Why is my underfloor heating losing pressure?
Wet underfloor heating (UFH) is a set of plastic pipe loops buried in screed or sitting in a spreader plate system under the floor, all fed from a manifold. Like a radiator system, it is sealed. If the pressure keeps falling, water is getting out somewhere, and in a UFH system the possible places are fairly limited: the manifold and its fittings, the pump and mixing set, the connections where each loop joins the manifold, the loop pipework under the floor, or the boiler or heat pump that supplies it.
The loops themselves are usually one continuous length of pipe with no joints under the floor, so a leak in a loop almost always has a physical cause. A screw or nail driven into the floor is the classic one, often from fitting skirting, door thresholds, kitchen units or a new floor finish. Damage during the original screed pour and poorly made repair joints are the other usual suspects. That is useful to know, because it tells you where to look first: near walls, door openings and anything fixed to the floor since the heating went in.
Is the underfloor heating on its own circuit or shared with the boiler?
Before chasing a leak, work out how the system is plumbed, because it changes which gauge you should be watching.
- UFH run from a combi or system boiler through a mixing unit. The UFH shares water with the radiators. The boiler gauge shows the pressure for the whole system, so a drop could be in the radiator circuits or the underfloor loops.
- UFH run from a heat pump. The heat pump system usually has its own pressure gauge, often near the indoor unit or cylinder. The UFH manifold may be the main or only emitter.
- A manifold with its own gauge. Some manifolds carry a small pressure gauge. It is useful for isolation tests, but it only reads correctly while that manifold is connected to the rest of the system.
If radiators share the circuit, it is worth reading our guide to what to do when a boiler keeps losing pressure as well, because the leak may not be under the floor at all.
Can the leak be at the manifold?
Yes, and it is the first place to check because you can see it. Take the cover off the manifold cabinet or look at the manifold on the wall and check each of these with a torch and a dry tissue:
- The compression or push-fit connections where each loop pipe joins the flow and return bars.
- The flow meters or balancing valves on top of the flow bar. The small sight glasses can weep at their seals.
- The valve bodies under the actuator heads on the return bar. Pull an actuator off gently and look at the valve pin for moisture or crusting.
- The pump, its unions and the blending or mixing valve.
- The automatic air vents and drain cocks at the ends of the bars.
- The floor directly beneath the manifold, where slow drips often collect unseen at the back of the cabinet.
A white or green crust, a tide mark or a slightly damp screed under the manifold is enough to point the search there. Tightening fittings on a pressurised manifold without knowing the fitting type can make things worse, so leave repairs to a heating engineer.
How do you isolate underfloor heating loops to find the leak?
Most UFH manifolds allow each loop to be shut off at both ends, which means the loops can be tested one at a time. Done properly, this is the single most useful step in any UFH leak investigation, because it turns a whole floor into one pipe.
- Record the starting pressure with the system cold and the loss rate over a few days, so there is a baseline.
- Shut the manifold's main flow and return isolating valves so the manifold is separated from the boiler or heat pump side.
- Close every loop at both ends: the flow meter or lockshield on the flow bar and the valve on the return bar.
- Connect a test pump and gauge to the manifold fill and drain points.
- Open one loop at a time, pressurise it and hold it, watching the gauge. A sound loop holds. A leaking loop drops.
- Repeat for every loop, labelling each one against the room it serves.
- Test the primary side separately if all the loops hold, because the leak may be on the pipework between the boiler and the manifold.
Test pressures and hold times should follow the pipe and manifold manufacturers' data. The installation standard for water based underfloor heating, BS EN 1264, calls for circuits to be pressure tested before the screed is laid, and an installer's commissioning sheet, if you have one, will usually record the figures used. A loop test on an existing floor is a controlled job with the right test pump and gauge, and it is not something to improvise with the boiler filling loop.
How does a thermal survey find the leak within a loop?
Once the leaking loop is known, the search area is one room or one zone. The loop is then run hot, on its own if possible, and the floor is scanned with a thermal imaging camera. A healthy loop shows as an even pattern of warm lines, spiralling or snaking across the room. A leak shows as a warm spread that breaks away from that pattern, often strongest at one point and fading outwards as the escaping water spreads through the screed.
Timing matters. If the floor is scanned too early, the pipe pattern has not come through yet. If it is scanned after hours of running, the whole floor is warm and small differences wash out. Running the loop from cold and watching the pattern develop gives the clearest picture, and a second scan later helps confirm it. The floor finish makes a big difference to what the camera can see.
| Floor finish over the screed | How clearly the pipe pattern shows | Notes |
|---|---|---|
| Ceramic or stone tiles | Clear | Dense, conductive finishes show the loop layout and leaks well |
| Vinyl, LVT or thin laminate | Usually clear | Glossy surfaces can reflect heat sources, so the angle of view matters |
| Engineered timber | Moderate | Timber slows and spreads the heat, so allow more time |
| Carpet and underlay | Weak | Thick underlay insulates the surface. Lifting a corner or using other methods may be needed |
| Spreader plates under a timber floor | Moderate | The plates even out heat, which can blur a small leak |
For the background on how the cameras work with UFH, our earlier underfloor heating leak detection guide covers the basics of the survey itself.
What if thermal imaging cannot see the leak?
Sometimes the loop is known to be leaking but the thermal image is inconclusive, usually because of a thick floor finish or a very slow leak. There are two further tools. Acoustic listening can pick up escaping water under pressure, although plastic pipe carries sound poorly, so the microphone has to be close to the leak. Tracer gas is often the better option for UFH: the leaking loop is drained, filled with a hydrogen and nitrogen mix, and the floor is swept with a detector that picks up the hydrogen as it comes through the screed and the joints in the floor finish.
Combining the loop test with one of these methods is what lets the repair be made through a single small opening instead of lifting a whole room.
What does an underfloor heating leak repair involve?
With the leak marked, the floor finish and screed are opened over a small area directly above it, the damaged section of pipe is cut out and a coupling suitable for that pipe type is fitted. The loop is then pressure tested before the screed is made good and the finish relaid. The size of the opening depends on the floor build-up and the depth of the pipe, but a precise location keeps it to the minimum.
If a loop has been damaged in several places, or a repair would sit under a kitchen island or fitted unit, an engineer may suggest abandoning that loop and supplying the room another way. That decision is easier when the survey report shows exactly where the damage is.
When should you call a leak detection specialist?
If you have found a weeping fitting at the manifold, a heating engineer can usually deal with it. Call a specialist when the manifold is dry and the pressure still drops, when a loop test has identified a leaking circuit but not the spot, when there is a warm or damp patch on the floor, or when you need an insurer-ready report.
Our underfloor heating leak detection service combines loop isolation, thermal imaging and tracer gas where needed. Under-floor leak detection is priced at £300 to £700 depending on the floor area, the number of loops and the methods required, and every survey is no find, no fee. Call 020 3912 0200 to talk it through.
Frequently asked questions
How do I know if my underfloor heating is leaking?
The main signs are a system that needs regular topping up, a floor area that feels warmer or damper than the rest, lifting or staining of the floor finish, and one room or zone heating poorly compared with others.
Can a leak in underfloor heating be found without lifting the floor?
Usually, yes. Isolating the loops identifies the leaking circuit, and thermal imaging or tracer gas then pinpoints the spot, so only a small area is opened for the repair.
Should I keep running underfloor heating with a leak?
If the loss is slow and there is no visible damage, running it for a short time is usually acceptable, and a thermal survey needs the system working. If water is appearing at the surface, isolate that loop at the manifold and get it looked at.
What pressure should underfloor heating be at?
It depends on how the system is supplied. Where the UFH runs from a sealed boiler or heat pump circuit, it shares that system's pressure, and the boiler or heat pump manual gives the recommended cold figure.
Can I use a leak sealer in underfloor heating?
Check with the pipe and manifold manufacturers first. Sealers can collect in flow meters, valves and narrow pipework, and a physical puncture from a screw or nail will not be sealed reliably.
