Central heating leak detection behind a falling boiler gauge
Central heating leak detection works out which part of a sealed heating system is leaking water, a common reason a boiler keeps needing its pressure topped up, across radiator, boiler, underfloor and buried heating pipework. Cambrian Leak Detection carries out this work in homes across Wales.
What makes a sealed heating system lose pressure?
A sealed heating system loses pressure when water leaves the circuit: Vaillant UK counts a leak, most often at a radiator valve or another system component, among the most common reasons, and bleeding radiators or a faulty pressure relief valve let water out too.
A sealed heating system is a closed circuit in which the boiler, its pipes and the radiators share the same water, and Ideal explains that the boiler's gauge reports on that whole circuit rather than on the boiler by itself. When the needle drops, water has gone somewhere.
The four boiler makers cited here, Baxi, Ideal, Vaillant and Worcester Bosch, agree on one line: on a sealed system a gauge below 1 bar counts as low. They also agree the needle rises as the water heats, and each leaves the exact working range to the boiler's own manual and gauge markings.
- Vaillant UK
- Names bleeding radiators, a leak from the boiler or the system, and pipes and radiators that corrode or loosen over time as the most common reasons for lost pressure; where no leak turns up, it points to the boiler itself or a concealed leak elsewhere on the system.
- Worcester Bosch
- Suggests starting with the joints and radiator valves you can see, then thinking about underfloor pipework, and last checking the pressure relief valve's discharge pipe outside, where a drip may mean the valve is faulty.
- Ideal
- Links repeated top-ups to a possible small leak; its threshold is in the next section.
- Baxi
- Agrees with the other three that below 1 bar is low, and points owners to the boiler's own guidance for the working range.
If the boiler itself may be leaking, Vaillant advises having it inspected by a Gas Safe Registered engineer and never taking the casing off yourself. Whether a leaking boiler can stay in use is a question for that engineer and the manual.
Sources: Ideal, Topping up the pressure on your Ideal boiler; Hafren Dyfrdwy, Report a problem; Worcester Bosch, Greenstar i Troubleshooting; Vaillant UK, What pressure should my boiler be?; Baxi, What should my boiler pressure be set at?; Vaillant UK, Why is my boiler losing pressure?; Worcester Bosch, The heating system is constantly losing pressure.
How many top-ups a year suggest a heating leak?
Top-ups once a month or more suggest either a small leak or a pressure relief valve at fault, according to Ideal, which treats topping up once or twice a year as generally not a concern.
That threshold is one maker's guidance, so read it beside your own boiler's manual. A top-up now and then is, on Ideal's account, generally nothing to worry about. Needing one at least once a month leaves two suspects: a small leak somewhere on the circuit, or a pressure relief valve that is letting water go.
Telling the two apart starts outside. Worcester Bosch's check is to look at the valve's discharge pipe where it leaves the house: a drip there may mean the valve is faulty. A dry pipe and a falling gauge leave a leak somewhere on the circuit, and damp patches or rust marks around radiators are the place to look first.
Sources: Ideal, Topping up the pressure on your Ideal boiler; Worcester Bosch, The heating system is constantly losing pressure; Vaillant UK, Why is my boiler losing pressure?
Where do central heating leaks hide in a house?
Central heating leaks hide in the pipework nobody sees: around radiator valves and other system components, under tiled or concrete floors, inside walls, in underfloor heating loops and inside the boiler itself.
Radiator valves and system components
Vaillant says a leak from the heating system most often starts at a radiator valve or another system component.
Under tiled or concrete floors
Heating and hot water pipes run beneath solid floors, and thermal imaging is used there because hot water escaping from them warms the floor above.
Inside walls
Pipework hidden in a wall shows nothing from the room, and Vaillant points to a concealed leak on the system when nothing shows.
Underfloor heating loops
Worcester Bosch asks owners to think about underfloor pipework, and Grant UK covers the loops of an underfloor system losing pressure.
Inside the boiler
Vaillant names a leak from the boiler among the common causes; its advice on who inspects a boiler that may be leaking is in the first section.
The hot and cold supply pipes run through the same floors and walls, so a damp patch near a radiator can also come from one of the hidden leaks on the water pipes.
Sources: Vaillant UK, Why is my boiler losing pressure?; Worcester Bosch, The heating system is constantly losing pressure; Grant UK, Why would an UFH system lose pressure?; Testo, Practical Guide Heating; Teledyne FLIR, Can thermal imaging see through walls?; The Building Conservation Directory, Thermal Imaging in the Investigation of Solid Masonry Structures; Fluke, Why thermal imaging is essential for detecting moisture damage.
Which tests locate a heating leak beneath floors or inside walls?
Tests that locate a heating leak beneath floors or inside walls start with pressure testing, which shows whether a circuit holds and, on underfloor heating, which loop is leaking, then thermal imaging to find the warm patch escaping hot water makes, with tracer gas and acoustic listening where heat alone does not show it.
| Method | What it shows | Its limit |
|---|---|---|
| Pressure testing | Pressure draining away from a section that has been sealed and tested. | Plastic pipe expands slightly when pressurised and any trapped air must be let out first, so a gauge that drops is not, on its own, proof of a leak. |
| Thermal imaging | Temperature differences on a surface. | The camera reads surfaces only, so a pipe or leak behind them shows only where it changes the surface temperature, and a cool patch may turn out to be a draught; other checks confirm what it suggests. |
| Tracer gas | Hydrogen coming up through the surface from the leak. | The gas can take very different times to reach the surface, depending on what lies above the pipe. |
| Acoustic listening | The sound a pressurised pipe makes where water is leaving it. | A quiet leak, or a pipe with no pressure in it, gives nothing to hear; for those, Sewerin recommends tracer gas. |
When an underfloor system loses pressure with no sign of where, Grant UK suggests testing the loops one at a time under pressure to find the leaking one. A gauge that falls during a test is not proof on its own, because plastic pipe swells slightly under pressure and trapped air has to be let out first.
Cambrian Leak Detection sets the order of tests for each system, and the home page lays out the leak detection methods used across Wales side by side. Not every leak can be pinned down: Fluke notes that a thermal image points to a probable zone without always settling the question, so some breaking into walls or floors may still be needed.
Sources: Grant UK, Why would an UFH system lose pressure?; Welsh Water, Water Leaks Code of Practice; Fluke, Why thermal imaging is essential for detecting moisture damage; Northumbrian Water, Guidance note 3: Pressure testing of new water service pipework; Wunda, Phase 3: Preparing and connecting the pipe to the manifold; Financial Ombudsman Service, decision DRN-4125471; Ideal, Topping up the pressure on your Ideal boiler; Teledyne FLIR, Can thermal imaging see through walls?; Testo, Practical Guide Heating; The Building Conservation Directory, Thermal Imaging in the Investigation of Solid Masonry Structures; Sewerin, VARIOTEC 460 Tracergas; INFICON, Find Hidden Water Leaks Fast; INFICON, Hydrogen leak detectors; Sewerin, Listening stick W50; Sewerin, AQUAPHON A 200; Sewerin, Stethophon 04.
Is a dripping radiator valve a job for leak detection or a plumber?
A dripping radiator valve is a plumber's job when the drip can be seen, because the leak is already found; leak detection is for pressure loss with no visible cause.
The boiler makers' own checks need no tools: compare where the needle sits with the gauge's markings and the manual, run an eye along the pipework for drips, weeping valves or wet patches, and look at the pressure relief valve's outside pipe. A weeping valve you can see is something a plumber can tackle directly.
If every visible joint is dry and the gauge still falls, the leak is hidden, and that is where tracing earns its place. Where a hidden leak has damaged the home, the policy's trace and access cover for a heating leak may pay for finding and reaching it; the policy wording decides.
Sources: Vaillant UK, What pressure should my boiler be?; Ideal, Topping up the pressure on your Ideal boiler; Worcester Bosch, The heating system is constantly losing pressure; Admiral, What is trace and access cover?; Confused.com, Trace and access.
Is low boiler pressure the same as low water pressure at the taps?
No: low boiler pressure is the pressure inside the sealed heating circuit, shown on the boiler gauge, while pressure at the taps comes from the mains supply, which Hafren Dyfrdwy suggests checking by timing the cold kitchen tap filling a 1-litre jug, around 7 seconds being normal.
To run that jug check, first make sure all taps and appliances are off, then open the cold kitchen tap fully and time the fill. A slow jug says something about the water coming into the house; it says nothing about the heating circuit, and a full gauge says nothing about the taps.
A big drop in water pressure is on Welsh Water's list of signs that a private supply pipe may be leaking, along with damp ground and lush grass. The page on a leak on the supply pipe outside covers who owns that buried pipe and how a leak on it is found.
Sources: Hafren Dyfrdwy, Report a problem; Ideal, Topping up the pressure on your Ideal boiler; Welsh Water, Network Leakage.