Concealed Pipe Detection

Tracer Gas Leak Detection London

Tracer gas is what you reach for when sound will not travel and heat will not show: plastic and PEX pipework that absorbs leak noise, circuits cast into screed or slab, and buried runs under drives and planting. The mixture is 5% hydrogen in 95% nitrogen, well below hydrogen's flammable threshold, and it works because the hydrogen molecule is small enough to migrate up through screed, tile beds, soil and micro-cracks to a detector at the surface.

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Quick answer

Tracer gas is a second stage method rather than a first reach. The circuit is drained, a hydrogen and nitrogen mix is introduced, and the gas migrates up through screed or soil to a surface detector. It needs the system out of service and it needs migration time, so it is scheduled deliberately rather than deployed on arrival, and a peak has to reproduce after ventilation before it counts.

What builds the fee for a tracer gas survey

Stage of the survey	What sets the fee	What lengthens it
Establishing that the prerequisites are met	Whether the circuit can actually be drained and taken out of service	Systems that cannot be interrupted during occupied hours
Draining, purging and charging the section	Volume of the section and how it is valved	Circuits with no drain point, needing a temporary one
Allowing migration through the build up	Depth and density of what sits between the pipe and the surface	Deep screed, thick soil cover and sealed floor finishes
Surface detection and mapping of peaks	Area to be swept and how many candidate peaks appear	Ventilation paths that carry gas away from the breach
Reproducing a peak after ventilation	Whether a single peak is decisive or several must be resolved	Peaks that do not return, requiring recharge and a second sweep

We do not publish a price list, because the figure is built from the property rather than picked from a menu. The detection fee is confirmed in writing before an engineer travels, and repair is quoted separately once there is a located point to quote against.

It is a second stage method, not a first reach, and any firm treating it as a default is charging you for the most involved option before the cheap ones have been tried. Gas cannot be introduced until the circuit has been shown to fail a pressure decay test, isolated from everything else, and drained down, because the pipe has to hold gas rather than share space with water. That preparation is most of the work.

What the detector gives you is a concentration gradient, and reading it is the skill. Gas can travel along a service route and surface some distance from the breach, and permeable finishes spread the signal. So a peak is mapped, the area ventilated and the gas re-introduced to see whether the same peak returns in the same place. A point that reproduces is a finding. A point that does not is a reason to keep working.

Tracer gas is quoted on the preparation more than the scanning: which circuit has to be drained, how it will be refilled and dosed, and how long the construction needs for the gas to surface. All of that is set out before the work is authorised.

What you get

Definitive for non-metallic pipes (PEX, polyethylene) where acoustic methods are ineffective.
Pinpoints leaks under concrete floor slabs, in cavity walls, and beneath landscaped areas.
Essential for pressurised closed-loop systems like underfloor heating and chilled water lines.
Minimally invasive, protecting historic fabric in listed buildings and high-spec interiors.
Effective in complex multi-layer builds common in Central London commercial properties.
Provides unambiguous evidence of leak location for insurance and contractor reports.

How it works

A method, not a guess

Confirm the circuit actually fails

A pressure decay test on the isolated circuit has to show loss before any gas is bought or injected. This step also rules out the boiler components and valves that mimic pipe leaks, which is the most common reason a tracer gas job should not go ahead at all.

Drain down and prepare the circuit

The circuit is drained, open ends are capped and the injection point is chosen at a manifold, valve or drain-off that gives the gas access to the whole run. On heating systems we plan the refill and inhibitor dose at this stage rather than after.

Inject and allow migration time

The 5% hydrogen mix is introduced at working pressure and given time to find its way out and up through the construction. Screed and slab take longer than a timber void, and rushing this stage is how a survey produces a diffuse signal and no usable point.

Scan for the gradient, then reproduce it

The surface is scanned methodically with a sniffer probe to map the concentration gradient rather than stopping at the first reading. The area is then ventilated and the gas re-introduced to confirm the same peak returns before the point is marked on the building.

Before you book anyone

Six things to know before you book tracer gas leak detection in London

Tracer gas is a second stage method, so check the first stage happened

Gas is the most involved and most expensive thing in the toolkit, and it should only be reached for once a pressure decay test has shown a specific circuit failing. Booking tracer gas as an opening move means paying for drain down, consumable gas and a methodical surface sweep on pipework that may not be leaking at all. Ask the firm what test has to fail before they will inject anything. If the answer is none, they are selling equipment time rather than a diagnosis.

Draining the circuit has consequences worth planning for

Gas and water cannot occupy the same pipe, so the circuit has to come down before injection. On a heating system that means the refill, the air purge and a fresh corrosion inhibitor dose are all part of the job, and on a large system that is time and materials somebody has to account for. Ask whether the inhibitor is included and who is responsible for the refill. This is the part most commonly left out of a headline detection price.

Confirm the mix and ask what happens to the gas afterwards

The professional mixture is 5% hydrogen in 95% nitrogen. At that concentration it is inert, non-toxic, leaves no residue and sits below the point at which hydrogen becomes flammable in air, and it is accepted for use in potable systems and occupied buildings. The safety case depends on the operator using that certified mix rather than something improvised, and on the system being properly vented afterwards. Both are fair questions to ask before anyone connects a regulator to your pipework.

Ask how they handle a peak that does not reproduce

Hydrogen is the smallest molecule in the atmosphere, which is why it works and also why it wanders. Gas can travel along a duct or service route and surface some distance from the breach, permeable screed spreads the signal across a wide area, and a sealed floor finish can suppress it over the true point and let it emerge at the nearest grout line. A single peak is a candidate, not a conclusion. The correct answer to this question is that the area is ventilated and the gas re-introduced to see whether the same peak returns in the same place.

Migration time is part of the method, not padding on the invoice

Gas needs time to find its way out of the pipe and up through the construction, and a deep screed pour with a sealed finish takes considerably longer than a suspended timber void. Scanning too early produces a faint, diffuse reading that is easy to misplace, which is the most common reason a tracer gas survey produces an approximate answer. If a quoted visit duration looks short for a solid floor, ask how much of it is allowed for migration.

On a communal riser, sort the authorisation before the visit

Tracer gas is frequently the only method that will work on riser pipework in a block, and it needs that riser isolated, which means the freeholder or managing agent authorising both the isolation and the access. Establish which sections come out of service and for roughly how long, so notice can go to residents in advance. Surveys of communal pipework fail far more often on access arrangements than on technique.

Compare like for like

Tracer gas is a second stage, not a first reach

Gas reaches what acoustic and thermal cannot, and it is also the method most often deployed prematurely, because it looks decisive. It needs the circuit drained, it needs migration time, and it produces peaks that have to be earned rather than accepted. Here is how that plays out three ways.

What to check	A visit that starts by scanning	A firm that finds and opens on one price	London Leak Detection 247
Prerequisites before charging the system	Gas goes in because the previous method drew a blank and something has to happen.	Gas goes in because it produces a mark quickly and a mark supports an opening.	The circuit has to be drainable, isolatable and confirmed as the failing one by decay testing first. Gas applied to a circuit that was never shown to be losing water locates nothing and costs a day.
Migration time	Not allowed for. A sweep taken too early finds nothing and the survey is called inconclusive.	Compressed to fit the visit, which produces a peak somewhere and a scope of works.	Time is allowed according to what sits above the pipe. Deep screed, soil cover and sealed finishes all slow migration, and the wait is part of the method rather than an inconvenience to be shortened.
How a peak is treated	The first peak is the answer, and gas escaping through a joint or a service penetration reads identically to gas escaping at a breach.	The strongest peak becomes the opening, whether or not it reproduces.	A peak has to reproduce after ventilation before it counts. Gas takes the easiest route to the surface, which is frequently a crack or a penetration rather than the point directly above the pipe.
Ventilation control	Uncontrolled, so a draught carries the gas away and the sweep reads clean over a live leak.	Uncontrolled in the other direction, with accumulated gas in a void producing a broad false reading.	The space is ventilated and settled deliberately between sweeps, and the sequence is recorded, because a peak that appears once and never returns is an artefact rather than a location.
Interaction with the building	Occupants and neighbouring demises are an afterthought.	Handled only as far as the works require.	The mix is inert and non toxic, but the system is out of service while it is in use, so the window is agreed in advance with whoever holds the demise, and the affected occupiers are told what is happening and for how long.
What is written afterwards	A location and no method.	A scope of works.	The circuit charged, the concentration and the sweep pattern, which peaks appeared, which reproduced after ventilation, which were discounted and why, and the marked point dimensioned from two fixed features.

Recurring questions

The arguments that come up again and again

Tracer gas attracts a distinct set of questions in UK property discussion, partly because the word hydrogen alarms people and partly because it is the most involved method in the toolkit. The useful themes are about when it should be reached for, what preparation it drags with it, and how to tell a real reading from a wandering one.

Homeowners asking whether putting hydrogen into their pipes is safe

The question appears whenever tracer gas is mentioned, usually from people who hear hydrogen and picture something flammable in a drinking water supply. The answers are consistent: the professional mixture is 5% hydrogen in 95% nitrogen, which is inert, non-toxic and well below the concentration at which hydrogen becomes flammable in air, and it is accepted for potable systems. Our take: the reassurance is correct and the caveat is worth keeping. The safety case rests on the certified 5/95 mix being used and the system being vented properly afterwards, so ask the firm to confirm both.

Accounts of tracer gas surveys that found nothing

A recognisable pattern involves gas being deployed as the opening move, the survey returning no usable reading, and the fault later turning out to be inside the boiler rather than in any pipe. Our take: this is a sequencing failure rather than an equipment failure. Gas should only follow a pressure decay test that has shown a specific circuit losing pressure. Reaching for the most expensive method before the cheapest test is how people end up paying for a search of pipework that was never leaking.

Surprise at what draining the system brings with it

Owners quoted for tracer gas are often unprepared for the associated work, particularly on heating systems where the circuit has to come down and then be refilled, purged and re-dosed with corrosion inhibitor. The complaint is usually that this appeared as an extra rather than as part of the job. Our take: ask specifically whether the refill and the inhibitor are included and who is responsible for them. On a large system this is a genuine cost and it is the item most commonly missing from a headline detection price.

Confusion about gas surfacing somewhere the pipe clearly does not run

People are understandably sceptical when a detector peaks in a spot with no pipework anywhere near it. The explanation is that hydrogen is the smallest molecule in the atmosphere and will follow the easiest route out, which can mean travelling along a duct or service void and emerging at a grout line or expansion joint some distance away. Our take: this is exactly why a single peak is not a conclusion. Ventilating the area and re-introducing the gas to see whether the same peak returns is the control that separates a finding from an artefact.

Difficulty getting communal pipework isolated in a block

Discussions among leaseholders and managing agents make clear that tracer gas on riser pipework fails far more often on logistics than on technique: the riser has to be isolated, residents have to be given notice, and someone has to authorise both. Our take: settle the authorisation and the access window before the visit is booked rather than on the day. Establish which sections come out of service and for how long, so notice can go out in advance and the survey is not abandoned halfway.

Questions

Asked before every booking

Is the gas safe to use in an occupied flat and on drinking water pipework?

The mixture is 5% hydrogen in 95% nitrogen: inert, non-toxic, leaves no residue and sits below the concentration at which hydrogen becomes flammable. It is the standard mix for potable systems. The two things that actually matter are that the operator is using the certified 5/95 mix rather than something improvised, and that the system is vented properly afterwards. Ask any firm to confirm both.

Why does the system have to be drained first?

Because gas and water cannot occupy the same pipe. If water remains in the run, the gas cannot reach the breach at the pressure needed to push through the construction, and you get a weak or absent signal from a pipe that genuinely leaks. Draining is inconvenient, particularly on a large heating system, which is precisely why the pressure test comes first and confirms the circuit is worth draining.

How long does the gas take to come through a screeded floor?

Longer than through a timber void, and the depth of the pour and the floor finish both matter. We allow migration time rather than scanning immediately, because scanning too early produces a faint reading that is easy to misplace. On a deep pour with a sealed finish the wait can be a significant part of the visit, which is one reason screeded jobs take longer to survey than accessible ones.

What can make tracer gas point at the wrong place?

Gas following a service route, a duct or a joint and surfacing away from the breach; a highly permeable screed spreading the signal over a wide area; a sealed finish suppressing it over the true point and letting it emerge at the nearest grout line or expansion joint. This is why a single peak is not a conclusion. Reproducing it after ventilation, and corroborating with moisture or thermal data, is what turns it into one.

Can it be used on a communal riser in a block?

Yes, and it is often the only method that will work there, but it needs the riser isolated and that means the managing agent or freeholder authorising the isolation and the access. We will tell you which sections need to come out of service and for roughly how long, so the notice to residents can go out before the visit rather than during it.

Is tracer gas safe in drinking water pipes and occupied buildings?

The professional mixture is 5% hydrogen in 95% nitrogen: inert, non-toxic, odourless, non-corrosive and leaving no residue. At that concentration it sits below the level at which hydrogen becomes flammable in air, and the mix is accepted for use in potable systems and occupied properties. The two conditions that matter in practice are that the operator is using the certified 5/95 mix rather than something improvised, and that the system is vented properly once the survey is finished.

Why does the system have to be drained before tracer gas?

Because gas and water cannot share the pipe. If water remains in the circuit the gas cannot reach the breach at the pressure needed to push out through the construction, and the result is a weak or absent signal from pipework that genuinely leaks. Draining is the inconvenient part, especially on a large heating system, which is exactly why a pressure decay test should confirm the circuit is worth draining before anyone starts.

How long does the gas take to surface through a solid floor?

Longer than through a suspended timber void, and the depth of the pour and the floor finish both matter. Migration time has to be allowed rather than skipped, because scanning too early produces a faint, diffuse reading that is easy to place in the wrong spot. On a deep screed with a sealed finish the wait can be a meaningful part of the visit, which is one reason solid floor surveys take longer than accessible ones.

When is tracer gas the right method rather than acoustic or thermal?

When the pipe is plastic or PEX, which absorbs the leak noise acoustic work depends on. When the leak is too small or too slow to generate a detectable sound. When there is no temperature differential for a thermal camera to read, which covers cold supply pipework and any system that is not running. And when the pipework is cast into screed, slab or buried ground where nothing else will reach it.

Can tracer gas point at the wrong place?

It can, which is why a single peak is not a conclusion. Gas can travel along a duct or service route and surface away from the breach, a permeable screed can spread the signal over a wide area, and a sealed floor finish can suppress it over the true point and release it at the nearest grout line or expansion joint. The control for this is to map the gradient rather than stop at the first reading, ventilate, re-introduce the gas and confirm the same peak returns in the same place.

Water going somewhere it shouldn’t?

Tell us the symptoms and your postcode. We’ll confirm the visit, the fixed detection fee and the arrival window before you commit to anything.

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