How Leak Detection Works: The Methods, What Each One Reads, and Where Each One Fails

What leak detection actually is

Leak detection is the process of establishing that water is escaping from a pressurised or draining system, working out which system, and locating the escape point precisely enough that someone can open one small area and find it. Each stage is a separate piece of work and they fail in different ways.

What people usually mean by leak detection is only the third stage, the locating. In practice the first two matter more, because a survey that goes straight to locating on the wrong system finds nothing and still takes three hours. A damp patch on a kitchen ceiling can come from a bath waste, a shower tray, a heating pipe, a supply pipe, a soil stack, a roof or condensation, and those are six different investigations.

It is also not damp diagnosis. A wall can be wet because a pipe is leaking, because water is coming in from outside, because a bridged damp proof course is drawing moisture up, or because a room produces more water vapour than it can ventilate. The instruments overlap but the reasoning does not, and our guide on telling damp and a leak apart deals with that distinction.

Before any instrument: proving the leak exists

Every competent survey starts by establishing that something is actually losing water, because a good proportion of call outs where the customer is certain there is a hidden leak turn out to be something else. Two cheap tests do most of that work.

The meter test

Close every tap and appliance, note the meter reading including the smallest dials, leave the property untouched, and read it again. Movement means water is going somewhere. Then close the internal stop tap and repeat: if the meter still moves, the loss is on the supply pipe between the meter and the stop tap, usually underground. If it stops, it is inside. That single step splits the job in two before anyone unpacks a case. Our guide to reading a water meter to check for a leak sets out how to do it and how to read the dials.

The pressure test

On internal pipework the equivalent is a static pressure test. The circuit is isolated, brought to a test pressure with a hand pump and a calibrated gauge, and watched. BS EN 806-4, which covers water supply systems inside buildings, describes a tightness test in which the system is filled with water, fully vented and held at 1.1 times the maximum design pressure with no drop permitted. The standard is explicit that the test uses water and not air, because compressed air in a pipe stores a dangerous amount of energy.

A heating circuit is tested the same way, and it is often the clearest result of the day. A sealed system that holds pressure overnight is not leaking, whatever anyone has been told, and one that drops steadily with the boiler isolated has narrowed the fault to the pipework rather than the appliance.

Test	What it proves	What it cannot tell you
Meter reading with everything off	That a pressurised system is losing water	Which circuit, or where
Meter reading with the internal stop tap closed	Whether the loss is inside the building or on the buried supply pipe	Anything about waste, drainage or roof water
Circuit by circuit isolation	Which circuit the loss belongs to	The position of the defect along that circuit
Static pressure test to BS EN 806-4	That a given circuit is or is not tight	The size or nature of the failure
Controlled water test of one fixture	Whether that fixture is the source	Anything about the fixtures you did not test

Acoustic detection: listening to water escaping under pressure

Water forced through a small defect in a pressurised pipe makes noise. The turbulence at the orifice, and the impact of the jet on the surrounding material, produce vibration that travels along the pipe wall and through the ground or structure. Acoustic detection is the business of finding where that noise is loudest.

Two families of equipment do it. A ground microphone is a sensitive contact microphone placed on the floor surface, heavily filtered and amplified, moved in a grid while the operator listens for the peak. A correlator is different: two sensors are clamped to the pipe at accessible points either side of the suspected leak, and the instrument cross correlates the signals to find the time difference between the leak noise arriving at each. From the distance between the sensors and the speed of sound in that material, it computes the source position. That is why the operator must enter the pipe material, diameter and length accurately. It is the fastest method when conditions suit it, and usually the first thing tried on a pressurised pipe.

What acoustic detection cannot do

Acoustic methods need pressure and they need noise. Both of those can be absent.

• No pressure, no signal. A waste pipe, a soil stack, a shower tray, an overflow or a roof leak produces nothing useful, because nothing is being forced through a small hole.
• Plastic pipe is much quieter than metal. MDPE and barrier pipe damp vibration heavily, which shortens the distance the signal travels. Long plastic runs are the classic case where acoustic work narrows the area but does not pinpoint it.
• Very small and very large defects both go quiet. A slow weep may generate almost nothing. A catastrophic burst can drop pressure so far that the jet noise collapses.
• Deep pipes and loose ground absorb it. A pipe under a metre of soil is a much harder target than the same pipe under a concrete path.
• Background noise wrecks it. Traffic, a pump, a cistern refilling, the property's own boiler. Some of the hardest jobs in London are done at night because the street is quieter.
• Sound travels along the pipe. The loudest surface point is above the pipe, not necessarily above the defect, and on a run that changes depth the two can be some distance apart.

Our page on acoustic leak detection in London covers how this is applied in practice.

Thermal imaging: reading temperature, not water

A thermal camera does not see water and it does not see through anything. It measures infrared radiation emitted by a surface and converts it to an apparent temperature map of that surface. What makes it useful for leaks is that escaping water changes the temperature of the material it saturates, and that change eventually reaches a surface a camera can see.

That is why it is extremely effective on hot water and heating leaks and much weaker on cold ones. A leaking heating pipe under a screed produces a warm plume that spreads along the leak path and shows as a clear, shaped anomaly. A cold supply leak under the same screed produces a small temperature depression, sometimes a fraction of a degree, sometimes none at all once the water has equalised with the slab.

The technique has a formal basis. BS EN 13187 set out the qualitative infrared method for detecting thermal irregularities in building envelopes, and has been superseded by BS EN ISO 6781-1. The point those standards make is that thermography is qualitative. It identifies anomalies for investigation. It does not tell you what the anomaly is.

What thermal imaging cannot do

• It cannot see through a wall or a floor. It reads the surface. If the construction is thick, insulated or has an air gap, the anomaly may never reach the surface.
• It needs a temperature difference. A cold leak in a cold void in a cold house gives nothing. Sometimes the answer is to run the hot circuit and image again.
• Reflective and low emissivity surfaces lie. Gloss paint, tiles, polished stone, foil backed board and glass reflect infrared from elsewhere in the room, including the operator.
• Underfloor heating dominates everything. On a heated floor the loops are the only thing the camera wants to show you, and a leak has to be read against that pattern.
• It cannot distinguish sources. A cold patch can be a leak, condensation, a cold bridge, missing insulation or an air path. That judgement comes from a moisture meter and the building.
• Recent sun, radiators and occupancy contaminate the image. A wall that has had sun on it is unusable until it settles.

More on how it is used in practice is on our thermal imaging leak detection page.

Tracer gas: making an invisible leak detectable

When a leak is too small, too quiet or too cold for the other methods, the answer is to give it something detectable to emit. The system is drained, isolated and charged with a tracer gas, typically five per cent hydrogen in nitrogen, which is non toxic and, at that concentration, non flammable. Hydrogen is the smallest molecule there is, so it escapes through defects water barely weeps from, and it is far lighter than air, so it rises through screed, soil and floor construction to the surface where a sensitive detector picks it up. It is the most definitive method for a small leak in a buried pressurised pipe, and the one that closes jobs the other two only narrow.

What tracer gas cannot do

It is slow. The system has to be drained, which on a heating circuit means refilling, re dosing inhibitor and bleeding afterwards. The gas then has to reach the surface, which through a thick screed with a sealed floor finish can take a long time or not happen at all. A fully bonded vinyl or resin floor, or a membrane, can hold the gas beneath it, and the detector then reads a seam or a service penetration rather than the leak. It is also unusable on anything that cannot be sealed and pressurised, which rules out open waste systems and roofs. Our tracer gas leak detection page explains the preparation it requires.

Moisture measurement, hygrometry and cameras

The methods above find the pipe. These find the water, a different problem and frequently the one that decides what happens next.

Instrument	What it physically measures	What it is good for	Where it misleads
Pin type resistance meter	Electrical resistance between two pins in the material	Comparative wetness readings in timber and plaster	Salts, foil, metal mesh and carbon all read as wet
Non invasive capacitance meter	Dielectric change a short distance below the surface	Quick surveying of a large area without damage	Reads pipes, wires and dense material as moisture
Endoscope or borescope	Direct visual access through a small hole	Confirming a suspicion before opening anything up	Only sees what the hole points at
Drain camera	Video inside a waste or soil pipe	Cracks, displaced joints, root ingress, blockages	Cannot show what is happening outside the pipe wall
Dye testing on drainage	Whether water from a fixture reaches a specific point	Proving a shower tray, waste or gully is the source	A negative result only rules out that one fixture

Moisture mapping is what turns a located leak into a scope of works. Knowing the leak is at a particular joint is useful. Knowing how far the saturation extends into the floor void is what determines whether the room needs drying, and PAS 64, the code of practice for mitigation and recovery of water damaged buildings, is built around measuring and recording that rather than estimating it.

Matching the method to the symptom

Nobody chooses a method in the abstract. They choose it from what the property is doing.

Symptom	Likely systems	Usual first method	Usual confirming method
Meter turns with the internal stop tap closed	Underground supply pipe	Acoustic, ground microphone and correlator	Tracer gas on a drained, capped supply
Warm patch on a solid floor	Heating flow or return, hot water	Thermal imaging	Pressure test on the isolated circuit
Boiler pressure falling, nothing visible	Heating circuit, or a discharging relief valve	Pressure test with the boiler isolated	Thermal imaging, then tracer gas
Stain on a ceiling under a bathroom	Waste, shower tray, seal failure, supply	Controlled water testing of each fixture in turn	Moisture mapping and endoscopy
Damp wall at low level, no meter movement	Possibly not a leak at all	Moisture profile and hygrometry	External inspection, drainage and gully checks
Wet patch in a garden or drive	Supply pipe, or drainage	Acoustic on the supply, dye on the drainage	Tracer gas on the supply pipe

The methods work together, not in competition

The common misconception is that these are alternatives and a good engineer picks the right one. They are stages. Acoustic narrows a supply pipe leak from forty metres to two. Thermal confirms the anomaly behaves like water rather than a void. Tracer gas resolves the last half metre through a slab. Moisture mapping defines how far the damage has travelled. A survey producing a single point from a single instrument, with nothing corroborating it, is one to ask questions about, because the cost of being wrong is a floor opened in the wrong place. How this runs as a job is set out in our article on what a leak detection survey involves.

What a properly done detection leaves behind

The output of leak detection is not a hole in a floor. It is a decision someone can act on, documented well enough that a plumber, a loss adjuster and a managing agent can all use the same piece of paper.

• Which system is leaking. Named specifically: cold supply, hot supply, heating flow, heating return, waste, soil or external drainage.
• Where, with a reference someone else can find. Measured from two fixed features, not described as "under the hall".
• How it was established. The methods used and the ones tried that gave nothing, because that stops the next person repeating them.
• The confidence level. Whether the point is pinpointed or narrowed to an area, and what would close the gap.
• The extent of the damage. Moisture readings and the affected area, separately from the leak location.
• What access is required. What has to be opened, and anything that stops it, such as a screeded floor, a neighbour's ceiling or suspected asbestos in a building predating 2000.

Detection is a process of elimination carried out with instruments that each read one physical property, none of which is water in a pipe. Knowing what each is actually measuring, and what it cannot measure, is the whole skill.