Leaking Radiator? Where It's Coming From and What to Do

Find the actual source first

Dry everything completely, then lay kitchen roll at four places: under the valve on each side, around the spindle where the head screws on, and along the bottom seam of the panel. Come back in an hour. Whichever paper is wet tells you which of four very different jobs this is, and they range from a 50p olive to a new radiator.

A weeping valve or nut

The most common and the least serious. The compression nut where the valve meets the tail has loosened or the olive has hardened. A quarter turn with two spanners, one holding the valve body so you do not twist the pipe, stops most of them. If it still weeps, the olive needs replacing, which means draining down.

A leaking spindle

Water appearing from under the plastic head when the valve is turned is a failed gland. Some valves have a gland nut you can nip up; many modern ones do not and the valve gets replaced. Either way it is a valve job, not a radiator job.

A pinhole in the panel

A rust-coloured weep from the bottom of the panel itself is corrosion from the inside out, and there is no repair worth having. Sealant additives buy weeks, not years, and they can clog the boiler while they do it. This one is a replacement radiator, and the cause is usually system sludge from missing inhibitor.

Stop it now, then deal with it

Close both valves, the thermostatic one and the lockshield at the other end, counting the turns on the lockshield so it goes back to the same balance point. That isolates the radiator without draining the system. Bowl underneath, towels down, and it will hold until an engineer arrives.

Why it matters more than it looks

A slow radiator leak drops system pressure, so the boiler gets topped up with fresh oxygenated water again and again. That oxygen is what corrodes the rest of the radiators. One weeping tail ignored for a year is how a whole system fills with sludge.

Valve, union or panel, how to tell them apart

Dry everything with kitchen roll, then watch where the first drop reappears. Water at the spindle under the plastic head means the gland packing has failed, and that is often fixed by tightening the gland nut a quarter turn or repacking it with PTFE. Water at the union nut where the valve meets the radiator means the joint has loosened, usually from expansion cycling over years. Water on the underside of the panel itself, with no fitting above it, means the steel has corroded through from the inside. The first two are repairs measured in minutes; the third is a replacement.

Why the bottom of the panel is where they fail

Magnetite sludge is heavier than water, so it settles along the bottom of every radiator in the system. Sitting against the steel it holds moisture and oxygen against one spot, and eventually eats a pinhole from the inside out. That is why leaks appear at the bottom of a panel far more often than the top, and why a radiator can look perfect externally right up until it weeps. It is also why the leak is a system message rather than a component failure.

What to do in the first ten minutes

Put a container underneath and towels around it. Close both valves, the thermostatic head clockwise to zero, and the lockshield at the other end with a spanner, counting the turns so it can be reset to the same position afterwards. That isolates the radiator without draining the system. Do not force a valve that has scaled solid: snapping a spindle turns a drip into an open pipe, and in a flat that becomes the ceiling below. If a valve will not move, leave it and call.

Repair, replace, or treat the whole system

A leaking valve or union is a repair, typically within an hour. A pinholed panel means that radiator is finished. But replacing it alone treats the symptom, the same corrosion is working on every other radiator on the circuit. The proper response is a system water test, a power flush if the sample is dirty, correct inhibitor dosing and a magnetic filter fitted permanently. In Greater Manchester, where soft Pennine water is mildly aggressive to steel and copper, this matters more than it does in hard-water London, where scale rather than corrosion is the dominant failure.

Leaking Radiator? Where It's Coming From and What to Do, your questions