Central Heating Leak Sealer: Does It Work?

What leak sealer actually does

Central heating leak sealers are liquids you add to the water in the system. The best known in the UK are Fernox Leak Sealer F4, sold in a 500ml bottle, and Sentinel Leak Sealer, sold in a 1 litre bottle, plus aerosol versions such as Fernox F4 Express that inject through a radiator bleed valve. Once mixed in, the product circulates with the system water until it reaches a point where water is escaping and meeting air. There it reacts and builds up a plug that closes the hole.

Each bottle is dosed for a system of a stated size, and the label says how much water it treats and how long to circulate it before judging the result. On an open vented system it goes in through the small feed-and-expansion tank in the loft. On a sealed system it goes in through a dosing point on a magnetic filter, an injector, or a towel rail with the bleed valve removed.

When it works, and when it will not

Leak sealer is designed for weeps: pinholes and hairline gaps where you lose a small amount of water slowly. The classic case is a boiler that drops from 1.5 to 0.8 bar over a week, with no visible water anywhere, because a pipe is weeping under a concrete floor or inside a wall.

• Likely to help: a slow pressure loss of a fraction of a bar per day with no visible leak, on a system in otherwise good condition
• Unlikely to help: a drip you can see from a radiator valve, a compression joint or a bleed screw. These are mechanical faults. A new olive, a repacked valve gland or a new valve fixes them properly
• Will not help: a split pipe, a burst, or water running from the boiler. The gap is too big to bridge
• Should not be used for: a leak inside the boiler casing. That is a boiler repair
• Wrong fix: a pressure relief valve dripping outside. The pressure is climbing too high when the heating runs, usually because the expansion vessel has lost its charge, and the water dumped there is what drops the gauge when it cools

If the pressure is dropping, first rule out the pressure relief valve. Look at the copper pipe that leaves the boiler and goes through the outside wall. If it drips or has a white crust at the end, the problem is not a leak in the pipework.

The risks before you pour it in

Leak sealer is not risk-free, and the risk sits with the boiler rather than the pipes. Modern combi boilers have very narrow waterways in the plate heat exchanger and small passages in the pump and diverter valve. The makers of the sealers say their products do not block these when dosed correctly. Many boiler engineers are more cautious, and the conditions of some manufacturer guarantees exclude damage caused by additives in the system water. If your boiler is still under its guarantee, read the terms or ask the manufacturer before you add anything.

The other risk is that the sealer hides a problem that keeps growing. A weep under a floor is usually corrosion, and corrosion does not stop because one pinhole has been plugged. Water can also be soaking into a ceiling or a timber floor before the sealer closes the hole. If you have damp patches, staining or a musty smell, the leak needs finding rather than sealing.

Finally, think ahead. When a new boiler goes in, the system water should be cleaned first, and an engineer will want to flush out old sealer and sludge along with it.

How to use it properly if you decide to

If the leak is hidden, slow and on a system out of guarantee, sealer is a reasonable thing to try before lifting floors. Follow the bottle; these are the points people miss.

• Dose for the size of your system. Too little and it will not seal; the label gives the ratio
• Circulate it. Run the heating for as long as the label says, with some heat on, so the product reaches every part of the circuit, then leave the system running as normal
• Keep the pressure up. The sealer only works while water is being pushed out through the hole, so on a sealed system keep it at about 1 to 1.5 bar cold
• Give it time. Watch the gauge for a week. If the pressure still drops at the same rate, the leak is too big or in the wrong place for sealer
• Top up the inhibitor. Sealer is not a corrosion inhibitor, and most systems with a weep have too little inhibitor in them already, which often shows as dark water around the leaking radiator

What an engineer does instead

Before reaching for sealer, an engineer works through the places water can leave the system. First the pressure relief valve and the expansion vessel charge. Then every radiator valve and visible joint, the towel rails, the filling loop and the cylinder coil connections. Then the boiler itself. Most pressure-loss calls end at one of those, not under the floor.

If the leak is genuinely hidden, the engineer can isolate parts of the system and watch which section loses pressure, which narrows the search to one run of pipe. A section of pipe under a floor can often be abandoned and rerun along a skirting board for less than the cost of digging up concrete.

Our guide to a boiler losing pressure with no visible leak covers the checks you can make yourself. When you want it found, book central heating repair, or boiler leak repair if the water is coming from the boiler. The £60 call-out covers the visit and the repair price is agreed with you on site before any work starts. Gas work is carried out by Gas Safe registered engineers.