Electronic leak detection explained: the tools and the survey

Electronic leak detection is a loose term. It covers a set of instruments, each measuring something different: pressure, sound, temperature, gas, moisture. None of them sees water directly through a floor. What a skilled technician does is combine readings from several, each one narrowing the search, until the evidence points to a spot small enough to open with a single tile or a short section of plaster rather than a trench across the kitchen.

This article explains each method, what it does well and where it struggles, and then follows a typical survey from start to finish. The water leak detection section has related guides on finding and confirming leaks.

Start by confirming there is a leak

Equipment is the second step, not the first. Before anyone arrives, it helps to know that water is actually being lost and, ideally, roughly how fast. The simplest check is the water meter: turn off every tap and water-using appliance, note the reading, wait, and read it again. The water meter leak test sets this out properly, including how to deal with toilets and storage tanks that refill on their own.

The visible clues matter too: a heating system that keeps losing pressure, a warm patch on a floor, a musty smell, swelling skirting, a stain that slowly grows. The guide to signs of a hidden water leak covers what to look for. Small leaks add up. The US Environmental Protection Agency's WaterSense programme estimates that leaks in an average household can waste nearly 10,000 gallons of water a year, and much of that comes from leaks nobody has noticed.

The instruments, one by one

Pressure-drop testing

The circuit under suspicion is isolated and pressurised, then a gauge is watched over a fixed period. If the pressure falls, something is escaping. By closing valves one at a time, a technician can separate the cold supply from the hot, or one heating zone from another, and establish which circuit is leaking. On a sealed heating system, the boiler's own gauge is a rough version of the same test.

Good at: proving a leak exists and identifying the circuit.

Weak at: location. A pressure test tells you whether and which, never where.

Acoustic detection

Water escaping from a pressurised pipe makes a noise, a hiss or rush that travels along the pipe and through the material around it. Technicians listen with ground microphones placed on the floor at intervals, or with contact probes touched to pipes and fittings, through an amplifier with filters that cut out unhelpful frequencies. On longer runs, such as a supply pipe under a garden or drive, a correlator uses two sensors at different points on the pipe and compares when the leak noise reaches each one. With the pipe's length and material entered, it calculates where the noise is coming from.

Good at: mains-pressure leaks in metal pipe, and long buried runs.

Weak at: low-pressure systems such as sealed heating, which are often too quiet to hear; plastic pipe, which carries sound poorly; and noisy homes, where fridges and pumps may need switching off. Sound can also travel along a pipe and seem loudest some distance from the leak.

Thermal imaging

A thermal camera shows surface temperature, not water. It is most useful on hot pipes. With the heating or hot water running, a leak from a buried pipe tends to show as a warm bloom spreading across the floor or wall, distinct from the clean line of the pipe itself. Underfloor heating loops appear as stripes, and a leak usually looks like a smudge that breaks the pattern.

Good at: heating pipes, hot water pipes, underfloor heating, and quickly tracing where pipes actually run.

Weak at: cold water leaks, which only show where evaporation cools the surface; shiny surfaces such as polished tiles, which reflect heat from elsewhere; and rooms with sunlight, draughts or thick floor coverings. The images need interpreting, and a warm pipe run is easy to mistake for a leak.

Tracer gas

The pipe is drained or isolated, then filled with a gas mixture, typically 5% hydrogen in 95% nitrogen. The mix is not flammable at that concentration. Hydrogen molecules are tiny and light, so the gas escapes through the leak and works its way up through screed, concrete, tile joints and many paved surfaces. A sensitive sniffer probe moved slowly over the surface picks it up where it emerges.

Good at: silent leaks, low-pressure systems, plastic pipe, underfloor heating loops and buried supply pipes where acoustics fail.

Weak at: speed, because the gas needs time to migrate; very dense or sealed surfaces, which slow it down; and well-ventilated spaces, where it disperses. It also means taking the pipe out of service while the test runs.

Moisture meters and damp mapping

Pin-type meters measure electrical resistance between two probes pushed into a material, and they are most reliable in timber. Pinless meters use a sensor pad on the surface and give relative readings without making holes. Taking readings on a grid and plotting them produces a damp map, showing where the moisture is concentrated and which way it is spreading.

Good at: defining the wet area, showing whether it is drying out after a repair, and providing evidence for a report.

Weak at: finding the source. Water runs along joists, down walls and across slabs, so the wettest point is not always above the leak. Salts in old plaster can give falsely high readings, and condensation or rising damp can produce patterns that resemble a leak.

Cameras and borescopes

A borescope is a thin camera on a flexible or semi-rigid probe. Passed through a small drilled hole, a gap behind a bath panel or an access hatch, it lets the technician look at pipes and joints in voids without opening them up. Larger drain cameras do the same job inside waste pipes, which is a different kind of leak but often part of the same investigation when a ceiling stain sits under a bathroom.

Good at: confirming what other methods suggest, by showing the actual fitting.

Weak at: anything inside solid screed or masonry. It needs a void to look into.

Dye tests

Not every leak is a pipe. A few drops of food colouring or a dye tablet in a WC cistern will turn up in the pan after a while, without flushing, if the flush valve is letting water through. A meter test would show that water is being lost, but not that the toilet is the culprit. Coloured or fluorescent dye run into a shower tray, along a bath seal or down a waste can show whether a ceiling stain below comes from the shower enclosure rather than the supply pipework.

Good at: cheap, simple confirmation of WC, shower and bath leaks.

Weak at: pressurised supply leaks, where it does not apply.

How a typical survey runs

Firms differ, but a careful survey usually follows something close to this order.

1. History. The technician asks when the problem started, what has been noticed, what the meter readings show, what work has been done in the house recently, whether there is an insurance claim, and whether any drawings or photos of pipe routes exist.
2. Walk-round. A visual check of stains, the stopcock and meter, the boiler pressure, visible pipe runs, the underfloor manifold if there is one, and any obvious damp.
3. Confirm and isolate. A meter check with everything off, then pressure-drop tests on each circuit in turn to identify which one is leaking.
4. Narrow the area. Thermal imaging for hot water and heating circuits, plus moisture mapping on the affected surfaces, to reduce the search to a room or a section of floor.
5. Pinpoint. Acoustic listening along the suspected route, or tracer gas if the pipe is plastic, low-pressure or silent.
6. Cross-check. A good technician looks for two methods agreeing before marking a spot. One reading on its own can mislead.
7. Mark and, if agreed, verify. The spot is marked with measurements from fixed points. Sometimes a small inspection hole and a borescope confirm it before the main opening is made.
8. Restore. Systems are refilled, heating repressurised and bled, water turned back on, and the rooms left as they were found.
9. Report. Photos, readings and conclusions are written up and sent on.

Some technicians only locate leaks; others also repair and make good. Find out which before the visit.

What a survey report should contain

A useful report lets someone else, a plumber or an insurer, act on it without repeating the work. Look for:

• the address, the date, and the name of the technician;
• the circuits tested and how each one was isolated;
• the methods used and the readings, such as starting and finishing pressures with times, a plotted moisture map, and labelled thermal images;
• the leak position, measured from fixed points such as a wall or door frame, with photos of the marking;
• how confident the technician is, and why;
• the likely cause if it could be seen, and the size of opening needed for a repair;
• anything that was not tested, and the reason.

Questions to ask before you book

• Which methods do you expect to use for this type of pipe and system?
• Do you only locate the leak, or also repair and make good?
• How do you charge if the leak is not found, or turns out to be outside the property?
• Will the report be in a form an insurer is likely to accept?
• How large an opening is a repair likely to need?
• Will the water or heating need to be off, and for how long?

The general advice in the guide to choosing a plumber applies here as well: a written scope, clear terms, and evidence of relevant insurance and qualifications, which vary by country.

A note on trace and access insurance

Many home insurance policies include what is often called trace and access cover. In general terms it may contribute to the cost of finding a hidden leak and of opening up and making good the area afterwards. It frequently does not cover repairing the leaking pipe or fitting itself, and there may be limits and an excess. Wording varies widely between insurers and between policies, so read your own before booking anyone. Some insurers want to be told first and may appoint their own contractor. Keep meter readings, photos and the survey report together, because they make any claim easier to support.

Before the technician arrives

• Do the meter test and write down the readings and the times you took them.
• Note when symptoms appear, for example only when the heating is on, or after showers.
• Clear access to the stopcock, meter, boiler, manifold and any hatches.
• Find any photos or drawings of underfloor loops or pipe routes.
• Resist the urge to open up walls or floors yourself. Fresh holes can confuse readings and may complicate an insurance claim.

A good survey is a process of elimination. The instruments help, but what you are paying for is the method and the judgement behind them.