Why the timing is the useful part
Most boiler pressure advice ignores when the drop happens, which is a shame, because the timing rules out causes for free.
Two of the three usual suspects need the system to be hot. An expansion vessel fault shows as a wide swing, high while the heating runs and low once it cools, which is Ideal's description of what happens when the diaphragm tears and water fills the air gap. A pressure relief valve lifts when pressure rises past the top of the safe band, which again happens with the boiler firing. Neither of those mechanisms is doing anything at three in the morning in a house with the heating off.
A leak does not care. Water under pressure escapes through a hole whether the pump is running or not. If the gauge is measurably lower at seven in the morning than it was at eleven the night before, and the system sat cold in between, you are looking at a loss rather than a swing.
The three-night test
- Last thing, with the heating off and the system cooled, read the gauge and write down the number and the time. Photograph the dial if the needle sits between markings.
- First thing, before the heating comes on, read it again. Same dial, same angle, same light.
- Repeat for three nights. One night is noise. Three nights is a rate.
Then read the result:
| Overnight change | What it means |
|---|
| No measurable change three nights running | Not a static leak. Your loss is happening while the system is hot. Look at the vessel and the relief valve instead. |
| A small but repeatable fall every night | A genuine leak, and a slow one. Usually a weep on a joint, a valve gland, or a pinhole under a floor. |
| A large fall every night, or it will not hold at all | An active leak. Isolate the heating circuit and stop running it until it has been found. |
| Falls overnight only in cold weather | Check outside before assuming anything. A frozen or blocked condensate pipe backing up is a different fault, and it does not move the gauge. |
Ideal put the correct cold reading between 1 and 1.5 bar, so measure against that rather than against wherever your needle happens to sit.
Where an overnight loss usually comes from
Gravity tells you which floor to search
A static system under pressure loses water at its weakest point regardless of height, but what you can observe is where the water ends up. On a cold system with no pump running, the water leaves slowly and soaks rather than sprays, so it accumulates directly below the failure. That is worth knowing because it makes the evidence local. A damp patch that appears overnight and dries during the day sits under the leak, not three rooms away.
The usual sites, in the order worth checking
- Radiator valve glands and tail nuts, particularly on valves that have been turned recently.
- Towel rails, which take more movement than a radiator and are usually on the wettest floor in the house.
- Compression joints in airing cupboards and under bath panels, where nobody looks.
- Buried runs in screed. Nothing visible, and the only clue is the gauge.
- The filling loop itself. A loop left slightly open masks a leak by feeding it, and a loop with a failed washer can pass the other way.
The one overnight cause that is not a leak
If the boiler fires overnight for hot water or a frost thermostat, the system was not cold, and the whole logic above changes. A programmer that runs the heating for an hour at five in the morning gives you a vessel swing that looks exactly like an overnight loss. Check the programme before you check the pipework. If the boiler ran, repeat the test on a night with the programmer off entirely.
What you should not do
- Do not top up every morning without recording it. Ideal note that repeated refilling dilutes the inhibitor in the system, and it also destroys the only measurement you have.
- Do not open the boiler case. Under regulation 3 of the Gas Safety (Installation and Use) Regulations 1998, work on a gas fitting must be done by a competent, registered person.
- Do not lift floors speculatively. A three-night rate and a pressure test narrow the search far more cheaply than a crowbar.
When not to call us
If your three nights show no change and the loss only appears after the heating has run, this is a vessel or relief valve fault and you want a Gas Safe heating engineer with parts on the van. Baxi are explicit that recharging or replacing the vessel and cleaning or replacing the safety valve are registered-engineer work. We would only be able to tell you the same thing, on the clock.
What we do when the loss is real
The circuit is isolated and pressure-tested, which converts your three nightly readings into a measured loss rate and proves whether it is on the heating side at all. Thermal imaging follows the pipe runs through screed, tile and plasterboard and shows where the thermal pattern breaks. On a drained circuit, tracer gas is pushed through the pipe and picked up at the surface, which is what finds a pinhole under a poured floor. Then the floor is opened once, the pipe repaired or re-run, and the screed, boards and finish put back. Detail on the boiler and heating leak repair page, more of this cluster under boiler pressure, and the neighbouring cases at boiler losing pressure and boiler pressure keeps dropping with no visible leak.
Charged at £150 per hour, agreed before we attend, and if we do not find it you do not pay for the looking.
