Sometimes. Leak sealer can close a tiny weep or pinhole, usually within 1 to 24 hours, and the makers rate it only for small, inaccessible leaks. It plugs the fault rather than fixing it. Bigger leaks stay, an overdose can block the boiler, and a sealed leak gives your insurer nothing to see when you claim.
You have topped the boiler up again, and the bottle on the merchant's shelf promises to sort it for less than a call-out. Search for leak sealer central heating advice and you get four brands and a lot of forum arguments. Here is one: when sealer earns its keep, when it makes things worse, and what it means for an insurance claim.
On this page
- How central heating leak sealer works
- What leak sealer can fix, and what it cannot
- How to put leak sealer into a central heating system
- The risks: overdosing, masking and your boiler guarantee
- How to tell whether the leak sealer worked
- Why a sealed leak is a problem for an insurance claim
- When to find the leak instead
- Leak sealer or professional leak detection?
- Frequently asked questions
How central heating leak sealer works
Every mainstream product does the same job the same way. Fernox Leak Sealer F4, Sentinel Leak Sealer, Adey MC4 and Sika P32 are liquid polymer emulsions. You add the liquid to the system water, the pump carries it round, and it stays in suspension until it reaches the point where water is escaping. There, where warm water meets air on the outside of the pipe, the polymer drops out and forms a plug. Fernox describes the product finding "the area of oxygen ingress" and forming a polymer to seal the pinhole. Sentinel calls the result "a soft seal", and Sika "a soft barrier". Heat speeds the reaction, so you run the heating after dosing. Fernox and Sika say it is effective within 1 to 24 hours, Sentinel says normally within 24 hours, and the liquid then stays in the system for good.
One line of small print is worth knowing. Fernox states that the sealing efficiency of F4 "has been intentionally limited to prevent blockage of safety devices, pumps, etc." That is a manufacturer saying its product is weak on purpose. A sealer strong enough to close a real leak would close a heat exchanger too.
What leak sealer can fix, and what it cannot
The makers agree on the target. Fernox rates F4 for "small, internal inaccessible leaks", and Sentinel, Adey and Sika all describe small leaks and weeping joints. None of them publish a figure for the biggest leak the product will close, so treat any number you see online as a guess.
In practice that gives sealer a fair chance on a corrosion pinhole in a copper pipe (our guide to pinhole leaks in copper pipes explains how they start), a compression fitting that weeps once the system is hot, a radiator seam or leaking radiator valve gland, or a slow pressure drop with no visible water anywhere.
It has little or no chance on a leak you can see dripping, a split pipe or failed joint, a leak inside the boiler, or a system that needs topping up every few days. Heating and Plumbing World's guide lists large leaks, cracked heat exchangers, gushing leaks and leaks in pumps or valves as jobs sealer will not do. If the pressure is dropping fast, our guide on why a boiler keeps losing pressure covers the causes to rule out before you buy anything.
How to put leak sealer into a central heating system
If you want to try a bottle, follow the label; most of the trouble comes from guessing the dose or the method.
Dose by system volume
Fernox, Adey and Sika all rate a 500 ml bottle for a typical domestic system of roughly 100 litres, which they put at ten to twelve radiators, and Sentinel rates one litre for up to ten radiators. Fernox's current bottle says up to 130 litres and 16 radiators, so read the label in your hand. Count your radiators and do not round up.
Get it into the water and run the heating
On a sealed system, which most combi boilers run, the makers list a towel rail, a radiator, an in-line magnetic filter or a dosing vessel on the filling loop. On an open vented system you add it to the feed and expansion tank, but Fernox is clear that you must then draw water off at a drain cock and return it to the tank until all the sealer is in the circuit. Left in the header tank it does nothing. Then switch on the pump and, in Fernox's words, "apply at least a small amount of heat". Sentinel says to run the system for at least 15 minutes. Give it up to 24 hours before you judge the result. One exclusion: Fernox and Adey both say not to use it in single feed indirect cylinders, the Primatic type, because heating water and tap water share a cylinder.
The risks: overdosing, masking and your boiler guarantee
A second bottle "to be sure" is the most common mistake. The polymer plugs wherever water meets air, and a system carries air in more places than the leak: automatic air vents, the tops of radiators, and the narrow waterways of the boiler's heat exchanger. Heating and Plumbing World warns that too much sealer "can clog pump impellers, foul heat exchangers, and leave sludge in radiators". A blocked heat exchanger is one of the dearest repairs on a modern boiler. Have the magnetic filter cleaned at the next service and tell the engineer what was added.
Masking is the quiet risk. The gauge holds, so you stop worrying, and the pipe carries on corroding under the plug. The trade guides call sealer a stopgap, and when the seal gives way you are back where you started with a leak that has had months longer to develop.
Manufacturers take different positions on your boiler guarantee, so check your own boiler's installation manual first. Vaillant's ecoTEC plus manual tells the installer not to use unsuitable "corrosion inhibitors, biocides or sealants" and lists the additives it has tested, which are cleaners and inhibitors only. Worcester Bosch's manuals say that "normally the addition of sealing agents to the system water is not permitted as this can cause problems with deposits left in the heat exchanger", and that the company supports Fernox F4 only "in cases where all attempts to find a system micro leak have failed". Read that last line again. The boiler maker's own position is find the leak first, sealer last. If your boiler is under guarantee or on a cover plan, that wording is worth a phone call before you dose.
How to tell whether the leak sealer worked
Note the gauge reading when the system is cold, before you add the sealer. Run the heating for the day, then read it cold every morning for a week. If the reading holds for a fortnight, the sealer has plugged a weep for now; keep checking monthly, because a soft seal can let go. If it drops more slowly than before, there is more than one weep, or one bigger than the product is rated for, and a second bottle is where the damage begins. If it drops at the same rate, the water is going somewhere sealer cannot reach, such as a pressure relief valve discharging outside or a failed expansion vessel. Our guide to a combi boiler losing pressure covers both.
While the plug holds, the leak is hidden. That is fine if you never need to prove where it was. It is a problem if you do.
Why a sealed leak is a problem for an insurance claim
Most UK buildings policies include trace and access cover, which pays to find a hidden leak and to reach it, including making good the floor or wall opened to get there. Our guide to whether trace and access is covered by home insurance explains the cover. The claim is settled against evidence: a report showing where the leak was, how it was found and what had to be opened.
A leak sealed from the inside cannot give you that. There is no wet patch to photograph, no moisture reading, no thermal image of a warm plume under the floor, and the plug may hold long enough that a survey finds nothing to report. If the seal later fails and damage appears, you are claiming for a leak you knew about months earlier and chose to plug. That is a harder conversation with a loss adjuster than "we found it, here is the report".
When to find the leak instead
Finding a heating leak does not mean lifting floors on a hunch. Our central heating leak detection service works the other way round: pinpoint first, then open one neat access point in the right place. The circuit is drained and filled with a tracer gas, a safe hydrogen and nitrogen mix, which escapes at the leak and rises through the floor or wall to a detector; our tracer gas leak detection page explains the method. Thermal imaging picks up the warm bloom a heating leak leaves in a screed or behind plasterboard, and acoustic equipment hears water escaping under pressure. Between them they locate a weep the size of a pinhole, with a report written for your insurer. For the simple checks to try first, see how to find a leak in a central heating system.
The order matters. Find it, then fix it. Sealer is what the boiler makers themselves put last, once every attempt to find the leak has failed, and a leak in a heating circuit can be found.
Leak sealer or professional leak detection?
| Question | Leak sealer | Professional leak detection |
|---|---|---|
| What happens to the leak | Plugged from the inside; the fault stays | Located, then repaired at the source |
| Leaks it handles | Small weeps and weeping joints only | Any size, including under floors and behind walls |
| Evidence for a trace and access claim | None | Insurer-ready report included |
| Effect on the boiler | Safe at the labelled dose; an overdose can block the heat exchanger | None |
| What it costs | A bottle, plus the repair you still need | A fixed quote up front; no find, no fee on residential work, subject to terms |
Frequently asked questions
Does leak sealer work on a combi boiler that keeps losing pressure?
Only if the pressure loss comes from a tiny weep in the pipework or a radiator. The makers rate sealer for small, inaccessible leaks and weeping joints, effective within 1 to 24 hours. A faulty pressure relief valve or a failed expansion vessel also drops the pressure, and sealer does nothing for either.
How long does leak sealer last?
No manufacturer publishes a lifespan. The liquid stays in the system permanently, but the plug is described by Sentinel as a soft seal and by Sika as a soft barrier, and the trade guides call it a stopgap. It can hold for months, and it can let go when the system is drained or corrodes further.
Will leak sealer damage my boiler?
At the labelled dose, the makers say it will not block pumps or air vents. Overdosing is where the harm starts, because the polymer can settle in the heat exchanger. Vaillant warns against unsuitable sealants and Worcester Bosch says sealing agents are normally not permitted, so check your boiler's manual and guarantee first.
Which is the best central heating leak sealer?
There is no meaningful winner. Fernox F4, Sentinel Leak Sealer, Adey MC4 and Sika P32 are all polymer emulsions that seal where water meets air, dosed at roughly one bottle per 100 litre system, and rated for small weeps only. Buy whichever your boiler manufacturer names, if it names one, and dose it as the label says.
Can I use leak sealer for a radiator that weeps at the valve?
You can, and a weeping valve gland or radiator seam is the kind of small leak the makers have in mind. It is also one of the easiest leaks to fix properly, often by tightening the gland nut or replacing the valve, so a repair is usually the better first move. Sealer is meant for leaks you cannot get at.
Should I use leak sealer before making an insurance claim?
No. A trace and access claim is settled on evidence of where the leak was and how it was found. A sealed leak leaves no wet patch, no moisture reading and no thermal image, and a survey after dosing may find nothing to report. Find the leak and get the report first.
Boiler losing pressure and no idea where the water is going?
DCI Leak Detection finds hidden central heating leaks across Cornwall and Devon with tracer gas, thermal imaging and acoustic listening. We find your leak with minimal disruption and give you an insurer-ready trace and access report. No find, no fee on residential work, subject to terms. Not sure you have a leak at all? Try our free one-minute leak checker first.
Call Dickie on 07822 025 911 Fast response across Cornwall & Devon · Insurance-approved reports