How to Fix a Leaking Radiator Valve

A leaking radiator is usually one of three problems: a loose valve, a perished olive, or corrosion in the radiator body. The first two are fixable with basic tools in under an hour. The third needs the radiator replaced. This guide helps you work out which one you have and walks through the fix for each.

Step 1: Identify exactly where the leak is

Dry the whole radiator and the floor around it with a towel. Wait 5–10 minutes with the heating on, then look closely. The leak is almost always at one of five points:

• The TRV (thermostatic radiator valve) head — usually means the internal valve cartridge is failing
• The TRV body at the union nut — loose connection or worn olive seal
• The lockshield valve at the other end of the radiator — same causes as the TRV body
• The radiator body itself — pinhole corrosion, usually fatal for the radiator
• The bleed valve at the top corner — over-tightened or worn seal

Marking the wet spot with a felt pen helps confirm exactly where the water is coming from.

Step 2: Turn off the central heating

Switch the heating off at the thermostat and wait at least 30 minutes for the system to cool. Working on a hot radiator risks scalding and makes the joints harder to disturb cleanly. Never apply tools to fittings that are still hot to the touch.

Step 3: Close both radiator valves

Turn the TRV head fully clockwise to close it (you may need to remove the head to access the body if your TRV is the modern type that doesn't close fully). Then close the lockshield valve at the other end — pop the plastic cap off, use a small adjustable spanner, and turn clockwise until firm. Count the number of turns and write it down — you'll need to reopen it the same number of turns to keep the radiator balanced.

Step 4: Drain a small amount of water

Place a bowl or tray under the leaking joint. Loosen the union nut at that end by half a turn. Water will trickle out — let about half a litre escape to relieve the pressure at the joint. Tighten the nut back up before continuing.

Step 5: The fix — three options

Option A: Tighten the union nut

If the leak is at a union connection, often the nut has just worked loose over time. Use a spanner to firm it up by quarter turns. Don't crank it hard — over-tightening crushes the olive and makes the leak worse. After each tightening, dry the joint, run the heating for 10 minutes, and check.

Option B: Replace the olive

If tightening doesn't work, the brass olive seal inside the joint has perished. Drain the radiator (close both valves, open the bleed valve at the top, then loosen the union nut and let the water out into a bowl). Once empty, unscrew the union nut fully, slide off the old olive, fit a new one (5p part from any plumbing merchant), and remake the joint. Wrap fresh PTFE tape clockwise around the male threads before tightening.

Option C: Replace the valve

If the leak is from the valve body itself — water seeping from around the spindle — the internal seals have failed. A new TRV costs £15–£40. Drain the system enough to remove the old valve, remove the tail (the brass fitting screwed into the radiator) with a radiator spanner, and fit the new valve and tail with PTFE tape. This is more involved than the olive change but still a 30-minute job for a competent DIYer.

When the radiator itself is leaking

If water is coming from the radiator body — not from any joint — the radiator has internal corrosion. There's no economic repair. Internal sealant products (such as Sentinel SystemSafe or Fernox F4) can sometimes plug very small pinhole leaks temporarily but they're a stop-gap, not a fix. The radiator needs replacing.

A like-for-like radiator replacement costs £150–£350 fitted in Peterborough including a new valve set and a few litres of inhibitor. See our central heating services for the full scope.

Refill and repressurise

After any repair, slowly open the lockshield valve (same number of turns as before — that's why you counted), then open the TRV. Go to the boiler and check the system pressure. It should be 1.0–1.5 bar cold. If it's dropped below 1 bar, repressurise via the filling loop (see our re-pressurising guide). Bleed any trapped air via the bleed valve at the top corner of the radiator.

When to call a plumber

• The leak is from the radiator body, not a joint
• You can't isolate the leak (closed valves don't stop the drip)
• The valves are seized and won't turn
• The leak is inside a wall or under a floor — never excavate yourself
• The repair worked but the radiator now won't heat (probably air trapped or valve closed)

If you'd rather have a plumber handle it, our plumbing repairs team covers radiator leaks across all PE postcodes. Most callouts are completed in a single visit and we carry the common olive sizes and replacement TRVs on every van.