Quick answer: Dry the radiator thoroughly, then wrap toilet tissue around each joint, valve, and seam to find exactly where water reappears. A leak at the valve is usually a worn spindle packing, fixed by tightening or repacking the gland nut. A leak from the radiator body itself, such as a pinhole, usually means the radiator needs replacing rather than repairing.
A leaking radiator looks alarming, but in most cases the source is a straightforward valve or joint problem rather than a failure of the radiator itself. Working out exactly where the water is coming from is the key step, since the fix depends entirely on that location.
First: Stay Calm and Protect the Floor
Place old towels or a bucket underneath the leak immediately to catch water and protect flooring. This buys you time to work out what is actually wrong without adding water damage to the problem.
Locating the Source of the Leak
- Dry the entire radiator and surrounding pipework thoroughly with a towel, since a wet radiator makes it impossible to tell where new water is coming from.
- Take a piece of toilet tissue or kitchen roll and methodically press it against each joint, valve, and seam in turn.
- Watch for where the tissue becomes wet, which pinpoints the exact source rather than leaving you guessing.
Radiator leaks generally come from one of three areas: the valve or its connections, a joint or union nut, or the radiator body itself, most often through corrosion-related pinholes.
Fixing a Leaking Valve
A leaking valve is usually caused by worn spindle packing, the material that seals around the valve spindle where it moves. This type of leak often shows up when the valve is partially open and can sometimes be stopped just by closing it fully.
- Turn off the radiator using the valve at each end. Note how many turns it takes to close the lockshield valve, so you can return it to the same setting afterwards.
- Drain the system down to a level below where the leak is occurring, following your heating system’s manual for the correct drain point.
- Place towels and a bucket beneath the valve to catch any remaining water.
- Use a spanner to tighten the gland nut slightly and check whether this stops the leak.
- If tightening alone does not work, undo the gland nut fully and wrap PTFE tape into the spindle area before retightening.
- Check for leaks once the system is refilled and the radiator is back up to pressure.
Fixing a Leaking Joint or Union Nut
If the leak is coming from a pipe joint rather than the valve itself, the fix usually involves resealing rather than repacking.
- Drain the heating system down below the level of the joint.
- Loosen and remove the union nut connecting the pipework.
- Clean the threads thoroughly, then apply fresh PTFE tape or a suitable jointing compound.
- Reconnect and tighten the nut, then refill the system and check for any remaining leaks.
Isolating a Radiator Without Draining the Whole System
In many modern wet central heating systems, you can isolate a single radiator using the valves at each end, allowing repairs without draining the entire house.
- Close both the supply valve and the lockshield valve fully. Do not force them, since a stuck valve forced too hard can snap the spindle and cause a far larger leak.
- Check whether the valve fully isolates by opening the bleed valve slightly. If water still trickles out, this indicates a passing valve rather than a failed one, meaning it does not fully seal even when closed.
- Slowly open the bleed valve using a radiator key to release internal pressure safely before loosening any joints, since this prevents a sudden release of water.
- With a tray or bowl positioned under the connection, carry out the repair on the isolated radiator without affecting the rest of the heating system.
Pinhole Leaks and Corrosion
A pinhole leak, typically a small, steady weep from the flat surface of the radiator rather than a joint, indicates internal corrosion caused by sludge building up inside the heating circuit over time and gradually thinning the steel. Unlike a valve or joint leak, this cannot be repaired, only patched temporarily.
- A temporary sealant can be added, but only in systems with a feed-and-expansion tank. Sealants should never be used in a fully sealed heating system, since they can cause blockages elsewhere.
- Corroded steel with a pinhole leak ultimately needs the radiator replaced, since the underlying metal thinning will continue and the leak is likely to return or worsen.
- After replacing a corroded radiator, add a corrosion inhibitor to the refilled system to help protect the rest of the radiators from the same issue developing.
Sealed vs Open-Vent Heating Systems
Before using any sealant product, check what type of heating system you have. Feed-and-expansion tank systems, common in older UK homes with a tank in the loft, can generally tolerate a temporary sealant. Fully sealed, pressurised systems cannot, since sealant residue can circulate and cause blockages in pumps or valves elsewhere in the system. If you are unsure which type you have, it is worth checking with a heating engineer before adding any product to the system.
Do You Need to Top Up the System Afterwards?
If you lost a noticeable amount of water while draining or repairing, check your boiler pressure gauge once everything is reconnected. Most combi boiler systems should sit between 1.0 and 1.5 bar when the heating is off, and can be topped up via the filling loop if needed, following your boiler’s manual.
When to Call a Heating Engineer
Call a professional if a valve is seized and will not turn without excessive force, if you are dealing with a sealed system and unsure about sealant compatibility, if the radiator has multiple leaks suggesting widespread corrosion, or if repairing the valve would affect a warranty you want to keep intact.
Related Guides
Frequently Asked Questions
Can you fix a leaking radiator without draining the whole heating system?
Yes, in many cases. Most wet central heating systems let you isolate a single radiator using its own valves, allowing you to carry out repairs without draining the entire house.
What causes a radiator to develop a pinhole leak?
Pinhole leaks are usually caused by internal corrosion, where sludge accumulation inside the radiator gradually thins the steel until small holes form.
Is it safe to use radiator sealant in a sealed heating system?
No. Sealant is only suitable for systems with a feed-and-expansion tank. In fully sealed, pressurised systems, sealant residue can circulate and cause blockages elsewhere.
How do I know if my radiator leak is from the valve or the radiator body?
Dry the radiator fully, then press tissue against each valve, joint, and the radiator’s flat surfaces in turn. A leak at a joint or valve will show as damp tissue there, while a leak from the body itself, often a pinhole, will show up on a flat section rather than a connection point.
Final Thoughts
Most radiator leaks trace back to a worn valve seal or a loose joint, both of which are manageable repairs once you have located the exact source with a simple dry-and-tissue test. A pinhole leak from corrosion is the one exception that calls for replacement rather than repair, so it is worth ruling that out early before spending time on a valve fix that will not solve the actual problem.