A boiler that needs topping up every week is losing water somewhere, and the bottle of leak sealer on the shelf at the merchant looks like the cheap way out. Sometimes it is. A liquid sealer can close a pinhole weep that nobody could reach anyway. It will not close a joint that is pulling apart, a split pipe or a leak inside the boiler, and a sealer added to the wrong system can leave you with a blocked heat exchanger as well as the original leak.
This guide covers what these products actually do, which leaks they suit, how they are added to the sealed systems most London homes now have, how long to give them, and the signs that the leak needs finding instead.
How does central heating leak sealer work?
The water in a heating circuit is largely starved of oxygen. Where a pinhole or a hairline weep lets water out, air meets the escaping water at that one point. Internal leak sealers are liquids that circulate harmlessly through the system and react where they meet that oxygen, forming a plug in the hole from the inside. Fernox, one of the best known makers, describes its F4 product as seeking out the point of oxygen ingress and forming a polymer that seals the pinhole.
Two things follow from that. The sealer has to reach the leak, so it only works while the pump is circulating it. And it can only plug a gap small enough for that plug to hold, which means weeps and micro leaks, not anything you could see dripping from across the room.
Which leaks can a sealer actually fix?
| Leak | Sealer likely to help? | Why |
|---|---|---|
| Slow pressure loss, no drip anywhere you can see | Sometimes | This is the case these products are made for: a small, hidden weep under a floor or inside a wall. |
| Pinhole in a radiator panel | Briefly, at best | A radiator corroded through from the inside has thin metal all around the hole. The next pinhole is usually close behind. |
| Weeping valve spindle or union nut | No | These are mechanical seals. Tightening, repacking or replacing the part is the fix, and our radiator valve guide covers which is which. |
| Compression or soldered joint pulling apart | No | The gap moves every time the pipe heats and cools. A plug will not hold it. |
| Split or burst pipe | No | Far too large. Isolate it and have it repaired. |
| Water dripping from the boiler itself | No | That is a component inside the casing, a job for a Gas Safe registered engineer. Our guide to a boiler leaking water explains the likely parts. |
| Pressure relief pipe dripping outside | No | That is the system over-pressurising, not a hole. See why the relief pipe drips. |
In other words, a sealer is a reasonable try when the pressure falls slowly, nothing is visibly wet, and the boiler itself is dry. Everything else on the list has a better answer.
Before you add anything: three checks that cost nothing
1. Make sure the water is actually leaving. A sealed system can lose pressure with no leak at all when the expansion vessel has lost its air charge. The tell is a pressure gauge that swings widely between cold and hot, often with the relief pipe outside dripping when the heating is up to temperature. Sealer does nothing for that.
2. Look where water goes when you cannot see it. Under the boiler, along exposed pipes in cupboards and airing cupboards, at every radiator valve with a dry tissue wrapped round it, and on the ceiling below any bathroom or radiator upstairs. A brown ring on a ceiling is often the first visible sign of a heating leak in a first floor void.
3. Time the loss. Note the pressure cold in the morning, top up to about 1 to 1.5 bar if needed, and note how many days it takes to fall back. A loss measured in weeks suits a sealer. A loss you can watch within a day or two usually means a leak too big for one, and our guide to a boiler that keeps losing pressure walks through narrowing it down.
Can leak sealer damage the boiler or void the warranty?
It can, and this is the question to answer before the bottle is opened. The sealing reaction is meant to happen at the leak, but a boiler has narrow waterways in its heat exchanger and small parts in its pump and valves, and any trapped air in the system is another place oxygen meets the sealer. Manufacturers of the sealers state their products do not block boilers when dosed correctly. Some boiler manufacturers' warranty terms restrict chemicals that can be added to the heating circuit, so check the documents for your boiler before dosing one that is still under warranty.
Three situations where we would not add one at all: a boiler within its warranty period, a system that has not been cleaned in years and runs black water from the bleed valves, and any system where the leak could be reached and repaired in an afternoon.
How do you add leak sealer to a sealed system?
Follow the instructions on the product you buy, because the method differs between liquid bottles and pressurised injectors. The general steps for a sealed system, which is what a combi boiler runs, are:
- Turn the heating off and let the system cool completely.
- Check the dose. Fernox rates its 500ml F4 as treating 100 litres of system water, or up to 10 single radiators. Larger homes may need more than one.
- Add it through a radiator, usually the highest one, either by draining down slightly and introducing it through the bleed point with an injector, or through a magnetic filter if one is fitted.
- Top the system back up to its normal cold pressure through the filling loop, then close the filling loop fully.
- Bleed air out of every radiator. Trapped air is where sealer can react in the wrong place.
- Run the heating with the pump on so the product circulates through every circuit, including any that are normally shut off.
Open vented systems with a small feed and expansion tank in the loft are dosed through that tank instead, drawing water off at a drain cock so the product is pulled into the circuit.
How long does it take, and how do you know it worked?
Fernox quotes 1 to 24 hours for its F4 product to take effect. Give it that, then go back to the log you kept before dosing. If the pressure now holds for twice as long as it used to, it has done something. If it holds for a month, it has probably sealed the weep.
If the pressure still falls at the old rate after a few days of running, the leak is not one a sealer can close, and adding a second bottle rarely changes the answer. At that point you have spent the price of the product and the leak still needs finding.
What happens if leak sealer does not work?
Then the question becomes where the water is going, and on a heating circuit that is a solvable problem without lifting floors on a guess.
A leak detection survey on a heating system usually starts by isolating circuits and pressure testing each one, so the loss is pinned to one section of pipe rather than the whole house. Thermal imaging then follows the hot pipe routes under floors and behind plaster with the heating running, and a leak shows as a warm bloom spreading out from the line of the pipe. Where the pipe is buried deep in screed or the leak is too slow for thermal to show, the circuit can be drained, filled with tracer gas and the gas picked up at the surface. Our guide to the signs of a central heating leak covers what each symptom points to.
Leak Detect London surveys heating leaks across London. A thermal imaging survey runs £150 to £250 and a combined survey using thermal, acoustic and tracer gas £250 to £500, with the figure confirmed before we start. If the heating is losing pressure and a sealer has not stopped it, call 020 3912 0200 with your pressure log to hand.
Frequently asked questions
Does central heating leak sealer actually work?
On small hidden weeps and pinholes, often yes, because it plugs the hole from the inside where oxygen meets the escaping water. On failing joints, valve seals, split pipes or leaks inside the boiler, no. Those need a repair.
How long does central heating leak sealer last?
A seal that holds after the first weeks can last as long as the pipe around it. The catch is that the pipe or radiator which developed one pinhole is often corroding elsewhere, so a sealed system can start losing pressure again from a new point.
Can I use leak sealer in a combi boiler system?
Combi boilers run sealed systems, and most sealers have a version for sealed systems. Check the boiler's warranty terms first, because some restrict what can be added to the heating water, and make sure the system is bled of air before the sealer circulates.
Will leak sealer fix a boiler that is dripping water?
No. Water dripping from the boiler comes from a part inside the casing, such as a pump seal, pressure relief valve, condensate trap or the heat exchanger. That needs a Gas Safe registered engineer, not a chemical.
Is it cheaper to use a sealer or have the leak found?
For a very slow loss with nothing visibly wet, a sealer is cheaper to try first. If the pressure is dropping within a day or two, or you can see staining on a ceiling, finding the leak is usually cheaper than repeated bottles and the water damage building up while you wait.
