A leaking radiator is one of the more common calls we get — and in most cases, it is straightforward to fix once you know where the water is actually coming from. The tricky part is diagnosing it correctly. Water can travel along pipework before it drips, which means the visible damp patch is often not directly above the source.
This guide covers every likely location for a radiator leak, what causes each one, what you can safely do yourself, and when to stop and call a plumber.
The Four Most Common Radiator Leak Locations
Before you grab a towel and a wrench, spend two minutes identifying where the leak is actually coming from. Feel along every fitting while the heating is running — warmth and moisture together will lead you to the source.
Valve Gland (TRV or Lockshield)
Moisture around the valve body or the gland nut. Usually a slow weep rather than a drip. Very common on older valves.
Bleed Valve
The small brass screw at the top corner of the radiator. Can weep if the washer is worn or if the valve has been over-tightened and cracked.
Pipe Joint (Below or Behind)
Where copper pipework connects to the valve or the wall. A failed solder joint or compression fitting. Requires draining to repair.
Radiator Body (Corrosion)
A pinhole through the panel itself, usually caused by internal sludge or lack of inhibitor. Rust spots on the surface are the tell-tale sign.
Before You Start: Dry Everything and Wait
Wipe the radiator and all fittings completely dry with a cloth. Lay paper towels or tissue under each suspect area. Run the heating for 20 minutes, then check each area. This confirms exactly where the moisture originates — not where it ended up.
How to Fix a Leaking Radiator Valve
Radiator valves are the most frequent source of leaks. There are two valves on every radiator: the thermostatic radiator valve (TRV) on one side, and the lockshield valve on the other. Either can develop a gland leak.
What a gland leak looks like
Moisture appears around the spindle — the moving part of the valve that you turn. On a TRV, this is where the valve head connects to the body. On a lockshield, it is the small adjustable cap. The leak is usually a slow weep, not a drip.
Fix 1 — Tighten the gland nut
Locate the gland nut
On a traditional manual radiator valve, the gland nut is the small hexagonal nut directly below the valve head or cap. On a TRV, it sits just below the thermostatic head. You may need to remove the TRV head first to access it — most TRV heads unscrew or pull off with a twist.
Tighten a quarter-turn
Use an adjustable spanner to turn the gland nut clockwise by no more than a quarter-turn. Do not overtighten — this can crack the valve body or make the spindle stiff to operate. Wipe the area dry and check again after 30 minutes with the heating on.
If the leak continues — repack the gland
If tightening does not stop the weep, the gland packing has failed. To replace it: close both radiator valves (turn the TRV to zero; close the lockshield fully, counting the turns so you can reopen it the same amount). Use a small container to catch any drips while you undo the gland nut, remove the old packing, wrap new PTFE tape several times around the spindle, and reassemble. This keeps the radiator in circuit — no full drain required.
How to Fix a Leaking Bleed Valve
The bleed valve (also called the air vent) is located at the top corner of every radiator. It is used to release trapped air from the system. A worn or damaged bleed valve is a straightforward fix.
If the bleed valve is weeping around the screw
Insert a bleed key and tighten the screw clockwise until it is firmly seated. Do not overtighten — the soft brass will strip easily. If the valve still weeps, the internal rubber washer has failed and the valve needs replacing.
Replacing a bleed valve
To replace the bleed valve without draining the whole system, you can isolate the individual radiator using both valves, then use a bleed valve replacement tool. These allow you to swap the valve with minimal water loss. If you are not confident doing this, a plumber can replace a bleed valve in under 30 minutes.
Old bleed valves can seize in place. If you force a stuck bleed valve, the valve body can shear off, leaving a hole in the radiator. If you cannot turn it smoothly with the correct key, stop and call a plumber. A replacement valve costs a few pounds. A replacement radiator is £200–£400.
How to Fix a Leaking Pipe Joint
Pipe joints fail for several reasons: original solder joints degrading over time, compression fittings that have worked loose, or pipework that has been knocked or stressed. A joint leak usually appears as a drip from below the radiator or from a fitting concealed behind skirting board.
This type of repair requires draining the affected circuit or the whole system — so it is not a job to do without some preparation. The steps are:
- Isolate the radiator (or drain the system if the joint is on a main flow or return pipe)
- Allow the pipework to cool completely before touching any joint
- If a compression fitting has worked loose, tighten the nut a half-turn and check
- If a compression fitting is leaking from the olive (the brass ring inside), it needs replacing — tightening further will not seal it
- If a solder joint has failed, the pipe section needs cutting out and re-soldering or replacing with a push-fit fitting
If the leaking joint is concealed behind a wall, under a floor, or in a position where you cannot comfortably access both sides with tools, call a professional. Attempting to repair a joint in a confined space often leads to a worse leak. This is a £120–£250 job for a plumber and typically takes less than two hours.
How to Deal With a Corroded Radiator Body
If the radiator panel itself is leaking — a pinhole in the body rather than at any fitting — the radiator needs replacing. There is no reliable long-term fix for a corroded radiator body. Leak sealant products exist, but they are a temporary measure at best and can cause problems elsewhere in the system by coating heat exchanger surfaces.
Signs the radiator body is the source
- Orange or brown rust marks on the surface of the panel
- Moisture appears on the flat surface of the radiator, not at any connection point
- Black sludge (magnetite) visible in the water when you drain the radiator
- Cold spots at the bottom of the radiator alongside a slow leak
Why corrosion happens
The most common cause is insufficient inhibitor in the central heating water. Inhibitor is a chemical treatment that prevents the steel panels from reacting with the water inside. It degrades over time and should be checked annually as part of a boiler service. In hard water areas like Slough, limescale also accumulates inside the system and accelerates wear on components.
If one radiator has corroded, it is worth checking the inhibitor level across the whole system before fitting the replacement — otherwise the new radiator will be exposed to the same corrosive conditions.
When to Turn Off the Heating and Call a Plumber
Call a plumber if any of these apply
- The leak is an active drip rather than a slow weep
- Water is reaching the floor, skirting, or the ceiling below
- The leak source is a pipe joint, not a valve or bleed point
- The radiator body itself is corroded through
- The boiler is losing pressure repeatedly (indicates a system leak rather than a single radiator issue)
- You cannot isolate the radiator — valves are seized or broken
- The leak is on a first or second floor and below is a bedroom or living space
- Multiple radiators show signs of rust or sludge (system-wide issue — requires a power flush)
Radiator Leak Repair Costs
Typical Pricing Guide
Tighten gland nut / minor valve adjustment: £80–£120 (if part of a call-out)
Replace TRV valve head or repack gland: £100–£180 including parts
Replace bleed valve: £80–£150
Re-make a leaking pipe joint: £120–£250 depending on access
Replace a corroded radiator panel: £200–£400 (standard single or double panel, parts and labour)
Power flush (where sludge is present): £300–£500 for a typical UK home
Prices are guide rates for Slough and Berkshire. Final cost depends on access, parts required, and whether draining is needed. All Manako Plumbing quotes are fixed price before work begins.
Preventing Radiator Leaks Long-Term
Most radiator leaks are preventable with basic annual maintenance. The key steps are:
- Annual boiler service: includes a system pressure check, inhibitor test, and visual inspection of radiators and valves
- Keep inhibitor levels up: should be topped up every two to three years in a typical system
- Bleed radiators when cold spots appear: trapped air causes uneven heating and increased corrosion risk
- Do not paint over radiator valves: paint can hide early valve deterioration and seize the gland mechanism
- Check system pressure monthly: a combi boiler should sit between 1.0 and 1.5 bar at rest. Repeated drops indicate a leak somewhere in the circuit
Related Guides and Services
- Heating Repair — Manako Plumbing — radiator valve replacement, power flushing, and heating system repairs
- General Plumbing Services — pipe repairs, leak detection, and general plumbing work
- Boiler Servicing — annual service includes inhibitor check and system pressure test
- Cold Radiators? How to Bleed a Radiator — step-by-step guide to bleeding air from the system
- Low Water Pressure — if your boiler pressure keeps dropping, there may be a system leak
Got a Leaking Radiator in Slough or Berkshire?
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Get a Free QuoteFrequently Asked Questions
Why is my radiator leaking from the bottom?
A leak from the bottom of a radiator usually comes from one of three places: the thermostatic radiator valve (TRV) connection to the radiator, the lockshield valve on the opposite end, or a pipe joint where the copper pipework connects underneath. Run your finger around each fitting when the heating is on and feel for moisture. Tightening the valve connection or wrapping the valve tail with PTFE tape can sometimes resolve a minor leak. If the leak is from the pipe joint itself, call a plumber — this requires draining and re-making the joint.
Can a radiator leak cause damp or structural damage?
Yes, if left unaddressed. Central heating water is treated with inhibitor and can cause staining, damp patches, and damage to flooring, skirting boards, or ceilings below. If the radiator is on an upper floor, a slow drip can go unnoticed until it saturates the ceiling below. If you can see a water stain but no obvious source, check the radiator above. Any sustained leak that contacts timber joists or structural elements should be treated as urgent.
Is it safe to keep using the heating with a leaking radiator?
It depends on the severity and location of the leak. A very minor valve gland weep — where moisture appears but does not drip — can be monitored short term while you arrange a repair. However, running the boiler with a significant leak will gradually reduce system pressure, which will eventually cause the boiler to lockout on a low pressure fault. It will also cause water damage to the surrounding area. If the leak is active (dripping), switch off the heating and call a plumber promptly.
How do I stop a radiator valve from leaking without turning off the heating?
For a gland nut leak on a manual valve, you can try tightening the gland nut (the small nut just below the valve head) by a quarter-turn while the system is running. Do this carefully — too much force can worsen the leak. For a TRV pin-seal leak, the only safe fix is to isolate the radiator using its lockshield valve, allow it to cool, and then replace the TRV head or the valve body. Attempting to work on a pressurised, hot radiator carries a burn risk.
What causes a radiator to rust or corrode?
Central heating systems without inhibitor are highly prone to internal corrosion. Oxygen in the water reacts with the mild steel radiator panels, producing magnetite — a black sludge that settles at the bottom of radiators and blocks pipework. External corrosion on the radiator body typically appears as orange-brown rust spots on the surface. This is often caused by a minor internal pinhole leak that allows water to weep through the panel. Once the radiator body is corroded through, it cannot be reliably repaired and should be replaced.
How much does it cost to fix a leaking radiator?
Tightening a valve gland nut or fitting a new valve head: £80–£150. Replacing a thermostatic radiator valve (TRV): £100–£200 including parts. Re-making a leaking pipe joint: £120–£250 depending on access and whether the system needs draining. Replacing a corroded radiator panel: £200–£400 for a standard single or double panel (parts and labour). In all cases, if sludge is present and multiple radiators are affected, a power flush may be recommended alongside the repair: typically £300–£500 for a whole-house flush.