What the vessel is doing while you are not thinking about it
Vaillant describe the expansion vessel as a chamber divided by a thin rubber sheet, with system water on one side and nitrogen on the other, sitting there to absorb the pressure rise as the boiler heats the water. Water expands by a small percentage between cold and operating temperature. In a sealed system with nowhere to put that extra volume, the pressure rise would be enormous. The vessel gives it somewhere to go: the gas compresses, the diaphragm moves, and the gauge climbs only a little.
When the gas charge escapes, or the diaphragm perforates and water crosses into the gas side, that cushion disappears. Ideal put the consequence plainly: if the rubber bursts or tears, water fills the air gap and the pressure in the system drops.
Why it looks exactly like a leak
Because from the homeowner's chair the symptom is identical. You look at the gauge, it is below where it should be, you top it up, and a week later you are doing it again. Every article you find tells you to hunt for a leak, so you take radiator covers off and crawl under the bath, and find nothing, which is precisely what a vessel fault produces.
There is a second reason the confusion sticks. A late-stage vessel fault does lose water. Once the hot peak overshoots the safe band, the pressure relief valve lifts and lets the excess out through the pipe outside. So you now have a genuine, measurable water loss, a dry house, and a completely misleading conclusion.
The symptoms that actually separate them
| Symptom | Expansion vessel | Leak |
|---|
| Gap between cold and hot readings | Wide. The needle travels a long way when the heating runs. | Normal. Ideal describe roughly 1 to 1.5 bar cold and around 2 bar hot. |
| Overnight, heating off | Steady. Nothing is escaping while the system is cold. | Falls. A hole works at any temperature. |
| Discharge pipe outside | Wet after the heating has run, once the fault is advanced. | Dry, unless the leak is coincidental. |
| Boiler lockouts | Sometimes at both ends: high pressure hot, low pressure cold. | Low pressure only. |
| Damp, staining, warm floor patches | None. Ever. | Eventually, though it can take months under screed. |
| Age | Vaillant put vessel life at five to ten years, shorter in hard water. | Any age, though corrosion-driven pinholes favour older systems. |
The test you can do, and the one you cannot
You can do the readings. Cold reading before the heating comes on, hot reading after an hour with all radiators open, discharge pipe checked while the system is still hot, then a second cold reading the following morning. Four numbers and one observation, and they distinguish the two faults more reliably than anything an engineer can tell from the doorstep.
You cannot test the vessel itself. Checking the gas charge at the vessel's air valve, and seeing whether water comes out of it because the diaphragm has gone, means working inside the boiler case on most combis. Regulation 3 of the Gas Safety (Installation and Use) Regulations 1998 requires work on a gas fitting to be carried out by a competent, registered person, and Baxi and Ideal both send this job to a Gas Safe registered engineer. Some systems have a separate external vessel on the pipework rather than one inside the boiler, which is a different situation, but the charge test is still a job for someone with a gauge and a pump.
What happens to a failed vessel
Two outcomes. If the diaphragm is intact and only the charge has leaked away, the vessel is recharged with air to the correct pre-charge for the system and the problem goes away. If the diaphragm has perforated, Vaillant are clear that the vessel is replaced. Where the boiler's internal vessel is undersized for a large system, an additional external vessel is sometimes fitted on the return instead, which is a design fix rather than a repair.
None of that is leak detection, and none of it is our work. We will say so on the phone, because sending a detection engineer to a vessel fault is an hour of your money spent confirming what your own gauge readings already said.
When the vessel is a red herring
The trap runs the other way as well. A house can have a tired vessel and a leak at the same time, particularly on an older system where hard water has done for the diaphragm and corrosion has done for the pipework. The tell is the overnight reading. Replace the vessel, then take three consecutive cold morning readings. If they still fall, the vessel was only half the story and the pipework is the other half.
That sequence matters for cost. Confirming a leak after the vessel has been sorted means the pressure test has a clean baseline, and a clean baseline makes the trace faster.
What detection adds once the vessel is ruled out
The circuit is isolated and pressure-tested, which establishes that a loss exists, sizes it, and narrows it to a branch. Thermal imaging then follows the pipe runs through screed, tile and plasterboard, reading the warm line of a flow pipe and the anomaly where water has soaked the substrate around it. Where thermal cannot resolve the point, the circuit is drained and tracer gas is pushed through the pipe, escaping at the failure and surfacing to be detected. The floor is opened at that point, once. Then the pipe is repaired or re-run and the screed, boards, tiles and decoration are put back by our own people rather than left for you to arrange, which is the whole argument of the boiler and heating leak repair page.
Related: boiler losing pressure, boiler pressure keeps dropping with no visible leak, and the wider boiler pressure cluster. Detection is £150 per hour, agreed before we attend, and if we do not find it you do not pay for the time spent looking.
