Do this first
- Container under it, towel round the pipe. Heating water is black with magnetite and it stains carpet, timber and grout permanently within the hour.
- Close both valves. Thermostatic head down to its off setting or the decorator's cap fitted, then the lockshield at the other end closed clockwise with a spanner, counting turns so the balance can be restored later.
- Dry the whole radiator and watch where the first bead reappears. This is the diagnosis. Do it before you start tightening anything.
- If both valves are shut and it is still leaking, the water in the radiator is what you are seeing. That will run until the panel is empty, so keep the container under it and be ready to change it.
- Do not top the boiler up repeatedly to keep the pressure up while it drips.
- Check the floor under the radiator and the ceiling below it. Radiator leaks are slow and old before they are noticed.
Valves and joints are fixable. The panel is not.
Everything hangs on whether the water is coming from a connection or from the radiator itself, and the answer is visible once the radiator is dry.
| Where the bead forms | What it is | Outcome |
|---|---|---|
| Under the valve head at the spindle | Dried gland packing | Tighten or repack. Minutes, no drain-down. |
| At a union or compression nut at either end | Olive or tail joint relaxed | Nip up or remake the joint |
| At the bleed valve at the top corner | Bleed screw seat or seal | New bleed plug |
| On the face or underside of the panel, often with a rust bloom or a brown tear-stain | Corrosion pinholing through from inside | Replacement radiator |
| From a blanking plug at a bottom corner | Plug seal failed | Reseal or replace the plug |
A bead forming on the panel itself is the one that ends the conversation. Corrosion works outwards from the inside, driven by oxygen in the system water, and by the time it has come through the steel the rest of that panel is the same age and in the same condition. Radweld and similar sealers can close a pinhole for a season, but they also circulate through the pump and the valves, and they do nothing about the next pinhole two feet along. The honest answer is a new radiator. If the drip is at a valve rather than the panel, a radiator leaking from the valve goes into that in more detail.
One thing worth doing while the system is exposed: if the water coming out is black and gritty, the system is carrying sludge, which is what accelerated the corrosion in the first place. That is an argument for a power flush and an inhibitor dose at the same visit, not a separate problem.
What the repair involves
A valve, union or bleed plug repair is done between the closed valves, tested hot, and the lockshield reset to the turns it was on so the rest of the house still balances. A radiator replacement means isolating, draining the panel, unscrewing the old tails, fitting new tails and valves to the new panel, hanging it on brackets set to the right height, refilling, venting every radiator in the house, re-pressurising and running the system to temperature to recheck.
The floor under a radiator is almost always the hidden part of the job. Slow drips sit in one spot for months, swelling chipboard, staining boards and rotting the top of a joist. We lift, dry and reinstate as part of radiator and valve leak repair, including skirting, floor finish and decoration, rather than handing back a fixed radiator and a ruined floor. If the boiler gauge has been dropping alongside this, read a boiler losing pressure as well, because a radiator rarely explains all of it.
When it is not us
- A quarter turn on a gland nut stopped it. Done. No call needed.
- Cold at the top and no leak at all. Air. Bleed it, top up to 1.0-1.5 bar cold, and carry on.
- Cold at the bottom and no leak. Sludge in the bottom of the panel. A flush question, not a leak question.
- The radiator is simply old and you want it changed. That is planned work, and any competent heating installer can do it. Worth pricing as a batch if several are the same age.
- You rent. Report it to the landlord in writing. Heating installations are theirs to keep in repair.
- Anything inside the boiler casing. Gas Safe registered engineers only, whatever the fault looks like from outside.