A leaking radiator valve usually drips from the gland where the spindle turns or the union nut where the valve meets the radiator. Dry everything, then do the tissue test to find the exact point. A weeping gland or loose nut is often a simple tighten or PTFE re-pack, but a leaking radiator body, or pressure that keeps dropping, points to a job for a heating engineer or leak detection.
A wet patch under a radiator, a rusty stain down a valve or a small morning puddle are all worrying to find, but most radiator leaks are slow and fixable. The trick is working out exactly where the water is escaping, because the fix for a weeping valve is nothing like the fix for a corroded radiator. This guide covers what to do first, the few places radiators actually leak, how to spot which one, the fixes that are safe to try yourself, and when a leak you cannot see needs a professional.
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What to do right now
Most radiator leaks are slow rather than dramatic, so you usually have time to act calmly. Work through these steps.
- Catch the drip. Slide a bowl, tray or thick towel under the leak to protect the floor and skirting. A folded towel also shows you overnight how fast the water is coming.
- Close the radiator off if you can. Most radiators have a control valve at one end and a lockshield, the capped one, at the other. Closing both isolates the radiator and often slows or stops the leak while you sort a fix. Turn valves gently, and do not force a seized one.
- Do not overtighten in a panic. Cranking hard on a nut or valve can crack the fitting or shear the spindle and turn a weep into a flood. Snug, not forced, is the rule.
- Watch the boiler pressure. On a sealed system a radiator leak slowly drops the pressure gauge. Note the reading before you top up, as it is a useful clue.
- Keep it dry and take a photo. Mop up, dry the radiator ready for the tissue test below, and photograph the damage in case you claim on insurance later.
Where radiators and valves leak
Radiators look like a single sealed unit, but water only escapes from a few predictable places. Knowing them makes the leak much easier to pin down.
| Leak point | What is going on |
|---|---|
| Valve gland or spindle | The gland is the part the spindle turns through. Its internal seal wears over time, and water weeps out around the spindle, often only when the valve is turned. This is the most common valve leak, on both manual valves and a thermostatic radiator valve (TRV). |
| Union or coupling nut | The nut where the valve joins the radiator tail. Inside is a small brass ring called an olive that forms the seal. If the nut works loose or the olive has gone, water seeps from the joint. |
| Thermostatic radiator valve (TRV) | A TRV is a manual valve with a temperature-sensing head on top. It leaks in the same places as any valve, the gland and the union nut, but the head can also seize or the pin stick. |
| Radiator body | Inside, untreated water slowly corrodes the steel. That rust can eat through and create a pinhole or weeping seam, usually low down or along a welded edge. There is no reliable lasting home fix. |
| Bleed valve | The small square-headed valve at the top corner, used to release trapped air. If it is not fully closed, or the seal has perished, it can dribble from the top of the radiator. |
| Pipe tail | The short pipe connecting the valve to the radiator, and the pipe feeding it from the floor. A leak on a radiator pipe here can look like a radiator leak when the pipe is the real culprit. |
How to find which point is leaking
The single most useful thing you can do is the tissue test, and it takes minutes.
- Dry everything. Towel-dry the whole radiator, both valves, the nuts and the visible pipe. A spread-out wet patch tells you nothing until you start from dry.
- Wipe each point with dry tissue. Press fresh kitchen roll or toilet tissue against each suspect spot in turn: the gland, the union nuts top and bottom, the bleed valve, the pipe tails. Check which wets first.
- Note when it leaks. A gland leak often shows when the valve is operated or the heating cycles. A body or pipe leak tends to be steady. A bleed-valve leak sits right at the top corner.
This matters because water runs. A drip pooling at the bottom may have travelled down from a leak higher up, so the lowest wet point is not always the source. A leak at the top of the radiator points to the bleed valve and the top union; a leak at the bottom usually means the lower valve, its union nut or the pipe tail.
DIY-safe fixes and when to call an engineer
Once you know the point, the right fix follows. Some are genuinely simple. Others are best left to a professional.
Leaks you can usually fix yourself
- Weeping gland. Pop off the valve head or cap and gently tighten the gland nut clockwise with a spanner, a quarter turn at a time, until the weep stops. If it still leaks, the spindle can be repacked: wind PTFE tape around the spindle, push it down into the gland with a small flat screwdriver, and refit the nut. Do not overtighten.
- Loose union nut. Hold the valve body steady with one spanner and nip the union nut up with another. Small movements only. If it keeps weeping, the olive inside has likely failed, which means draining the radiator down to re-seal or replace it, a step up in difficulty.
- Dribbling bleed valve. Make sure it is fully closed with a bleed key. A perished bleed valve can be replaced cheaply once the radiator is isolated and drained.
Leaks to leave to a heating engineer
- A leaking radiator body. Corrosion from the inside has no dependable DIY cure. The radiator usually needs replacing, which a heating engineer will do properly and safely.
- A seized or failed TRV that needs swapping. Changing a valve means draining part of the system and getting the seals right, so unless you are confident it is an engineer job.
- Anything near the boiler. By law in the UK, work on a gas appliance must be done by a Gas Safe registered engineer. Do not attempt boiler repairs yourself.
One thing people often miss: if a repair meant draining the system or losing a lot of water, top the corrosion inhibitor back up. Draining and refilling dilutes the inhibitor that protects the metal inside your radiators and pipes, and running without it invites rust and sludge. A standard domestic system of up to around nine radiators typically needs about 500ml, refreshed roughly once a year. Not sure where to shut the water off first? Our guide to finding the shut-off valve for your water walks you through it.
When a hidden central heating leak needs detection
Sometimes the radiators look fine and the floor is dry, yet the system keeps losing water. That is the classic sign of a leak you cannot see, often a buried pipe under the floor rather than the radiator itself.
The simplest test on a sealed system: turn the heating off for about 24 hours and recheck the pressure gauge. If pressure has dropped with the heating off, there is almost certainly a leak in the system. Resist the urge to keep topping up, because every refill adds air and speeds corrosion. Our article on why a boiler keeps losing pressure covers this, and the wider guide to finding a leak in a central heating system covers the full diagnostic.
When the leak is hidden, professional detection earns its keep. Rather than lifting floors on a hunch, a specialist combines methods to pinpoint the source first:
- Thermal imaging reads the warm trail a heating leak leaves under a floor or behind a wall.
- Acoustic detection listens for water escaping a pressurised pipe.
- Tracer gas is fed into the pipework and rises at the exact escape point, passing through wood, tile, vinyl, carpet and even a concrete slab.
- Pressure testing and moisture meters confirm there is a real leak and how far the damp has spread.
Together these turn guesswork into a targeted find, so any floor that does come up is a small, confirmed access point rather than a room full of lifted boards. Our central heating leak detection service across Cornwall and Devon is built for exactly this.
Frequently asked questions
Why is my radiator valve leaking?
Most valve leaks come from the gland, the part the spindle turns through, where the seal has worn and lets water weep out as the valve is operated. The other common spot is the union nut where the valve joins the radiator. Both are usually fixable, not a sign the whole valve has failed.
How do I know which part of the radiator is leaking?
Dry the radiator and both valves with a towel, then wipe each point in turn with dry tissue and watch where the water comes back first: the gland, the union nuts, the bleed valve, the pipe tails and the body. Water runs along the radiator, so the lowest drip is not always the source.
Can I fix a leaking radiator valve myself?
Often, yes. A weeping gland nut can usually be cured by gently tightening it with a spanner, and if that fails by repacking the spindle with PTFE tape. A loose union nut can be nipped up too. A leak from the radiator body, caused by corrosion, has no reliable home fix and is one for a heating engineer.
Why is my radiator leaking from the bottom?
A leak from the bottom is usually the valve, the union nut where the pipe meets it, or the pipe tail, because that is the lowest point where water collects. It can also be a corroded body weeping from a low seam. Do the tissue test to tell a valve leak from a body leak, because the fix differs.
My boiler pressure keeps dropping. Is a radiator leaking somewhere I cannot see?
Possibly. On a sealed system, turn the heating off for about 24 hours and recheck the gauge. If pressure falls with the heating off, there is almost certainly a leak, which may be a radiator, a valve or a buried pipe. Topping up repeatedly just adds air and speeds corrosion, so the leak is worth finding.
Do I need to add inhibitor after fixing a radiator leak?
If the repair meant draining the system or losing a lot of water, yes. Draining and refilling dilutes the corrosion inhibitor that protects the metal inside your radiators and pipes. A standard domestic system needs roughly 500ml topped back in, refreshed about once a year to keep sludge and rust at bay.
Losing water and cannot find the leak? We can
If your heating keeps dropping pressure or there is damp you cannot trace, we find hidden central heating leaks across Cornwall and Devon with thermal imaging, acoustic detection and tracer gas. Fast response, minimal damage, and the insurer-ready report if you need to claim.
Call Dickie on 07822 025 911 No Find, No Fee on residential leak detection (subject to terms)