Pro Plumbers · 6 November 2026
Leaking radiator valve: gland nut, valve body or pipe joint?
A wet patch on the carpet under a radiator, a damp skirting board or a brown stain on the ceiling below are all clues that a radiator valve may be leaking. The fix depends almost entirely on exactly where the water is coming out. This guide works as a triage: three likely leak points, how to recognise each, and what is sensible for you to try before calling an engineer. If you want background on what each valve does, our post on radiator valves explained covers that, so we will not repeat it here.
Do these first
- Turn the heating off and let the radiator cool. Hot water and hot metal make it harder and less safe to work.
- Put down a towel and a bowl. Fold a towel under the valve and place a shallow dish where the drip lands.
- Dry everything. Wipe the valve, the nuts and the pipe below with a dry cloth so you can see where fresh water appears.
- Isolate the radiator if the valves turn. Close the valve at each end of the radiator. One is usually the control or thermostatic valve; the other, often under a plastic cap, is the lockshield. Count the turns on the lockshield as you close it so it can go back to the same setting. If either valve will not turn freely, do not force it.
Isolating the radiator stops fresh water feeding the leak, though some water already in the radiator may still drip out.
Leak point one: under the cap or around the spindle
What it looks like: water seeping from the top of the valve, where the head or cap meets the body, often leaving a crust or a bead of water around the spindle. This usually comes from the gland nut or the packing around the spindle, which seals the moving part of the valve.
What you can try: on many valves, a small hexagonal nut sits just under the head. A gentle nip, a fraction of a turn with a correctly sized spanner while holding the valve body steady with a second spanner, can sometimes stop a weep. Never over-tighten. If a small nip does not stop it, stop there. Turning harder can strip the thread, crack the valve or make the leak worse.
When to stop: if the nut will not move, the valve turns with the spanner, or the drip continues, leave it and call. The packing may need replacing or the valve may be past repair.
Leak point two: from the valve body itself
What it looks like: water appearing from the main brass or chrome body of the valve rather than any nut, sometimes with green or white corrosion, pitting or a visible hairline crack.
What you can try: very little. A cracked or corroded body cannot be tightened into sealing. Keep the radiator isolated if possible and contain the drip.
What usually happens next: the valve is replaced. This means draining some or all of the system, fitting a new valve, refilling and checking for leaks. It is a job for radiator repair. If the radiator itself is old, rusting or the wrong size for the room, you might choose to replace both together through radiator installation and replacement.
Leak point three: the union nut or the pipe joint below
What it looks like: water at the large nut where the valve joins the radiator tail, or lower down where the valve meets the pipe from the floor or wall. The valve may look perfectly dry on top.
Why it happens: the radiator tail, the union or a compression joint on the pipe can lose its seal over time, particularly after the radiator has been taken off for decorating, or where the pipe has been knocked.
What you can try: hold a dry tissue around each joint in turn to confirm which one is wet. A very slight weep at a compression joint can sometimes be eased with a careful quarter turn, again using two spanners so you do not twist the pipe. If you are unsure, or the joint is old and stiff, do not attempt it. Resealing a union or tail usually needs the radiator isolated and partly drained, which is best left to an engineer.
After the leak stops: pressure and air
Any water lost from a sealed heating system lowers the pressure, and any work on a radiator can let air in. Once the leak is fixed and the radiator is back in use, check the boiler gauge against the range marked on it or in your manual. If it is low, follow our guide to repressurising a boiler. You may need to bleed the radiator once it is warm, then check the pressure again. If the gauge keeps falling, our post on a boiler losing pressure explains what that points to.
Water dripping down onto a boiler from a radiator above is easy to mistake for a boiler fault. Our guide to a boiler leaking water helps you tell the two apart.
Time to call us
Call rather than carry on if:
- water is spraying or running rather than dripping;
- the valves are corroded, seized or turn without closing;
- you cannot isolate the radiator;
- there is a damp patch or stain on the ceiling below;
- several radiators show leaks, which may point to a wider issue a heating engineer should look at.
Our Sevenoaks team takes calls on 01732 444555, Monday to Friday 7am to 10pm and Saturday to Sunday 9am to 8pm, or you can send details and a photo through our contact page. Our minimum charge is £90 + VAT, including the first hour on site. Larger jobs get a fixed, no-obligation price.