Do this first
- Prove the loss before you chase it. Top up to 1.2 bar cold, then detach or fully close the filling loop. A loop left cracked open feeds the system from the main and hides everything.
- Log the gauge. Note the cold reading morning and night for three days, with the heating on as usual. A number you can show is worth more than a description.
- Look at the discharge pipe outside, the small pipe through the wall near the boiler. A drip after the boiler fires, or a white limescale trail down the brickwork, means water is leaving there rather than into the building.
- Wrap dry kitchen roll round every radiator valve, tail nut and towel rail connection and leave it an hour. A weep too small to see will mark the paper.
- Walk the floors barefoot two hours after the heating goes off, and feel along the pipe runs. A patch still warm then is a hot pipe losing water under the screed.
- Do not open the boiler casing to look inside, and do not keep refilling daily while you investigate.
Three causes, three different jobs
Invisible is the normal state of these leaks, not an unusual one. A pinhole in a 15mm copper pipe under screed can empty a system weekly and produce no damp you can see for months, because the screed takes the water and the heating dries it again. There are three realistic explanations and they are one test apart.
1. A pinhole or weeping joint under a floor or inside a wall
The tell is a steady fall whether the heating runs or not, and a circuit that will not hold static pressure when it is isolated and pressure tested. Often there is a warm patch, a lifting board edge, a cracking grout line, or a skirting that has darkened. Thermal imaging finds most of these in an hour because the pipe is hot and the wet screed around it is hotter still.
2. A failed expansion vessel, which is not a leak at all
The tell is the swing. Pressure climbs well above normal as the system heats, then drops below where it started once it cools, and the discharge pipe outside is wet. The vessel's job is to absorb the volume change as water heats; once the diaphragm perforates there is nowhere for the expansion to go, so the relief valve opens and sheds water to protect the boiler. Nothing inside your house is wet. It needs recharging or replacing, and it is worth eliminating before anyone lifts a floor.
3. A passing pressure relief valve
Debris on the valve seat holds it slightly open, so the system bleeds away through the discharge pipe at normal pressures. The symptom looks identical to a vessel fault from the gauge, and the difference is found at the pipe: a valve passing on a cold, correctly pressurised system is weeping when it has no reason to.
| Observation | Most likely |
|---|---|
| Steady fall, discharge pipe bone dry | Leak on the circuit |
| Big pressure swing hot to cold, discharge pipe wet after firing | Expansion vessel |
| Slow fall, discharge pipe damp even when cold | Relief valve passing |
| Falls only after the pump runs | A joint that leaks under running pressure |
If a warm floor is your strongest clue, a warm patch on the floor goes into that on its own, and underfloor heating has a different test again.
What the repair involves
The visit is diagnostic before it is destructive. The circuit is isolated and pressure tested to confirm the loss is real and to size it, which on its own rules the vessel in or out. If the circuit genuinely will not hold, the run is traced with thermal imaging through screed and tile, acoustically where pipework is reachable, and with tracer gas on a drained circuit where the pipe is too deep or too cold to image. Only then does anything get opened, at one point rather than along a guess.
Repairs are cut-and-remake in copper, or a new vessel, valve or tail. Making good follows in the same visit list: screed, boards, tiles, plaster and decoration, which is what boiler and heating leak repair covers and what most detection-only firms leave behind.
When it is not us
- The gauge holds for a week with the loop detached. Then you had a filling loop passing, and the system was never leaking.
- Everything points inside the boiler. Heat exchanger, internal seals, the pump head: Gas Safe work only, and the casing is not to come off for anyone else. Book a heating engineer for the appliance.
- The boiler is in warranty. Use the manufacturer's scheme first so the claim stays intact.
- It is an open-vented system with a header tank in the loft. There is no pressure gauge to fall, and a dropping tank level is a different investigation.
- You rent, or you are a leaseholder with a communal heating system. Report it to the landlord or block management in writing. Heating installations are the landlord's repairing obligation, and communal plant is not yours to instruct on.