Do this first
- Turn the heating off and leave it off for at least four hours, then feel the patch again. A floor that is still noticeably warm long after the system has shut down is the whole symptom, and it needs the system cold to be meaningful.
- Mark the warm area in pencil or with low-tack tape, including where it is warmest. This is the most useful thing you can hand an engineer.
- Read the boiler pressure cold, and again the next morning. Write both down.
- Read the water meter with everything off, wait thirty minutes and read again, so you know whether you are also losing mains water.
- Check for a hot water cylinder or a hot supply pipe run in the same area before assuming it is the heating.
- Do not drill, screw or nail into that floor, and do not lift a section of it to have a look. You are standing over a live pipe of unknown position.
- Keep furniture and rugs off it so the area can be imaged properly when someone attends.
What makes a floor stay warm
Heat in a floor with the system off has to be coming from water that is still moving, or from a mass of wet screed holding heat. Three explanations cover almost all of them.
| Evidence | Cause |
|---|---|
| Warm patch, boiler pressure falling, no visible damp anywhere | A leak on a buried heating pipe: water escaping into the screed and carrying heat with it |
| Warm patch, pressure steady, house has underfloor heating | Normal residual heat in a screed, or a zone valve or actuator stuck open |
| Warm patch near a cylinder, a kitchen or a bathroom, pressure steady, meter moving | A hot water supply pipe leaking under the floor |
| Warm patch with a musty smell, lifting floor finish, cracking grout | A leak that has been running long enough to saturate the build-up |
| Warm patch over a boiler, flue or heated service void below | Not a leak at all |
The reason this symptom is worth taking seriously is that it appears long before damp does. A hot pipe losing water under a screed will read plainly on a thermal camera months before anything feels wet underfoot, because the wet screed conducts heat outwards far better than the dry screed around it. Acting on a warm patch is acting early, which is the difference between one small opening in a floor and a room stripped back.
Two associated readings sharpen it. A falling boiler gauge points at the heating circuit; a moving water meter points at a hot or cold supply pipe. Both moving usually means two separate things. If the gauge is the thing that brought you here, pressure dropping with no visible leak is the companion page, and if the house has underfloor heating, start instead with an underfloor heating leak.
What the repair involves
This is the symptom that non-invasive detection was made for. The system is brought to temperature, the floor is imaged with a thermal camera to map the heat signature and find its hottest point, moisture readings are taken across the same area to confirm the screed is wet rather than simply warm, and then the pipe is confirmed acoustically or with tracer gas on the drained circuit. Tracer gas is the one that finds what thermal cannot, because it works on a cold, empty pipe and comes up through the floor at the point of loss.
After that, the floor is opened once. The pipe section is cut out and remade, tested under pressure, and then the screed, the boards, the tiles, the finish and the decoration are put back. That is the whole of gas tracer leak detection followed by the repair and the making good, and it is why the detection and the reinstatement are one job here rather than two contractors.
When it is not us
- The house has underfloor heating and the pressure is steady. A screed holds heat for hours by design. That is the system working.
- A zone valve or actuator is stuck open and one area is always warm. That is a heating controls fault, not a leak, and a heating engineer sorts it.
- The warm area is directly over a boiler, a flue run or a heated void. Nothing is leaking.
- The floor is warm and the ceiling below is stained. Deal with the leak, but the ceiling may also be a separate source. See a water stain on a ceiling.
- Your insurer includes trace and access cover. Tell them before you instruct, because locating the leak and making good the opening may be claimable.
- You rent, or the heating is communal. Report it in writing to the landlord or the managing agent. Screed in a block is usually structure, and not yours to open.