Tracer gas leak detection pumps a safe mix of 5% hydrogen and 95% nitrogen through an emptied pipe. The gas is lighter than air and escapes through even the smallest crack, rising up through soil, screed or flooring to the surface, where a sensitive probe detects it. It works well on plastic pipes and low-pressure systems where acoustic methods struggle to pick up a clear sound.
In This Guide
What tracer gas leak detection actually is
Tracer gas leak detection finds leaks that other methods cannot always hear or see. Instead of listening for the sound of water under pressure, or scanning for a temperature difference with a thermal camera, the technique introduces a gas into the pipe itself and tracks where that gas escapes. Wherever the pipe has failed, gas comes out through the same crack the water has been leaking from, and it rises to the surface where it can be picked up with a handheld probe.
It is one of three main methods we use, alongside acoustic listening and thermal imaging, and it earns its place because some pipe materials and some leak types simply do not give acoustic equipment much to work with.
How the process works, step by step
Step 1: Isolate and drain the pipe
The suspect section of pipework is isolated from the rest of the system and drained of water, since the gas needs empty pipe to travel through freely.
Step 2: Introduce the tracer gas
A safe mix of 5% hydrogen and 95% nitrogen is fed into the pipe under gentle pressure. Because hydrogen is one of the lightest gases there is, it finds its way through gaps that water alone might seep through far more slowly.
Step 3: Scan the surface with a probe
A sensitive gas detector is swept across the floor, wall, or ground above the suspected pipe run. Where gas has escaped and risen through the material above it, the probe picks up a reading, pinpointing the leak to a small, specific area.
Reading the probe on-site
The detector gives a live reading as it passes over the floor or ground, with the signal strengthening the closer it gets to the escape point. Once the strongest reading is found, that spot marks the leak, usually within a very tight radius rather than a rough estimate.
Why hydrogen and nitrogen, and why it is safe
The 5% hydrogen, 95% nitrogen mix is chosen specifically because it sits well below the concentration needed for hydrogen to be flammable in air. It is inert, non-toxic, and does not react with the pipe material, the water system, or anything else in the property. This is a completely different situation from a gas supply leak: if you ever smell gas from a boiler or gas appliance, that is a gas emergency, and you should call the National Gas Emergency line on 0800 111 999 and evacuate. Leak detection firms do not handle gas emergencies, and tracer gas surveys are unrelated to a property's gas supply entirely.
Because the mix is safe to use in occupied buildings, we can run a tracer gas survey while a family is still living in the house, without needing anyone to leave or any part of the property shut down beyond the pipe being tested.
When tracer gas beats acoustic detection
Acoustic listening works by picking up the sound of water escaping under pressure. That works brilliantly on metal pipes carrying mains-pressure water, where the sound travels well and is easy to isolate. It works far less well on plastic pipework, which dampens sound rather than carrying it, or on low-pressure systems like central heating circuits, where there simply is not enough pressure behind the leak to make a clear noise.
Tracer gas does not rely on sound at all, so it steps in exactly where acoustic falls short. It also indicates a leak of any type, so even a very slow weep is caught, whereas acoustic detection needs enough pressure and flow to generate an audible signal.
Tracer gas works well for
- Plastic and MDPE pipework
- Low-pressure central heating circuits
- Leaks under concrete or screed floors
- Underfloor heating loops
Acoustic tends to suit
- Metal pipes under mains pressure
- Leaks with a clear, loud escape point
- Situations where the pipe cannot be drained
Where tracer gas is used
It is a common choice for slab leaks, since gas passes through a concrete floor far more readily than water does, rising up to the room above where the probe can find it without lifting the slab first. It is also well suited to underfloor heating, where plastic loops run beneath screed and the system operates at relatively low pressure, both factors that make acoustic detection unreliable. If a leak is suspected in a heating loop, this method often confirms the exact loop and location before anyone touches the floor.
Our tracer gas leak detection service covers exactly these scenarios, and it pairs well with thermal imaging on the same visit, since a warm patch flagged by the camera can be confirmed with a gas reading before any repair work starts.
Suspect a leak in plastic pipe or underfloor heating?
Tracer gas finds leaks that acoustic methods can miss, without draining your whole system on a guess.
Tracer gas versus other methods
| Method | Best for | Limitation |
|---|---|---|
| Tracer gas | Plastic pipes, low pressure, underfloor heating | Pipe must be isolated and drained first |
| Acoustic listening | Metal pipes, mains pressure, quick surface checks | Struggles on plastic or low-pressure systems |
| Thermal imaging | Spotting warm or damp zones fast | Confirms a suspect area, does not pinpoint alone |
In practice we rarely rely on just one method. Thermal imaging often narrows the search area first, and tracer gas or acoustic listening then confirms the exact point. Our guide on how thermal imaging finds hidden water leaks covers that side of the process in more detail, and our page on how acoustic leak detection works explains the listening method this technique often works alongside.
What to expect during a survey
A typical tracer gas survey starts with a conversation about where the leak is suspected and which pipe run feeds that area. The engineer isolates the relevant section, drains it, and introduces the gas. Scanning the surface above the pipe run usually takes anywhere from twenty minutes to an hour or so depending on the size of the area, and most jobs are completed same day. You get a written report at the end marking the exact location, which is accepted by insurers as evidence for a claim.
Frequently Asked Questions
Yes. The gas used is a safe 5% hydrogen, 95% nitrogen mix, well below flammable concentration, non-toxic, and inert. Surveys can be carried out while a property is still occupied without anyone needing to leave.
No. The gas is introduced through existing pipework and simply escapes through the crack already causing the leak. Nothing is cut, drilled, or lifted to run the test itself, which is why it is classed as a non-invasive method.
Acoustic listening relies on the sound of water escaping under pressure, which plastic pipes and low-pressure systems do not produce clearly. Tracer gas does not depend on sound, so it works reliably where acoustic detection struggles.
Yes, and it is one of the most reliable methods for slab leaks. The gas passes through concrete more easily than water does, rising to the surface where it is detected with a probe, without needing to break up the floor.
Most surveys are completed the same day, with the scanning stage typically taking between twenty minutes and an hour depending on how large the suspect area is and how the pipe run is laid out.
No, they are unrelated. If you smell gas from a boiler or appliance, that is a gas emergency: call the National Gas Emergency line on 0800 111 999 and evacuate. Leak detection firms do not attend gas emergencies.
Yes. Underfloor heating loops are plastic and run at low pressure, both of which make acoustic detection unreliable. Tracer gas is a common and effective method for locating the exact loop and point of failure.
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