How tracer gas leak detection works
What happens when an engineer uses tracer gas to find a leak under a car park, service yard or shop floor, where the method works best, and what it cannot do.
Some leaks refuse to make a sound. The water meter says you are losing water, or the boiler keeps losing pressure, but the pipe runs under a car park, a delivery yard or a tiled shop floor, and nobody can hear or see a thing. This is where tracer gas earns its place.
It is one of three main methods leak detection engineers use, alongside acoustic listening and thermal imaging. This guide explains how it works, when it is the right choice, and what you need to do so the visit goes smoothly.
The idea in one paragraph
A pipe that leaks water will also leak gas. Tracer gas detection makes use of that. The suspect section is isolated and, where needed, drained, then filled with a mix of hydrogen and nitrogen. Hydrogen is the lightest gas there is, so it slips out through breaks too small to hear and works its way up through soil, concrete, tarmac and the joints between paving slabs. An engineer then walks the pipe route with a detector sensitive enough to pick up tiny traces at the surface, and the strongest reading usually marks the leak.
Step by step
- Agree the section to test. The engineer uses the meter readings, any drawings, and the positions of stop taps and valves to decide which run of pipe to test. On a retail site that might be the supply from the boundary to the building, a branch to a customer WC, or a heating circuit under the floor.
- Isolate the pipe. The section is shut off at the stop tap or valves so the gas stays where it is put. Some pipes are drained first.
- Introduce the gas. The mix is fed in through a connection point, such as a tap, a drain-off valve or a fitting, and brought up to pressure.
- Give it time. The gas needs time to escape at the break and rise through the ground or floor above. How long depends on the depth, the surface over the pipe and the ground conditions.
- Sweep the route. The engineer moves a probe along the line of the pipe. On hard surfaces, readings are often taken at joints, cracks and the edges of covers, where gas tends to come up first, and sometimes through small test holes.
- Confirm and mark. The point with the highest reading is checked, often with a second method, and marked for repair. You get a report showing where the leak is and how it was found.
Where tracer gas works best
| Situation | Why tracer gas helps |
|---|---|
| Plastic supply pipes | Plastic generally carries leak noise less well than metal, so listening can struggle |
| Very small leaks | Gas can escape through a break that makes little or no noise |
| Low pressure pipework | Acoustic listening relies on water escaping under pressure |
| Heating circuits under the floor | A drained heating circuit can be gassed and swept like a supply pipe |
| Long runs under car parks and yards | The engineer can follow the route over a large area of hard standing |
| Noisy sites | Traffic, plant and trading noise do not drown out a gas reading the way they drown out a listening survey |
What tracer gas cannot do
It is a very useful method, and it still has limits worth knowing before the visit.
- It needs the pipe isolated. If the section cannot be shut off, because a stop tap is seized or missing, that has to be sorted first.
- It needs a rough idea of the route. The engineer has to sweep somewhere. Drawings, photographs from the original fit-out, and the positions of stop taps and covers all help.
- Sealed and deep surfaces slow it down. Thick sealed concrete, membranes and deep pipes can make the gas slower to reach the surface, and wind can scatter it outdoors.
- It finds the break and nothing more. It tells you where gas is escaping. It says nothing about the condition of the rest of the pipe, so the engineer may recommend other checks.
Before, during and after the visit
Before. Tell the engineer about the building: its age, any drawings, where the meter and stop taps are, and whether anything relies on a constant water supply, such as a coffee machine, an ice machine or produce misting. HSE advises that if a building was built or refurbished before 2000, you should assume asbestos is present, and that work disturbing the building’s fabric needs a more intrusive survey first. Have your asbestos information to hand before anyone plans to open a floor.
During. Expect the isolated section to be without water for a while. Plan around customer and staff toilets and hand washing, and brief the team on where the engineer will be.
After. Once the leak is repaired, the pipework is refilled and checked. Schedule 2 of the Water Supply (Water Fittings) Regulations 1999 says every water system shall be tested, flushed and where necessary disinfected before it is first used, and good practice after a repair follows the same steps. The same schedule treats pipes below ground, under a wall or foundation, or in a chase or duct as concealed fittings, and says fittings with joints must not be concealed. That is worth raising with whoever does the repair if a pipe is being re-routed. Water Regs UK publishes guidance on the regulations, and a WaterSafe approved plumber can advise on compliant repairs.
What the report gives you
A good tracer gas survey ends with a marked location and a written record: what was tested, how, where the readings peaked, and photographs of the marked point. That record is what an insurer or landlord will want to see, and it is what the repair contractor needs to open up the right spot. Our reports are written to be insurance-ready, and there is more on claims on our insurance claims page.
About ADI Leak Detection
ADI has worked on leaks for over a decade, with 25 engineers across England, Scotland and Wales. We are accredited by CIPHE, WIAPS, WaterSafe and SPATA, and carry £5m public liability cover. We aim to find and repair on the same day, although that depends on what we find.
Tracer gas is used across our water mains, underfloor and heating surveys, and you can see the whole process on how it works.
To book a survey, call 0333 567 2615.
Frequently asked
Is tracer gas used inside an open shop?
It can be. The gas is a hydrogen and nitrogen mix supplied in cylinders and handled under the engineer's own procedures. Ask for the risk assessment and method statement before the visit so your team knows what to expect and where the engineer will be working.
Will we lose water while the test runs?
Usually, for the section being tested. The pipe has to be isolated so the gas stays in it, so agree the isolation with the engineer in advance and think about customer toilets, staff hand washing and any equipment on that supply.
Can tracer gas find a leak on a heating system?
Yes. A heating circuit can be drained and filled with tracer gas in the same way as a water pipe.