Thermal imaging leak detection: what the camera can and can't see

A thermal camera measures the temperature of surfaces. It never sees water or pipes directly, only the effect they have on the surface.

It works well on leaking hot water lines under tiles and on wet plaster, which usually reads cooler than dry plaster.

Reflections, sun, draughts and framing all make patterns that look like leaks, so reading the image takes experience.

Heat spreads, so the image narrows the search to an area and a second method, such as acoustic gear or tracer gas, finds the exact spot.

Every object gives off infrared radiation, and the warmer it is, the more it gives off. A thermal camera has a sensor that measures that radiation across its field of view and turns it into an image, usually with warm areas shown in reds and yellows and cool areas in blues and purples. It is measuring the outside skin of whatever it is pointed at: the face of the tile, the paint on the plaster, the surface of the concrete. Nothing behind that skin is visible. What makes it useful for leaks is that water, and especially hot water, changes the temperature of the surfaces around it in predictable ways.

The strongest signal comes from a hot water line leaking into a slab or a wall. The escaping water carries heat into the concrete or plaster, and the surface above it warms in a shape that often follows the pipe and then spreads out from the break. Wet building materials are the second big win. As water evaporates from damp plaster or ceiling board it cools the surface, so a wet patch often shows as a cooler area than the dry wall around it, sometimes before any stain appears. Wet ceiling insulation also reads differently from dry, which helps trace a leak from an upstairs bathroom across a ceiling.

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It cannot see through walls, floors or ceilings. It cannot tell you how deep a pipe is. A cold water line that is close to room temperature makes very little contrast, so a small cold leak can be almost invisible to it. Carpet and underlay, floating timber floors and thick rugs insulate the surface and blur or hide the pattern. Foil insulation and some wall linings block it. Outside, a buried service pipe under a lawn or driveway is usually too deep and too close to ground temperature for a thermal camera to be much help.

Thermal imaging needs a temperature difference to work with. A wall that has been in full afternoon sun stays warm into the evening and can hide a leak or fake one. Air conditioning, heaters and fans stir the surface temperatures around. A shower used just before the scan wets the wall and leaves its own cool pattern. A good scan starts with a settled room, the heaters and cooling off, and then deliberately creates contrast, for example by running hot water through the line under suspicion and watching how the surface responds.

Shiny surfaces act like mirrors for infrared. Glossy tiles, glass, polished stone and stainless steel can reflect the heat of a window, a person or a light, and the reflection looks like a warm or cool patch on the wall. Timber studs and noggings behind plasterboard show as a regular grid. Gaps around windows and doors let in cold air and show as cool streaks. Damp from condensation on an outside wall on a cold morning looks like a leak but is not one. Each of these has a tell, which is why an image taken without context is so easy to misread.

Even a clear thermal image is showing where heat or moisture has gathered, not exactly where the pipe has failed. In concrete, heat spreads sideways and water can pool some distance from the break. In a ceiling, water runs along joists before it soaks in. So a plumber uses thermal imaging to find the route of the pipe and the area of the leak, then confirms the exact spot with acoustic listening gear or tracer gas before anything is opened. Sydney Water itself notes that finding leaks hidden in walls or underground takes specialist equipment, and in practice that means using more than one tool.

Thermal attachments for phones have made the technology cheap and easy to try, and they can show you an interesting picture of your house. The problem is not the camera, it is the reading. Without a known temperature contrast, a pipe layout and an understanding of reflections and framing, it is very easy to find a blue patch and cut a hole beneath it that finds nothing. By all means look, but treat anything you see as a clue for a plumber rather than a place to drill.

A saved thermal image, next to an ordinary photo of the same wall or floor, is good evidence of where a leak was before anything was opened. That matters to insurers, and it matters for Sydney Water's hidden leak allowance, which depends on a licensed plumber finding and repairing a leak you could not see or hear. Before and after images, showing the warm or cool pattern gone once the repair is done, round out the record.

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