Tracer Gas Leak Detection, Explained

What the method actually is

Every other detection method listens to a leak, looks at the heat it makes, or measures the damp it leaves behind. Tracer gas does something different: it replaces the water with a gas that can get out of the building, and then finds the gas.

The gas is a mixture of 5% hydrogen in 95% nitrogen. Sewerin, who make the detectors most UK leak engineers carry, describe it as non-toxic and non-corrosive, and safe to use on drinking water pipework. It is sold in the UK as a standard cylinder gas, the same blend the refrigeration trade has used for years. Hydrogen is the smallest molecule there is, so it passes through openings that water only weeps through, and it is buoyant, so once it is out of the pipe it goes up rather than sideways.

The part that matters for your floor: hydrogen will come up through asphalt, concrete, screed and soil. That is why the method is described as non-invasive. Nothing is lifted to find the leak. Something is lifted afterwards, at one point, to repair it.

How a tracer gas visit runs

1. Isolate and prove the loss. Before any gas comes off the van, the suspect circuit is isolated and pressure tested. This confirms there is a real loss and separates a leaking pipe from a failed expansion vessel, a passing valve or a shower tray that only leaks when someone stands in it.
2. Drain the circuit. This is the step people are not told about, and it is the method's biggest constraint. Water left in the pipe blocks the gas and keeps escaping at the leak, so the detector follows the water rather than the gas. The pipe has to be empty.
3. Charge the pipe with the mixture. The gas goes in through a regulator at a low, controlled pressure, usually via a drain point, a radiator tail or a manifold port. It fills the run and finds the same hole the water was finding.
4. Sweep the surface. The engineer walks the floor with a hydrogen probe, in a grid, moving slowly. Detectors of this class read from single parts per million upwards, so the first indication is often well before the strongest one.
5. Find the peak, then confirm it. A single reading is not a result. The engineer works out from the first hit in both directions to establish where the signal is strongest and where it falls away, then re-tests after airing the room to make sure the peak repeats.
6. Mark and open once. The floor is cut at the marked point, the leak is exposed, and the pipe is repaired. On a screed floor that is typically a single access hole rather than a lifted floor.

What tracer gas finds that acoustic and thermal cannot

Leaks under screed and concrete

A pipe buried in 60mm of screed under tile is acoustically dead and thermally muted. Acoustic kit needs the leak noise to travel to a surface the engineer can put a microphone on, and screed absorbs it. Thermal imaging needs the leak to change the surface temperature enough to see, which it often does not on a slow weep or a cold-water pipe. Gas does not care about either. It leaves the pipe, works its way up through the screed and arrives at the tile grout.

Leaks on plastic pipe

Plastic pipework, particularly MDPE on a supply run and barrier pipe in a heating screed, is the acoustic method's worst case. Leak noise in plastic pipes is heavily attenuated with distance and what survives is low frequency and narrow band, which is exactly the part of the spectrum that a building's own noise floor sits in. Most of London's post-1990 supply pipes and nearly all underfloor heating circuits are plastic. Tracer gas works the same on plastic as on copper, because the gas leaves through the hole either way.

Systems that make no noise at all

Acoustic detection needs pressure. A leak on an unpressurised system, a gravity circuit, a drain, or a heating system that has already emptied itself makes nothing to listen to. Sewerin market their tracer gas detector specifically for leaks in what they call noiseless and unpressurised areas, which is an honest description of the gap it fills. If your boiler gauge has dropped to zero and stays there, there is nothing for a microphone to hear, and gas is the method that still works.

What tracer gas cannot do

This is the part competitors leave out, and it is the part worth knowing before you book anyone.

• The system has to be drained. If the pipe cannot be isolated and emptied, the method cannot run. In a flat where the leak is on a communal riser, or on a live mains you are not permitted to shut, that is a real blocker and the answer is a different method.
• Gas takes the path of least resistance, and that path is not always vertical. Detector manufacturers are explicit about this: on a concrete slab the gas may surface at a crack, an expansion joint or the edge of the slab rather than directly above the leak. A good engineer treats a strong signal as a small search radius, not a cross on the floor, and confirms it with a second method before cutting.
• Sealed floor coverings slow it down. Poured resin, thick vinyl welded at the edges and some epoxy damp-proof membranes hold the gas under the floor and push it out somewhere else. It still works, but the survey takes longer and the reading needs more interpretation.
• Draughts wreck the readings. An open door, a running extractor or an underfloor void with good cross-ventilation dilutes the gas below the level that gives a usable peak. Rooms get shut up and settled before the sweep, which is why the engineer will ask you to stop opening windows.
• It finds the leak, not the reason. A pinhole found by gas may be one of several on a corroded run. Locating it does not tell you whether the pipe has years left, and an honest engineer will tell you when a repair is a patch on a system that wants re-running.
• Drinking water pipework needs flushing afterwards. Schedule 2 of the Water Supply (Water Fittings) Regulations 1999 requires that every water system is tested, flushed and where necessary disinfected before it is first used. A supply pipe that has had gas in it gets refilled and flushed through before it goes back into service.

Is it safe in an occupied house?

The blend is deliberately sized to be safe. At 5% hydrogen in nitrogen the mixture is supplied as non-flammable, and once it leaves the cylinder the concentration can only fall as it mixes with air, so it cannot build up into something combustible. It is not toxic and it does not corrode pipework. The practical precautions are the ordinary ones: the room is ventilated after the survey, and the engineer does not charge a system that has a gas appliance connected to it without isolating first.

When a different method is the right one

Tracer gas is not automatically the answer. If the leak is on a live, pressurised metal supply pipe in a quiet property, acoustic will usually find it faster and without draining anything. If a hot pipe is leaking under a tiled floor and the heating has been running, thermal imaging can narrow it to a metre in ten minutes. If the question is whether the wall is wet at all, that is moisture mapping, not gas. Most real visits use two or three methods in sequence, and gas is the one that finishes the job when the others have narrowed it but not pinned it.

What it costs, and what happens next

Detection is charged at £150 per hour, fixed before we attend. If we do not find the leak, you do not pay for the time we spent looking. The total depends on how long the job takes, because a single-room screed sweep and a whole-house supply trace are not the same job.

The difference here is what happens after the mark on the floor. Most London firms that own a tracer gas kit hand you a report and leave. We open the floor, repair the pipe, and put the screed, tile or board back. If you are claiming, the detection report is written to be read by a loss adjuster and the trace and access element of the claim covers the opening and the making good.

See gas tracer leak detection for how we run it, or pricing for the rate in full.