What is tracer gas leak detection?
Tracer gas leak detection finds a leak in a water pipe by replacing the water with a gas that can be picked up at the surface. The pipe is isolated, drained, and filled with a gas mix under pressure. Wherever the pipe is damaged, the gas escapes and works its way up through the ground, screed or building fabric. An engineer sweeps the surface with a detector, and the point where the reading peaks marks the leak.
It is used when the other main methods cannot do the job. Acoustic detection needs a leak that makes noise, and plastic pipe or a very slow leak may barely make any. Thermal imaging needs a temperature difference, which cold water pipes often do not give. Tracer gas depends on neither.
What gas is used, and why?
The standard gas for water leak work is a mix of 5% hydrogen and 95% nitrogen, often called forming gas. At that concentration the mix is classed as non-flammable under ISO 10156, and it is non-toxic. Nitrogen already makes up most of the air we breathe.
Hydrogen is used as the tracer because it is the smallest and lightest molecule. It passes through tiny openings, moves quickly through soil and porous materials, and rises. Hydrogen detectors are very sensitive, and because there is almost no hydrogen in normal air, a small amount stands out clearly against the background.
How does a tracer gas leak test work, step by step?
- Confirm there is a leak and on which section. A meter test, a pressure drop test or a loop test narrows the search to one pipe or circuit. Tracer gas works best when the engineer knows which run to fill.
- Trace the pipe route. The detector has to be swept along the pipe, so its path is established first by locating equipment, thermal imaging or plans.
- Isolate the pipe. The section is shut off at both ends. For a supply pipe, that usually means the boundary stop tap and the internal stop tap. For a heating circuit, it means closing the relevant valves and isolating the boiler.
- Drain down. Enough water is removed for the gas to reach the leak. On a supply pipe this may involve opening a tap or fitting at the low point. On heating, the circuit is drained through a drain cock.
- Inject the gas. The gas is fed in from a cylinder through a regulator, at a controlled pressure suited to the pipe.
- Wait. The gas needs time to leave the pipe and travel to the surface. That time depends on depth and the materials above.
- Sweep the surface. The engineer moves the detector probe slowly along the route, over joints in paving, cracks in screed, grout lines and the edges of floors, where gas tends to emerge.
- Pinpoint. Where readings rise, the area is checked more closely. On hard surfaces a small hole may be drilled to let the probe sample directly below the surface.
- Purge and refill. The gas is vented, the pipe is refilled and bled, and the system is put back into service.
Why does the pipe need draining down?
Gas cannot push through a pipe that is full of water. If the leak is under water, the gas has to displace that water before it can escape, and a slow leak may not let it do so in any reasonable time. Draining the pipe, or at least lowering the water well below the likely leak, gives the gas a clear path.
This is the main practical cost of the method. The water supply or the heating is off for the duration of the test, and draining and refilling take time. On a heating circuit, refilling means bleeding air and checking the inhibitor afterwards. That is part of why tracer gas is often used after other methods have narrowed the area, rather than as the first step.
Where does tracer gas work best?
- Plastic supply pipes. MDPE and other plastics carry sound poorly, so acoustic methods lose sensitivity. Gas does not care what the pipe is made of.
- Underfloor heating loops. Once loop isolation has identified the leaking circuit, gas pinpoints the spot under screed and tiles. Our underfloor heating leak detection surveys use it where thermal imaging is not conclusive.
- Heating pipes buried in concrete. Particularly where the floor finish blocks a clear thermal image.
- Very slow leaks. A weep that makes no noise can still let gas out.
- Long runs under hard landscaping. Driveways and paths with joints give gas a way to surface along the route.
Where can tracer gas fail or give misleading results?
No method works everywhere, and it is better to know the limits before a survey than after.
- Very deep pipes. The deeper the pipe, the longer the gas takes to surface and the more it spreads out on the way, so the peak reading becomes broad and weak.
- Saturated or heavy clay ground. Wet clay has few open pores for the gas to travel through. The gas may take a long time to appear, or find another route.
- Large sealed surfaces. A wide area of continuous tarmac, a sealed resin floor or a membrane can stop the gas rising directly. It then moves sideways and surfaces at the nearest joint, edge or crack, which may be some distance from the leak.
- Ventilated voids. In a suspended timber floor with airbricks, or a well ventilated duct, the gas is diluted and carried away by air movement, so the detector sees less.
- Windy conditions outside. Wind disperses gas at the surface and makes a steady reading harder to get.
- A pipe that cannot be isolated or drained. If a stop tap will not close, or a shared supply pipe cannot be shut off without affecting neighbours, the test may need to wait until that is sorted.
- More than one leak. Gas escapes at every defect. A strong reading at one point does not rule out another nearby.
An experienced engineer allows for these by checking where gas is likely to surface, sampling through small drilled holes where needed, and confirming the result with another method where possible.
How does tracer gas compare with acoustic and thermal methods?
| Tracer gas | Acoustic | Thermal imaging | |
|---|---|---|---|
| Needs the pipe in service? | No, pipe is drained and gassed | Yes, under pressure | Yes, and for heating, running hot |
| Plastic pipe | Works well | Reduced sensitivity | Works if there is a temperature difference |
| Very slow leaks | Often effective | May be too quiet | May be too small to show |
| Cold water pipes | Works well | Works well on metal | Often little contrast |
| Disruption during test | Supply or heating off for the test | Minimal | Minimal |
In practice these are used together. For the listening side, see our explainer on acoustic leak detection.
Is tracer gas safe to use in a home?
The 5% hydrogen and 95% nitrogen mix used for leak detection is non-toxic and classed as non-flammable. The volumes used for a domestic pipe are small, and the gas is vented after the test. The main safety points are practical ones: the cylinder is handled and secured properly, the pipe is pressurised within its limits, and the water supply or heating is restored correctly afterwards.
When should you call a leak detection specialist?
Consider tracer gas when a meter test or pressure test proves a leak but an acoustic or thermal survey has not found it, when the pipe is plastic, or when the leak is under a floor finish you do not want to lift. Tracer gas testing is £200 to £350, and a combined survey using thermal, acoustic and tracer gas methods is £250 to £500. The written report is included and every survey is no find, no fee. See our water leak detection page or call 020 3912 0200.
Frequently asked questions
What gas is used for tracer gas leak detection?
A mix of 5% hydrogen and 95% nitrogen. It is non-toxic and classed as non-flammable, and the hydrogen can be detected in very small amounts.
Does tracer gas leak detection work on plastic pipes?
Yes. It is often the best method for plastic pipes, because it does not rely on the pipe carrying sound.
Will my water be turned off during a tracer gas test?
Yes. The pipe being tested has to be isolated and drained, so the supply or heating on that section is off until the test is finished and the pipe refilled.
How accurate is tracer gas leak detection?
On a shallow pipe under permeable ground it can be very precise. Depth, wet clay and sealed surfaces can spread the gas, so results are confirmed by sampling and, where possible, another method.
Can tracer gas find a leak under a concrete slab?
Often, yes. The gas rises through the concrete and escapes at joints, cracks and edges. Where the slab is sealed, small sampling holes help locate the peak.
