Why Is My Radiator Leaking? Causes & Fixes

The Chartered Institute of Plumbing and Heating Engineering has guidance on diagnosing leaks on a sealed system. Radiator leaks are almost always small and slow, which is exactly why they cause damage — a weep that takes a week to show on a carpet has been feeding the floor underneath the whole time.

Finding the source takes about ten minutes with kitchen paper.

Find the Source First

1. Dry the radiator, both valves and the pipework completely.
2. Wrap or tuck dry kitchen paper around each of these separately: the union nut at each valve, the valve spindle under each head, the bleed valve, and the seams of the panel itself.
3. Run the heating for twenty minutes.
4. Check which paper darkens first. That is your leak.

This matters because the fixes are completely different, and water runs down and along before it drips.

Cause 1: A Weeping Valve Union

The nut where the valve joins the radiator, or where it joins the pipe. The most common leak and usually the easiest fix.

The fix: a quarter turn with a spanner, holding the valve body still with a second spanner so you are not twisting the pipework below the floor.

If a quarter turn does not stop it, do not keep tightening — the joint seals on a compression olive, and over-tightening crushes it out of shape and makes the leak permanently worse. At that point the joint needs remaking with a new olive, which means draining down.

Cause 2: A Leaking Spindle

Water appearing from under the valve head, around the spindle, usually only when the valve is being turned or shortly after.

The O-ring or gland packing around the spindle has perished. On some valves the gland nut can be nipped up slightly; on others the head assembly needs replacing.

Because the valve body stays in place, this can often be fixed without draining the system. Our guide to radiator valve types covers which is which.

Cause 3: The Bleed Valve

A drip from the bleed point at the top corner usually means it was not closed properly after bleeding, or the small brass spindle has been overtightened at some point and no longer seats.

Try closing it firmly first. If it still weeps, the bleed plug can be replaced — a small part, and on most radiators it comes out with the system still full if you are quick and have a bowl ready.

Cause 4: A Pinhole in the Panel

The one with no cheap fix. Radiators corrode from the inside out, and eventually a pinhole appears — usually along a bottom seam or a weld line where sludge has been sitting.

Signs: a rust-coloured stain running down the panel, and a drip that comes from the metal itself rather than any fitting.

The radiator needs replacing. Sealant additives and external repair putties buy days, not months, and a corroded radiator that has holed in one place will hole again elsewhere.

It is also a signal about the wider system, and in a hard water area — Anglian Water publishes local figures — internal corrosion progresses faster. Internal corrosion at that level means the inhibitor has been depleted for years, and other radiators are heading the same way. Worth checking the inhibitor level while replacing it.

What to Do Right Now

1. Put a container under it and towels around the base.
2. Close both valves — thermostatic clockwise to off, lockshield clockwise with a spanner. That isolates the radiator and usually slows or stops the leak.
3. Note how many turns you closed the lockshield, so it can be reset later.
4. Keep the floor dry. Chipboard and laminate swell irreversibly.
5. Check the boiler pressure — a leak drops it, and a combi will lock out below about 0.5 bar.

A leaking radiator is rarely an emergency once isolated. It becomes one if it is dripping onto electrics or through a ceiling.

Why Pressure Keeps Dropping

If you are topping the boiler up more than once or twice a year, there is a leak on the heating circuit somewhere — and a radiator weep is the most common culprit, because it evaporates off a warm panel without ever forming a puddle.

Look for rust staining or a white crusty deposit around valve unions. Both are the residue of water that has been evaporating there for months. Our guide to a boiler losing pressure covers the other causes.

Frequently Asked Questions

Can I use a leak sealer additive?

It can work on a genuine pinhole as a temporary measure, but it also risks clogging the boiler heat exchanger and pump. Most manufacturers discourage it. For a valve leak it does nothing at all.

Why does it only leak when the heating is on?

Heat expands the metal and raises system pressure, so a marginal joint opens slightly. It is still a real leak — it just needs the conditions to show.

Is a rust stain under the valve serious?

It means water has been evaporating there for some time. The joint needs remaking rather than tightening, and it explains gradual pressure loss.

Can a leaking radiator be repaired rather than replaced?

Valve and spindle leaks, yes. A holed panel, no — the corrosion causing it is throughout the radiator, not just at that point.

If the Panel Itself Has Gone

A corroded radiator needs replacing, and it is worth testing the system inhibitor at the same time so the new one does not go the same way. We cover Milton Keynes and the surrounding area for heating repairs and radiator replacement. Get in touch.