If you are topping up the boiler pressure every few days, a central heating leak detection kit looks like a sensible first move. A bottle of fluorescent dye and a small UV torch costs far less than a survey, and the idea is simple: put the dye in the water, let it circulate, and look for the glow where water is escaping. For some leaks that works. For the leaks that cause most slow pressure loss in London homes, it does not, because the dye can only show you water you can shine a torch on.
This guide explains how UV dye kits work, what they can and cannot find, what goes wrong when people hire thermal or acoustic detectors, and the point at which paying for a proper survey becomes cheaper than carrying on guessing.
What is in a central heating leak detection kit?
Most kits sold for heating systems are dye kits. You get a concentrated fluorescent dye designed to be added to the circuit water, and usually a small ultraviolet torch. The dye is close to invisible in normal light but glows brightly under UV. Where water has leaked and dried, it leaves a residue that shows up as a bright green or yellow mark when you sweep the torch over it.
The dye goes into the system the same way an inhibitor does: through a filter dosing point, a radiator bleed valve with a dosing tool, or at a radiator after partially draining it. On a sealed system you then repressurise, run the heating for a while so the dye spreads through every radiator and pipe run, and start looking. Read the kit instructions and the boiler manual before adding anything, since some manufacturers are specific about what goes into their heat exchangers.
Separately, some people search for an electronic central heating leak detector. That usually means one of three things: a moisture meter, a thermal imaging camera, or an acoustic listening device. Hire shops and some merchants rent the first two. Those are covered further down.
Which leaks will UV dye actually show?
Dye works on any leak point where escaping water reaches a surface you can see and get a torch to. In practice that means:
- Radiator valves, including the spindle, the union nut and the tail into the radiator.
- Compression and soldered joints on exposed pipework, such as pipe drops at skirting level or runs in an airing cupboard.
- Pipe and valve connections under the boiler and around the filter, pump and diverter valve where they sit outside the casing.
- Pinholes in a radiator panel, often at the bottom edge where sludge collects.
The honest limitation is that most of these leaks are also findable without dye. A leaking valve usually leaves a crust of white or green deposit, a tide mark on the floor or a damp carpet edge. What the dye adds is certainty on a slow weep where the residue is faint, and a way to tell a heating leak from a condensation drip or a spilt drink. If you want a checklist of the ordinary signs to look for before buying anything, our guide to the signs of a leak in a central heating system covers them.
Why can't dye find a leak under the floor or inside a wall?
Because the dye only shows where water ends up, and you need to see that spot. A leak on a pipe under a screeded floor sends water down into the screed and insulation below it. A leak on a pipe inside a stud wall runs down the back of the plasterboard. A leak under floorboards drips onto the ceiling void below or soaks into the joists. In each case the glowing residue exists, but it is behind a surface a torch cannot see through.
Sometimes the water does surface somewhere. A damp patch on a ceiling, a stain at the foot of a wall or a lifting floor edge can glow under UV. That confirms the damp is heating water, which is useful, but water travels along joists and pipe runs before it shows, so the stain is often a metre or more from the actual leak. Cutting in where the stain appears is how floors and walls get opened in the wrong place.
This matters because hidden leaks are the usual cause of a system that loses pressure with nothing visible anywhere. If you can see every joint and none of them glow, the dye has told you something, just not where the leak is. Our guide on why a boiler keeps losing pressure walks through the checks that narrow it down.
| Where the leak is | Will a UV dye kit find it? | Better approach |
|---|---|---|
| Radiator valve or visible joint | Usually | Dye or a dry tissue test, then repair the fitting |
| Pinhole in a radiator | Usually | Replace the radiator; the metal is thin around the hole |
| Pipework under floorboards | Rarely | Thermal imaging with the heating running, then lift one board |
| Pipework in a screeded floor | No | Thermal imaging, acoustic listening or tracer gas |
| Pipe inside a wall or boxing | Only if water surfaces | Thermal imaging and moisture mapping |
| Inside the boiler casing | Not safely | A Gas Safe registered engineer |
What goes wrong when you hire a thermal camera or acoustic detector?
The tools are real, but most of the skill is in reading them.
Thermal cameras show heat, not water. With the heating on, every hot pipe under a floor shows as a warm stripe. A leak shows as a warm patch that spreads beyond the line of the pipe, but so do a pipe running close under thin screed, a cluster of fittings, underfloor heating loops and sun on a floor near a window. People often find a warm spot and assume it is the leak, when it is simply where two pipes cross.
Timing changes the picture. A thermal scan taken as the heating comes up to temperature looks very different from one taken after it has been running for an hour. Leaks are often clearest in a narrow window when the surrounding floor is still cool.
Acoustic kit needs a quiet house and a trained ear. Listening for a heating leak means picking out the hiss of water escaping under pressure from the noise of the pump, the boiler, traffic and fridges. Without experience of what a leak sounds like through timber, tile or concrete, it is easy to settle on the loudest point rather than the right one.
Moisture meters read the surface. A pin or pad meter tells you the top few millimetres of a material are damp. That is useful for mapping how far water has spread, but damp moves, and the wettest reading is not always above the leak.
The risk in each case is the same: the tool gives you a confident answer and you open a floor or wall on the strength of it. If it is wrong, you pay for the damage and the survey anyway. Our guide to leak detection equipment explains how each method works when a surveyor uses them together.
Is there anything worth doing yourself before paying for a survey?
Yes, and none of it needs a kit.
- Check the pressure relief pipe. The copper pipe from the boiler to the outside wall should be dry. If it drips, the relief valve may be letting water out and the system is not leaking at all.
- Walk every radiator and exposed joint with a dry tissue or kitchen roll, pressing it under each valve and fitting while the heating is hot. It picks up a weep that you would miss by eye.
- Note the rate of loss. Write down the gauge reading when you top up and again each day. A system losing half a bar a day has a very different leak from one losing half a bar a month.
- Isolate sections if you can. If your system has zone valves or isolating valves, closing off one part and watching whether the pressure still drops can tell you which circuit the leak is on. Only do this if you know which valves are which.
If all of that turns up nothing, the leak is almost certainly concealed, and that is exactly where a dye kit stops being useful. A leak sealer is the other tempting shortcut at this stage. Our guide on whether central heating leak sealer works explains when it is worth a try and when it makes things worse.
When is a professional heating leak survey cheaper than guessing?
A standard survey with us starts from £150. Thermal imaging surveys are £150 to £250, acoustic surveys £150 to £300, and a combined survey using several methods is £250 to £500. For leaks under solid floors, under-floor detection is £300 to £700. No find, no fee applies to leaks on your own property.
Set that against the likely costs of guessing. Lifting a run of floorboards in the wrong room, chasing out plaster that did not need chasing, or breaking up a section of screed to find the pipe is not where the leak is will cost more to put right than the survey. So can weeks of topping up, since fresh water brings oxygen that speeds up corrosion inside radiators.
A reasonable rule: if you have checked everything visible and the pressure is still falling, the kit has done its job by ruling those points out. At that stage a survey that marks the leak on the floor, before anything is cut, is usually the cheaper route. Our central heating leak detection service is built for exactly this, and you can call us on 020 3912 0200 to talk through what you have already tried.
Frequently asked questions
Can I put UV leak detection dye in a combi boiler system?
Dyes sold for heating systems are designed to go into sealed systems, including those with combi boilers. Check the dye maker's instructions and your boiler manufacturer's guidance first, add it through a proper dosing point, and do not open the boiler casing to do it.
Does the dye need to be flushed out afterwards?
Follow the product instructions. Most heating dyes are intended to stay in the system at low concentration. If you drain down for a repair, it is a good moment to refill with fresh water and inhibitor.
Will a thermal camera from a hire shop find a leak under a concrete floor?
It can, but hire cameras are generally lower resolution and the results are easy to misread. Warm pipes, crossed runs and underfloor heating all look like leaks. If the reading is the only basis for breaking up a floor, get it confirmed first.
My system loses pressure but nothing glows under the UV torch. What now?
Check the pressure relief pipe outside. If it is dry, the leak is most likely on a concealed pipe under a floor or inside a wall, where dye cannot show it. That is the point to book a survey rather than open things up.
Is a leak detection kit worth buying at all?
It is worth it if you suspect a visible fitting and want to be sure before replacing a valve or radiator. It is not worth it for a system with no visible signs of water, because those leaks are almost always out of sight.
