Do this first
- Write down the reading and the time. Cold system, before you touch anything. The number matters less than how fast it falls.
- Top it up once, to 1.0-1.5 bar cold. Boiler off, open both filling loop valves slowly, watch the gauge, close both fully, detach the loop if yours is removable, restart.
- Do not keep topping up on a schedule. Every refill puts fresh oxygenated mains water into the system, which corrodes it from the inside and makes more pinholes.
- Check the outside discharge pipe. It is a small copper or plastic pipe through the external wall near the boiler. If it drips or has a limescale trail, the system is dumping water there rather than leaking into your house.
- Walk the visible circuit. Every radiator valve spindle and tail nut, both towel rails, under and behind the boiler, and any pipe visible in a cupboard or loft. Dry kitchen roll wrapped round a joint for an hour finds a weep your eye will not.
- Feel the floors. A patch of floor that stays warm hours after the heating has gone off is a hot pipe losing water under the screed.
- If you can smell gas, see scorching, or the boiler is leaking from inside its case, turn it off at the fused spur and leave it off. Do not remove the casing.
The three things it usually is
A sealed system is closed. In normal use it should not lose water at all, so a gauge that keeps falling has a cause, and there are only three common ones. How fast it drops, and when, tells them apart.
| Pattern | What it points at | The test |
|---|---|---|
| Falls steadily whether the heating runs or not | A genuine leak on the circuit, often a pinhole under a floor | Isolate and pressure test: a sealed circuit that will not hold static pressure is losing water |
| Climbs high when hot, drops below normal when cold, and the discharge pipe outside drips | Failed expansion vessel or a pressure relief valve passing | Watch the gauge through one full heating cycle; the swing is the giveaway |
| Falls only when the heating is running | A joint or pipe that only leaks once the pump raises pressure in that circuit | Run the heating with the filling loop detached and re-read cold |
The expansion vessel case is the one worth ruling out before anyone lifts a floor. The vessel absorbs the volume change as the water heats; when its diaphragm perforates, the system has nowhere to put the expansion, so the pressure swings wildly and the relief valve lets water out to protect the boiler. Nothing is leaking inside your house at all, and the fix is a recharge or a new vessel.
The rough guide on frequency: once or twice a year is normal, every month or two is a weep worth finding, weekly is an active leak, and daily means it is going somewhere fast. If you have looked everywhere and found nothing, read pressure dropping with no visible leak next, and the boiler pressure hub covers the fault codes that come with it.
What the repair involves
A detection visit starts by proving the loss is real and sizing it, because that decides everything after. The circuit is isolated and pressure tested; if it holds, the fault is the vessel or the relief valve and no floor gets touched. If it does not hold, the run is traced with thermal imaging through screed and tile, with acoustic listening on accessible pipework, and with tracer gas on a drained circuit where thermal cannot see. That locates the point of loss before anything is opened, so one board or one small area of screed comes up rather than a room.
The repair itself is a cut-out and remake in copper, or a new valve, vessel or radiator tail. Then the making good: screed reinstated, boards refixed, tiles matched where we can, plaster and decoration finished. That is the whole of boiler and heating leak repair rather than a hole and a phone number for a plasterer.
When it is not us
- You have topped up twice this year and that is it. That is normal, especially after bleeding radiators. Nothing to fix.
- The boiler itself is leaking inside the casing. Only a Gas Safe registered engineer may work on it, and it is an offence for anyone else to remove the casing. Turn it off and book a heating engineer for the appliance; we deal with the pipework outside it.
- A fault code that clears and stays cleared. Low pressure lockouts are a symptom. If the pressure holds afterwards, there is nothing to trace.
- Radiators cold at the top, pressure fine. That is air, and bleeding them is a five-minute job you can do yourself.
- Your boiler is under warranty and losing water internally. Call the manufacturer's scheme first. Bringing us in ahead of them can complicate a claim.
- You rent. The boiler and heating installation are the landlord's to keep in repair. Report it in writing and let them instruct.