the valve needs replacing, which is a quick job for a plumber and is the same kind of seal work behind a dripping tap.

A leak from the body of the radiator itself is the serious one, because it means corrosion has eaten through from the inside. Steel panel radiators rust internally when the system water is not protected, and that rust produces the black sludge called magnetite. Given enough years the corrosion works outward until it perforates the metal, usually low down on the panel or in a bottom corner where sludge and moisture sit longest. These pinhole leaks often start as a faint rusty tear stain running down the front of the radiator, then become an active weep or spray. You cannot reliably patch a corroded panel, because where one pinhole has appeared the surrounding metal is thin too, so a body leak almost always means the radiator is replaced rather than repaired.

The bleed valve at the top corner is another common leak point, and it is usually the easiest to deal with. The bleed valve is the small square nipple you open with a radiator key to release trapped air. If it was not closed fully after bleeding, or the tiny sealing washer inside has worn, it can seep from the top edge of the radiator. First check it is properly shut by closing it gently with the key. If it still weeps once closed, the valve itself needs a new washer or replacing, which is minor. A wet top corner after someone has bled the radiators is nearly always a bleed valve that has not been done up, not a crack in the panel.

Pipe joints and connections are the last place to look, sitting just below and behind the radiator. The tails, olives and compression fittings where the pipework meets the valves can weep if an olive has not seated, a nut has loosened with expansion and contraction, or an old soldered joint has failed. In Chelsea's period homes this often shows under floorboards or against skirting, so a damp patch on the floor near a radiator does not always mean the radiator is at fault, it can be the pipe below. If the wet area is on the pipe rather than the panel or valve head, treat it as a pipe leak and read our guide on central heating leak detection, because tracing a hidden joint under a floor needs care.

To find the true source, dry everything first, then watch where the water reappears. Wipe the radiator, the valves and the pipework completely dry with a cloth or kitchen paper, then wrap dry tissue around each suspect point: the gland nut, both valve unions, the bleed valve and the pipe joints. Leave the heating running and come back after twenty minutes. The tissue that dampens first tells you the origin. Working top down, a wet top corner is the bleed valve, dampness at the valve head is the gland or spindle, a rusty streak on the panel is corrosion, and water pooling on the floor with a dry radiator above is a pipe joint. This one test saves a lot of guesswork and tells a plumber exactly what to bring.

Contain the leak while you wait, but treat containment as temporary. For a valve or gland weep, close the valve at that end to reduce pressure on the seal, place a shallow tray or a folded towel under the drip, and empty it before it overflows. A radiator leak sealer added to the system is a stopgap at best and can clog valves and the boiler, so avoid pouring products in as a first move. Do not over tighten nuts in a panic, since cracking a brittle old fitting turns a slow weep into a flood. If the leak is a fast spray from a corroded panel, turn off the heating, and if you cannot isolate the radiator at its valves, shut the system down and be ready to isolate at the stopcock so you can drop the pressure.

A body leak is really a symptom of a sludged, unprotected system, which is why the fix is often bigger than one radiator. If corrosion has perforated one panel, the same corrosive water is circulating through every other radiator and the boiler, so replacing the leaking radiator alone leaves the underlying problem in place and the next pinhole is only a matter of time. This is where a proper system clean matters. A power flush pushes water and cleaning chemicals through the system at high flow to lift decades of magnetite out of the radiators and pipework, and it is usually paired with fresh inhibitor and a magnetic filter to slow the corrosion coming back. Our radiator power flush service explains what is involved. If several radiators are also cold at the bottom, that confirms system wide sludge rather than a one off fault.

Knowing when to replace a radiator rather than repair it keeps the cost sensible. Replace the panel when the leak is a corrosion pinhole in the body, when the radiator is heavily rust stained inside, or when it is cold at the bottom across most of its width even after bleeding and balancing. Repair, rather than replace, when the leak is at a valve, gland, spindle, bleed valve or pipe joint, since these are seal and fitting faults on otherwise sound metal. A single failed valve is a swap; a corroded radiator on a dirty system is a replacement plus a clean. A plumber will tell you honestly which side of that line your radiator sits on rather than defaulting to the bigger job.

Chelsea's older housing stock makes radiator leaks a frequent call. Many houses and mansion flats in SW3 and SW10 run heating on pipework and steel radiators that are decades old, sometimes on narrow microbore that sludges and blocks sooner than modern pipe, so internal corrosion and pinhole leaks turn up more often here than in newer homes. Original brass valves that have not been touched in years are prone to gland weeps the moment they are adjusted, and hidden pipe runs under period floors make joint leaks harder to trace. In some mansion blocks the heating is communal, so isolating a single radiator or planning a flush has to be arranged around the building and the managing agent. Emergency Plumber Chelsea works sympathetically on period systems and carries common valves and fittings for same visit repairs. Call 020 3002 6667.

Quick tips

• Dry every suspect point, then wrap dry tissue around the gland nut, valve unions, bleed valve and pipe joints; the first to dampen is your leak.
• A wet top corner after bleeding is almost always a bleed valve left open, not a crack; close it gently with a radiator key first.
• Only ever nip a gland nut a light quarter turn; forcing an old brass valve cracks it and turns a weep into a flood.
• A rusty tear stain running down the panel means internal corrosion; that radiator needs replacing, not patching.
• If one radiator has corroded through, protect the rest with a power flush, fresh inhibitor and a magnetic filter, or the next pinhole soon follows.

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