How do I fix a leaking radiator?
Finding where a radiator is leaking, the temporary fixes that hold, and when it needs a plumber.
- Difficulty
- intermediate
- Time
- 1 hr 30 min
- Read
- 2 min
- Safety
- warning
Short answer
Dry everything and find the exact source — valve, spindle, bleed point, or the radiator body itself. Valve and spindle leaks often stop with a quarter turn on the gland nut or new PTFE tape. A leak from the body means corrosion, and that means replacement.
Radiator leaks come from four places, and the fix differs completely for each. Water tracks along pipes and pools somewhere else entirely, so the first job is always finding the actual source rather than the puddle.
Safety
What you'll need
- Towels and a shallow container
- Adjustable spanner
- PTFE tape
- Radiator bleed key (optional)
- Leak sealer as a temporary measure (optional)
Step by step
- Turn the heating off and let the radiator cool.Working on a hot pressurised system risks scalding.
- Dry everything completely and put dry kitchen roll around each joint.The paper shows exactly where water reappears, which is the only reliable way to find the source.
- Wait and check: the source will be the valve body, the spindle, the bleed point, the union nut, or the radiator itself.Water runs along pipework, so the puddle is rarely under the leak.
- For a leak from the valve spindle, try a quarter turn on the gland nut.Just nipping it up compresses the packing and often stops it. Do not overtighten.
- For a leak at the union nut where the valve meets the radiator, undo it, wrap the thread with PTFE tape and reassemble.Drain the radiator first. Ten to fifteen wraps in the direction the nut tightens.
- For a weeping bleed valve, close it fully and, if that fails, replace it.They are inexpensive and need the radiator isolated and drained.
- For a leak from the radiator body itself, plan to replace the radiator.It is corroded from the inside. Sealant products buy time; they do not fix corrosion.
- After any work, repressurise a sealed system to the boiler's stated range and check for leaks again.Typically 1–1.5 bar cold, but check your boiler's guidance.
Tips
- A container and towels under the leak buys you time to plan rather than panic. A slow radiator leak is rarely an emergency.
- Repeated leaks and sludgy black water suggest system corrosion. A chemical clean and inhibitor is the underlying fix.
- Photograph the valve arrangement before dismantling anything — reassembly order matters.
Common mistakes
- Overtightening the gland nut — It crushes the packing and can make the leak worse or seize the valve.
- Assuming the puddle is under the leak — Water tracks along pipes. Dry everything and use paper to find the true source.
- Relying on leak sealer for a corroded radiator — It is a temporary measure. A pinholed radiator will keep failing.
Questions people ask
Why is my radiator leaking from the valve?
Usually the gland packing around the spindle has hardened, or the union nut has loosened. Both are straightforward: a quarter turn on the gland nut, or new PTFE tape on the union thread.
Can a leaking radiator be repaired or does it need replacing?
Valve, spindle and joint leaks are repairable. A leak from the radiator body itself is internal corrosion, and replacement is the only real fix.