Which valve turns the radiator off?

Close both: the thermostatic valve (TRV) at one end and the lockshield (the one under a cap) at the other. Note how many turns the lockshield takes so you can reset the balance afterwards.

Common plumbing problems

Radiator leaking: what's causing it and how to stop it

Guidance built with working UK plumbers · Last updated July 2026

A leaking radiator usually comes from one of three places: a valve (where the pipe meets the radiator), the valve spindle under the plastic cap, or a corroded pinhole in the radiator panel. Valve leaks are often a simple nip-up; a rusted-through panel usually means the radiator needs replacing.

Usually amber — contain it and work through it calmly. Heating water is often dark and stains, so get a towel and tray down. It only becomes urgent if it's flowing fast or you can't isolate the radiator.

Do this now

Find the exact spot. Dry everything, wrap tissue around each point, then watch where it re-wets: the valve body, the nut where the pipe meets the valve, the spindle under the cap, or the panel itself.
Leak at the pipe-to-valve nut → gently nip it up with a spanner, holding the valve body still so you don't twist the pipe.
Leak from the spindle (under the cap) → tighten the gland nut a quarter-turn, or re-pack the gland.
Leak from the panel itself (a pinhole) → it's corroding through. A system leak-sealant may buy time, but plan a replacement.
To isolate the radiator: close both valves — the TRV and the lockshield. Count the lockshield turns so you can reset the balance later.

Likely causes

Where it's wet	Likely cause
Nut where pipe meets valve	Loose compression nut
Under the valve cap	Worn spindle gland
The radiator panel itself	Corrosion pinhole — usually replace
Bleed point at the top	Loose or worn bleed valve

When to call a professional

Call a plumber for a pinhole in the panel (replacement), if you can't stop the leak, or if you're draining and refilling the system — it needs re-inhibiting to prevent future corrosion. LeakScan gives guidance, not a substitute for a qualified plumber.

Questions, answered straight

Is a leaking radiator an emergency?

Usually not — it's typically a slow, amber leak you can contain while you sort it. Fast leaks or ones you can't isolate are more urgent.

Why is the water brown or black?

Heating water carries inhibitor and rust from inside the system, so it's dark and stains. Normal for heating water — but a very rusty leak can mean the panel is corroding through.

Can I use radiator leak sealant?

It can hold a small pinhole temporarily, but it's a stop-gap. A corroded radiator will usually need replacing.

Which valve turns it off?

Close both — the TRV at one end and the lockshield at the other. Note the lockshield turns so you can reset the balance.

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