To fix a leaking radiator, first dry everything with kitchen roll and find exactly where the water appears: a leak at a valve nut or tail can usually be tightened or remade, a leaking bleed screw often needs a new screw or washer, but a pinhole or seam leak in the radiator panel itself cannot be repaired properly and the radiator needs replacing. Until then, put a tray underneath, turn both valves off and keep an eye on the boiler pressure gauge.
Most leaks are slow weeps rather than floods, which gives you time to trace them properly. The trick is patience: water runs along the underside of a radiator and drips from the lowest point, which is rarely where it is actually escaping.
Tracing the leak in five minutes
Do this with the heating off and the radiator cool enough to touch.
- 1
Dry every surface
Wipe the valves, the pipe tails, both ends and the full underside of the panel until they are completely dry.
- 2
Wrap tissue round each joint
A twist of toilet paper around each nut and the bleed screw shows the first damp spot far more clearly than your eye can.
- 3
Run the heating briefly
Some leaks only appear when the metal expands and the system is under pumped pressure. Watch the tissue, not the floor.
- 4
Check the panel face and seams
Run a dry finger along the welded top and bottom seams and look for rust spots or bubbling paint on the face, especially low down.
- 5
Look at the pressure gauge
A steady drop in pressure over days confirms water is leaving the sealed system, which helps rule out a leak from somewhere else in the room.
Where the water comes from, and what it means
| Leak point | Usual cause | Typical remedy |
|---|---|---|
| Nut between valve and radiator | A slightly loose union or a worn olive on the tail | Tighten carefully with two spanners; if it persists, the tail is remade after draining down |
| Spindle under the valve head | A worn gland packing or O-ring around the spindle | Nip the gland nut a fraction, or replace the valve |
| Bleed screw at the top corner | A damaged screw, missing washer or debris on the seat | Replace the bleed screw or its seal |
| Pinhole on the panel face | Internal corrosion eating through the steel from the inside | Replace the radiator — patches and sealants do not last |
| Along a seam | Corrosion or stress at the weld | Replace the radiator |
| Pipe below the floor | A joint or pipe under the boards, not the radiator at all | Lift the board and repair the pipework |
A leak at the valve itself rather than the radiator has its own guide: see our leaking radiator valve page.
Why radiators rust through from the inside
Steel radiators and water in a sealed system slowly react with each other, producing black iron-oxide sludge. In a healthy system this is held in check by inhibitor and a magnetic filter. Where the inhibitor has been diluted by repeated topping-up, the reaction speeds up, and the thinnest point of the steel eventually gives way — typically low down, where sludge settles.
That is why a pinhole leak is a warning about the whole system, not just one radiator. If the water you bleed out is black, or other radiators are cold at the bottom, the replacement radiator is likely to suffer the same fate unless the system is cleaned and re-dosed.
What not to do
- Do not rely on a leak-sealer additive as a permanent fix — it can also collect in the boiler and in narrow valves.
- Do not overtighten a brass nut; it can split the olive or crack the valve body and turn a weep into a steady leak.
- Do not keep topping the boiler back up indefinitely. Fresh water brings fresh oxygen and speeds up corrosion everywhere else.
- Do not remove a radiator from the wall without closing both valves and having towels and a tray ready.
Questions people ask
Can you fix a leaking radiator without draining the system?
A weeping nut or spindle gland can often be nipped up without draining. Remaking a tail, changing a valve or replacing the radiator needs at least that radiator isolated and emptied, and sometimes the system drained down.
Is a leaking radiator an emergency?
Usually not, but it should not be ignored. Contain the water, close both valves on that radiator and keep an eye on the boiler pressure. If water is getting near electrics or through a ceiling, turn off the electrics in that area and stop the water first.
Why does my radiator leak only when the heating is on?
Metal expands as it warms and the pump adds pressure, so a marginal joint or seal can open up only when the system is running. That pattern points to a fitting rather than a hole in the panel.
Will leak sealer fix a pinhole in a radiator?
It may slow it for a while, but the steel around the pinhole is already thin from corrosion. Replacing the radiator is the lasting fix, ideally along with checking the rest of the system for sludge.
Why does my boiler keep losing pressure after a radiator leak?
A sealed system loses pressure whenever water escapes. Even a slow weep that evaporates before it reaches the floor can drop the gauge over a few days.
Who does the work
- Gas Safe registered engineers — registration 554328
- Worcester Bosch accredited installer
- John The Plumb Ltd, company registration 10026483
Find the exact spot before deciding anything. A dry cloth and a few twists of tissue will tell you whether this is a five-minute tightening job or a new radiator, and that is the single most useful thing to know when you pick up the phone.
Radiator leaking?
Call 07891 410 960, Monday to Saturday 8am to 5pm. Tell us where the water appears and we will explain the likely fix.