Central heating leak sealer works sometimes, and only on a tiny weep. It forms a soft plug over the hole from the inside, so the pipe is still corroded and the plug can let go. When the pressure drop returns, the leak has usually grown past what sealer can hold. The fix is to find the leak and repair the pipe.
The bottle did exactly what it said on the label. You poured it in, ran the heating, and for four or five weeks the gauge sat where you left it. Then one morning it was back at 0.5 bar and you were reaching for the filling loop again. It is a very common story in Cornwall, especially in older houses where the pipework has been in the floor since the heating went in. This guide explains what sealer actually does inside your system, why the fix does not last, what a second dose risks, and the route that ends the topping up for good.
What leak sealer does inside the pipes
Every mainstream sealer works the same way. It is a liquid polymer that circulates with the system water and stays dissolved until it reaches a point where water is escaping and meeting air. There it reacts and forms a soft plug across the hole. Heat speeds it up, which is why the instructions tell you to run the heating after dosing. The manufacturers describe the target honestly: small, inaccessible leaks and weeping joints. None of them claim it will fix a leak you can see dripping.
Two details from the small print matter for your situation. First, the makers deliberately limit how strongly the product seals, because a sealer aggressive enough to close a real leak would also close air vents, pump seals and the narrow passages of a heat exchanger. Second, the trigger is contact with air, and a leak is not the only place a heating system meets air. Automatic air vents, radiator bleed points and the boiler itself all offer the same invitation.
Why it worked for a month and then stopped
A plug that held for a month did its job. What it could not do is change the reason the pipe failed. If the leak was a corrosion pinhole, the water that ate through the copper is still in the system, still working on the same pipe and on every other pipe in the loop. The hole grows, or a second one opens nearby, and the plug that fitted a pinhole cannot cover a larger gap.
There are a few common patterns behind the return of the pressure drop.
- The hole grew. Pitting corrosion does not stop because a polymer sat on top of it. Once the opening is larger than the plug, water finds its way round the edge.
- The plug moved. Sealer forms a soft barrier, not a weld. Thermal expansion, a pressure surge when the system fires, or simply the flow of water can dislodge it. This is also why the drop sometimes comes and goes.
- It was never one leak. A system with corrosive water rarely has a single weak point. The sealer closed the one that was leaking that week. Another one has since started.
- The leak was too big all along. If you were topping up every few days rather than every few weeks, the loss was beyond what sealer is rated for. The month of calm may have been partly luck and partly the plug slowing rather than stopping the loss.
- Fresh water undid it. Every top up brings in oxygenated water and dilutes the inhibitor. The system corrodes faster after a dosing cycle than before, not slower.
If you are unsure whether the loss is even a leak, central heating losing water with no visible leak sets out the other causes to eliminate first, from the pressure relief valve to the expansion vessel.
The risks of a second bottle
The instinct is to add another dose. That is where the low-cost shortcut starts to cost money.
| Where sealer can settle | What happens | What it costs you |
|---|---|---|
| Automatic air vents and bleed points | They gum up and stop venting air | Cold radiators, kettling, repeated bleeding |
| Heat exchanger passages | Debris narrows the waterways | Boiler faults, lockouts, a possible guarantee dispute |
| Radiator valves and pump | Soft polymer coats moving parts | Sticking valves, noisy pump, poor circulation |
| Radiator bottoms | Reaction debris joins the sludge | Cold spots low down, reduced output |
Dose by system volume, not by hope. A typical domestic bottle is rated for roughly 100 litres of system water, and a large house with many radiators may hold more, but doubling up on a small system is how vents and boilers get blocked. Several boiler manufacturers take a dim view of sealer residue found inside a heat exchanger during a guarantee inspection, so check your boiler’s terms before you add anything.
Sealer also complicates detection later. Tracer gas, one of the methods used to find a leak in a drained circuit, reacts with a sealed hole much as air does, so the plug can partly re-form during a survey. It is still possible to find the leak, it just takes longer.
What the returning drop is telling you
Read the gauge over a week with the heating on its normal programme and no bleeding. A slow creep down over ten days is a small leak that is still within reach of a straightforward repair once it is located. A drop of half a bar in a day or two is a joint or a split, and the plug was never going to hold it. A drop that only happens when the heating is hot points to a fitting that opens up with expansion, typically a compression joint or a radiator tail. Our guide on why a boiler keeps losing pressure explains the timing patterns in more detail.
Whatever the rate, the leak now has a history, and that matters for insurance. Many buildings policies include trace and access cover, which pays towards locating a hidden leak and making good the floor or wall that had to be opened. A sealed leak gives the insurer nothing to see. A located leak, photographed and documented, gives them a clear cause. Repeated dosing and months of topping up, on the other hand, can be read as damage that built up gradually, which is where claims get refused.
What to do instead
The lasting fix is a short sequence: find the leak precisely, repair the pipe, then restore the system water. In that order.
Find it without lifting the floor on a guess
A central heating leak detection survey starts by confirming the loss is on the heating circuit rather than the boiler or the mains, then narrows the search. Thermal imaging shows where hot water is reaching a floor or wall surface. Acoustic listening picks up the sound of the escape. On a drained circuit, tracer gas is introduced and traced to the point it surfaces. The readings are cross checked before a repair point is marked, so your plumber opens one small area rather than several.
Tell the engineer that sealer has been used and when. It changes how the survey is run and it is nothing to be embarrassed about. Most of the systems we see in Cornwall have had a bottle in them at some point.
Repair the pipe, not the symptom
With the leak marked, a plumber replaces the failed section or fitting. That is usually a small job once the location is known. If the survey finds pitting along a length of pipe rather than one hole, the plumber may advise replacing that run, which is still far cheaper than chasing the next pinhole every few months.
Deal with the water
A corrosion leak means corrosive water. After the repair, a flush and a fresh dose of inhibitor protect the rest of the system. Cornwall’s soft water is kind to kettles but it is slightly acidic, and a heating system that has been topped up for a year with the inhibitor diluted away corrodes faster than one that has been left alone. Ask your plumber to check the inhibitor level as part of the job.
When sealer is a reasonable stopgap
We are not against the bottle in every case. It has a narrow, genuine use: a tiny loss, pressure dropping over weeks rather than days, in pipework you genuinely cannot reach for now, as a bridge to a repair you have already planned. A holiday let near Padstow with a slow weep under a new kitchen floor and guests booked until the end of the season is the sort of situation where buying a few weeks makes sense. What sealer is not is a repair, and treating it as one is how a small problem becomes a saturated screed, a boiler fault and a difficult claim.
We provide leak detection across Cornwall, and the report you receive marks the repair point for your plumber and documents the cause for your insurer.
Frequently asked questions
Does central heating leak sealer work at all?
Sometimes, on a small weep or pinhole where the pressure drops over weeks rather than days. It forms a soft plug over the hole from the inside. It does not repair the pipe, it does nothing for larger leaks, and the manufacturers themselves rate it only for small, inaccessible leaks.
Why has the pressure drop come back after using leak sealer?
The plug covered the hole but the corrosion underneath carried on. Either the hole has grown past the plug, the plug has moved, or a second leak has opened on the same corroded pipework. Repeated topping up also brings in fresh oxygenated water, which speeds the corrosion up.
Can I add a second dose of leak sealer?
You can, but a second dose rarely holds longer than the first and raises the risk of blocking air vents, radiator valves and the boiler heat exchanger. If the first dose has failed, the leak is beyond what sealer is for. Finding and repairing it is the better use of the money.
Will leak sealer damage my boiler?
Used at the correct dose on a suitable system it usually does no harm. Overdosing, or dosing a system with a small water volume, can leave residue in the heat exchanger and vents. Some boiler makers treat sealer residue as a guarantee issue, so check your boiler’s terms before adding any.
Does leak sealer stop a leak being found later?
No, but it can slow the survey. Tracer gas reacts with a sealed hole much as air does, so the plug may partly re-form during testing. Thermal imaging and acoustic methods still work. Tell the engineer that sealer has been used so the survey can be planned around it.
End the topping up
If sealer bought you a month and the gauge is falling again, we can locate the leak on your heating circuit without lifting floors on a guess. You get a marked repair point for your plumber and an insurance-ready report.
