Central heating leak sealer: what it does, and why it makes the leak harder to find

How leak sealer works

Leak sealer is a liquid added to the heating, through a radiator on a sealed system or the header tank on an older open-vented one. It mixes with the water and goes everywhere the water goes. Where water escapes through a pinhole, the sealant reacts with the air and forms a plug over the hole from the inside.

On a tiny weep in a copper pipe or a radiator, that can stop the loss, at least for a while. The pipe is still corroded, the hole is still there, and whatever made it is still happening.

Why ADI does not recommend it

There are three problems, and the third one is why this guide exists.

• It is a temporary patch. The sealant masks the leak rather than repairing it. A pipe that has corroded through in one place is often thin elsewhere.
• It can harm the system. The same stuff that plugs a pinhole circulates through the boiler, the pump and every valve. Not every system is compatible, and ADI's guidance is that sealant can cause damage to heating components.
• It gets in the way of finding the leak. Tracer gas and pressure testing rely on gas or air escaping at the leak. Sealant partly blocking the hole changes that, and residue in the pipes can stop the tests giving a clear answer.

What it does to a leak survey

Part of a heating leak survey is draining the system and filling it with tracer gas or air under pressure. At the leak the gas escapes, rises to the surface and is picked up by a sniffer, and the gauge shows the pressure falling. A half-sealed pinhole lets out less gas, or none at all, which makes an already difficult job slower and less certain.

ADI's booking terms ask you to tell the office if sealant has gone in, and advise having it removed with a cleaning chemical or a power flush before testing. Even then, residues are known to stay in the system. That is why the office asks about it on the phone: it changes how the job is planned.

If you have already used it

You are not the first, and it is not a disaster. Tell the office when you book, say roughly when it went in and what product it was. If the pressure is still dropping, the sealant has not worked, and the leak still needs finding. If the pressure has stopped dropping for now, keep an eye on the gauge: it may start again.

What to do instead

Work through the checks that cost nothing first: the relief valve pipe outside, every radiator valve, a sheet of blue paper towel under the boiler. If those are dry and the pressure still falls, the leak is hidden, and the quickest way to a permanent fix is to find it and repair the pipe. The full list of checks.