Do this first
- Record the pressure cold, and again twelve hours later. Underfloor systems lose pressure slowly and the rate is the most useful number you can give an engineer.
- Find the manifold. It is usually in a cupboard, a plant room, under the stairs or in a utility, and it is where every loop in the house begins and ends.
- Look at it properly before you touch it. Dry the whole manifold, put paper underneath, and check the actuator heads, the flow meters, the isolation valves and each loop connection. A surprising share of underfloor leaks are at the manifold, in a cupboard, and are reachable without disturbing a single floor tile.
- Close each loop in turn. The isolating valves on the manifold let you shut loops one at a time. Close them all, top the system to its normal pressure, and watch. Then open one loop at a time, leaving each for a few hours.
- Note which loops heat and which do not, with the system running normally, and mark them on the manifold labels if they are not already.
- Do not drill, nail or screw into any floor that has heating in it, and do not lift a section of engineered board over a loop to look.
Manifold or screed, and the test that decides
This is the fork that matters, because one of these is a morning's work in a cupboard and the other involves opening a floor.
| Observation | Where the leak is |
|---|---|
| Pressure falls and one loop never reaches temperature while the others do | That loop, under the screed |
| Pressure falls and every loop heats evenly | The manifold, the primary pipework to it, or elsewhere on the heating circuit |
| Pressure holds with all loops isolated at the manifold, falls when one is opened | That loop, confirmed |
| Pressure falls with every loop isolated | Not the underfloor at all: primaries, a radiator circuit or the boiler |
| A warm strip or patch on the floor hours after everything is off | Water pooling around a hot pipe at the leak point |
The loop-isolation test is the whole diagnosis, and it costs nothing but patience. It also tells you something valuable when it comes back negative: if the pressure keeps dropping with every loop shut, the floor is innocent, and nobody should be lifting tiles. That result alone has saved a lot of people a lot of money. If a warm floor is your main clue rather than the gauge, a warm patch on the floor covers the thermal side, and the wider pressure question sits at the boiler pressure hub.
One more cause worth knowing about. Underfloor pipe is very rarely defective along its length; most screed leaks are at a point where something was driven into it. A kitchen island fixing, a skirting nail, a door threshold screw, a later bathroom fit-out. If the leak appeared shortly after somebody worked on the house, tell the engineer, because that is where the search starts.
What the repair involves
Detection on underfloor heating leans on thermal imaging, because the pipework is warm and the wet screed around a leak conducts differently from the dry screed around it. The loops are pressurised, isolated one at a time and imaged, then confirmed acoustically or with tracer gas on the drained loop where the floor build-up is deep or the finish is stone. What comes out is a pinpoint, not an area.
The repair is a small opening in the screed at that point, the damaged pipe cut out and a proper buried joint made, pressure tested before anything is closed, then screed reinstated, the floor finish relaid and the room redecorated where it was disturbed. Where a loop has been damaged in several places, it is sometimes better to abandon and re-route it than to keep patching. All of that sits inside underfloor heating leak repair, including the tiling and floor reinstatement that most detection firms do not touch.
When it is not us
- Pressure holds fine and one room is just cool. That is usually balancing, a failed actuator head, a stuck thermostat or a wiring centre fault. An underfloor or heating engineer, not a leak specialist.
- The system is electric underfloor heating, not water. A mat under tiles has no water in it. A fault there is an electrician's, and a damaged mat is tested with an insulation tester rather than a pressure gauge.
- Everything is cold and the boiler is in lockout. Fix the boiler fault first; you cannot test loops on a system that will not run.
- The floor is under warranty from a recent installation. Go back to the installer before anyone else opens it, or you may lose the cover.
- The manifold is in a communal plant room. That is block management's, and so is whoever is allowed to isolate it.
- You rent. Report it in writing. Space heating is the landlord's to keep in repair.