Nothing visible does not mean nothing is leaking
The visible part of a heating system is a small fraction of it. Radiators, valves and the boiler case are what you can inspect in ten minutes. The pipework connecting them runs under floorboards, through joist notches, in wall chases and, in most London flats built or refurbished in the last twenty years, buried in screed. A pinhole in a 15mm copper run under screed will drop a gauge weekly and show nothing at all above the floor, because screed holds water and a warm floor evaporates it as fast as it arrives.
So the first correction: dry surfaces rule out about a fifth of the possible leak sites, not the leak.
The three destinations, and how to check each
1. Into the building fabric
Under a floor, behind a wall, or into a void. The tell is not water, it is temperature and time. A warm patch on a floor with the heating off, a board that has cupped, a skirting that has darkened at its base, a cold room that never quite gets to temperature because a buried flow pipe is losing heat into wet screed. Ceilings below bathrooms and kitchens tell you late, after months.
2. Out of the relief valve discharge pipe
Almost nobody looks at this, which is why it is worth checking first. Vaillant describe the pressure relief valve as a safety device that lets pressurised water out through a pipe running from the boiler to the outside wall, or down towards the ground. Ideal say that outlet pipe should normally be dry. Go outside after the heating has run, find the small copper or plastic stub near the boiler's flue, and feel underneath it. Wet, or a pale limescale trail down the brickwork, means water has been leaving the system there.
3. Nowhere, because the vessel has gone
The case people miss entirely. Vaillant describe the expansion vessel as the chamber that absorbs the pressure rise when the boiler heats the system water, using a rubber diaphragm with gas on one side. Ideal put it bluntly: if the rubber bursts or tears, water fills the air gap and the pressure in the system drops. In the early stage of that failure nothing leaves the system at all. The gauge simply swings, high when hot, low when cold, and each time it reads low you top it up, which makes the next hot peak worse.
The half-hour that narrows it to one
- Cold reading, heating off overnight. Ideal put the correct cold figure between 1 and 1.5 bar.
- Hot reading, after an hour with all radiators open. Ideal describe a normal rise to around 2 bar. A jump towards the top of the dial is the vessel.
- Outside pipe, immediately after. Dry rules the relief valve out for this cycle.
- Cold reading again next morning. Lower than yesterday's cold reading means water has genuinely gone.
- Isolate what you can. If your system has isolation valves on separate circuits, closing one overnight and watching the gauge is the cheapest way to halve the search area.
Record the four numbers. Any engineer who turns up to a set of readings starts an hour ahead of one who has to generate them.
What makes a hidden heating leak different from a plumbing leak
Three things, and they all help you.
The water is hot, which makes thermal imaging genuinely useful rather than marginal. A wet patch of screed above a leaking flow pipe reads differently from dry screed around it, and the pipe run itself shows as a warm line that goes wrong at the failure point.
The system is sealed and can be isolated and pressurised to a known figure, so a pressure test does not just say whether there is a loss, it says how big. A circuit that holds for an hour is not the one you are looking for.
And the circuit can be drained, which is the condition tracer gas needs. A hydrogen and nitrogen mix pushed into an empty pipe escapes at the failure point and rises through screed, tile and floorboard to be picked up at the surface. That is how a leak under a poured floor gets located without the floor coming up first.
When the answer is not detection
- The outside pipe is wet. You need a Gas Safe heating engineer to establish whether the relief valve is passing or the vessel is over-pressurising it. Baxi name debris in the valve as one cause and say the fix is to clean or replace it.
- The cold-to-hot swing is wide. Same answer. Vessel recharge or replacement, and both Baxi and Ideal are clear it is a registered engineer's job.
- It dropped once and then held. You bled a radiator, or somebody did. Top up and stop watching the gauge.
- It will not hold at all, minutes after filling. That is a substantial open leak. Isolate the system and treat it as an emergency, not a diagnostic exercise.
Sending a detection engineer to a vessel fault wastes your money and our morning. We would rather tell you that on the phone.
What we do when it is ours
Isolate and pressure-test to confirm and size the loss, thermal-image the runs to narrow it to a zone, then tracer gas on the drained circuit to put a cross on the spot. The floor or wall is opened at that point, the pipe is repaired or re-run in copper, and the screed, board, tile or plaster goes back. That last part is the difference between us and a detection-only firm, and it is why the boiler and heating leak repair page reads the way it does.
If your pattern is a steady drop rather than a hidden one, start at boiler pressure keeps dropping with no visible leak or the wider boiler pressure hub. Rates are on the pricing page: £150 per hour, fixed before we attend, and no find, no fee.
