The cheapest underfloor leak is the one found before the screed
Once screed covers the loops, everything changes. Finding a leak means thermal imaging, tracer gas and a saw. Fixing it means breaking out floor, cutting a section, joining it, and reinstating. Finding the same fault the hour it happens, with the pipe still visible, means a coupler and ten minutes.
If you are mid-build, the pressure test is not a formality your installer signs off before moving on. It is the last moment the floor is cheap to open.
What the test is actually for
People treat it as one job. It is three, and they answer different questions.
| Stage | Question it answers |
|---|
| After the loops are laid, before screed | Are the pipe and the manifold connections sound as installed? |
| During the pour, pressure still on | Has anything been damaged in the last few hours? |
| After the screed has set | Did the loops survive the pour and the curing? |
Skip the middle one and you lose the only stage that tells you when a pipe was damaged. That matters later, when a leak appears and nobody can say whether it was the pipe, the installer or the screeder.
What a proper test involves
The sequence does not vary much between systems:
- Fill each loop individually from the manifold and purge it. Air left in a circuit will come out of solution over the following hours and drop the gauge, and you will spend a morning hunting a leak that was never there.
- Pressurise to the figure in the pipe manufacturer's instructions. That figure is set by the pipe and fitting system you have actually bought, and it is usually well above working pressure so that a marginal joint fails now rather than later. Do not take a number off a forum, and do not take it from this page — we are not going to invent one. It is printed in the installation manual, and the manual governs.
- Hold it for the period the manufacturer specifies before you read anything into the gauge. A short hold catches a spraying joint. It does not catch a slow weep, and it does not let temperature settle.
- Walk every loop. The gauge tells you the system is losing water. It does not tell you where. While the pipe is visible, your eyes are the best detection tool on site.
- Record the reading, the time and the temperature at the start and end of the hold. A reading with no temperature next to it is close to useless, for the reason below.
Some manufacturers permit an air test instead of water, usually where there is a frost risk before the heating is commissioned. Air behaves differently — it is far more sensitive to temperature and it will not show you a wet patch — so only do it if your own system's instructions allow it, and follow their method rather than adapting a water procedure.
Why the gauge moves when nothing is leaking
This is the single most common false alarm on site, and it costs people days.
Water expands as it warms and contracts as it cools. A sealed loop has nowhere to put that change, so the pressure moves with it. A slab that sat in the sun all afternoon and then cooled overnight can drop the gauge noticeably by morning. Nothing has escaped. The same volume of water is simply taking up less room.
Screed makes it worse in both directions. Cement curing is exothermic: the pour generates its own heat over the first hours, which pushes the reading up, sometimes enough to alarm someone who has just been told to watch for a drop. Then the slab cools over the following days and the reading falls back. A test read at 7am against a figure set at 3pm the day before is not comparing like with like.
Two habits fix this:
- Log the temperature with every reading — slab or ambient, consistently, the same way each time.
- Compare readings taken at the same time of day, once the slab has stopped moving thermally.
The signature of a real leak is different. Temperature movement is a reading that falls and then comes back. A leak falls and keeps falling, and it does not recover overnight. If the gauge has dropped and stayed down across a full warm-cool cycle, treat it as a loss and look for it.
Leave it under pressure through the pour
Of everything on this page, this is the part most often skipped and the part that pays for itself.
The pour is when the pipe is most likely to be hurt. Barrows, boots, rakes, mesh, a dropped tool, a screed rail pinned through a loop. Pipe under a set slab hides all of that. Pipe under pressure does not: puncture it and the gauge falls in front of the person who did it, while the pipe is still exposed and the screed is still wet enough to work.
So keep the pressure on. Put the manifold gauge where the screeding gang can see it, tell them what a falling needle means, and have someone glance at it through the pour. If it drops, stop and find the damage while it is reachable. A coupler now, or a floor breakout later. That is the choice.
Keep it on after the pour as well, through the set, with a further check once the screed has cured. It costs nothing and it closes the gap between the last person off site and the first person to blame.
What to photograph, and why it matters in five years
Disputes about underfloor heating surface a long time after the build, usually when a floor gets wet and three parties disagree about whose pipe it was. The people who win those arguments are the ones with a file.
Before the screed covers anything, take:
- Overheads of every room with the loops laid out, taken from enough angles that pipe runs can be traced afterwards. A phone photo from a step ladder is fine.
- Measurements written into the shots — a tape laid across the floor, or a marked-up plan photographed next to the real thing. This is what stops someone drilling into a loop when they fit a stair rail or a kitchen island three years from now.
- Close-ups of every manifold connection, with the loop numbering legible.
- The gauge itself: at the start of the hold, at the end, during the pour, and after the screed has set. Time-stamped, which phones do automatically.
- The pipe manufacturer's name and the product label on the coil. Later, that identifies the exact system and the exact test regime it required.
Alongside that, keep the installer's test certificate, the pipe manufacturer's instructions, and a dated note of who was on site for the pour. Back it up somewhere off the phone. A dispute years later turns on records, and the record is either taken in that one afternoon or it does not exist.
The same file makes a later leak cheaper. Called to a floor with a documented loop layout, we already know where the pipe runs and roughly where it does not, which narrows the search before any equipment comes out of the van.
When the pressure really has dropped
If you have ruled out temperature and the loss is real, work outward from the easy end:
- Isolate loops one at a time at the manifold. This is the advantage you have before the floor closes and the reason manifold valves exist. If the system holds with one loop shut, you have narrowed the problem to a single circuit.
- Check the manifold itself. A large share of "underfloor leaks" are at the connections, not in the pipe, and the manifold is the one part that stays accessible.
- Look for the obvious wound along the suspect loop. Pre-screed damage is usually a puncture or a crush, not a mystery.
Once screed is down, none of that is visible and the work changes. That is a detection job: thermal imaging to follow the warm pipe run through the slab, and tracer gas on a drained circuit where thermal cannot separate the loops. We have written both up in detail in thermal imaging leak detection and finding a leak under a concrete floor.
Where we fit, and where we do not
To be straight about it: we are not the people who install your underfloor heating or pour your screed, and a pre-screed pressure test is normally your installer's job under their own manufacturer's instructions. If your loops are still exposed and the system holds, you do not need us at all.
Where we come in is the other case — a system that will not hold, or a floor already closed over a fault. We pinpoint it through screed and tile, cut small, repair, and our own handymen and builders put the floor back. That work is on the underfloor heating leak repair page, with the symptoms that usually bring people to it set out under underfloor heating leak.
The commercial terms are the same as everything else we do: £150 an hour, agreed before we attend, no call-out fee, and if we do not find the leak you do not pay for the time spent looking. The full breakdown is on our pricing page. Where a leak has damaged a new floor, the detection visit comes with a written report prepared to support a trace-and-access claim — what the policy then covers is the insurer's decision, not ours.
