Central heating leak detection for a boiler gauge that keeps falling

Central heating leak detection traces the water a sealed heating system is losing to the place it escapes: a radiator valve, a joint, a pipe under the floor or an underfloor heating loop. Red Rose Leak Detection finds central heating leaks in homes and businesses across North West England.

Diagram: radiator pipes under a floor, with the leak marked where one pipe runs beneath it.

What makes boiler pressure keep dropping?

Boiler pressure keeps dropping because water is leaving the sealed heating circuit, and Vaillant UK names a leak as the commonest cause, typically at radiator valves and other parts of the system.

Water can leave a sealed heating circuit by four routes, and the boiler makers name each of them.

A leak at a radiator valve, a joint or another part of the system, the cause Vaillant UK puts first.
Bleeding the radiators, which Vaillant UK notes lets water out as well as air.
A leak inside the boiler itself, which Vaillant UK names where no other leak shows.
A faulty pressure relief valve, which Ideal lists beside a small leak as a reason for frequent topping up.

Ideal adds that repeated loss does not always mean the boiler is at fault, because it can start elsewhere in the heating system. That hidden leak on the pipework, under a floor or in an underfloor loop, is the one Red Rose Leak Detection traces.

What should the boiler pressure gauge read?

A boiler pressure gauge should read 1.0 to 1.5 bar with the heating off, in Ideal's figures, and rise while the heating runs; Worcester Bosch, Vaillant UK, Ideal and Baxi all treat a reading under 1 bar as low.

Boiler pressure, in each maker's own figures
Maker	Normal reading	Below 1 bar
Worcester Bosch	Around 1 bar with the heating off, rising to 1.5 bar or above while it runs	Needs topping up
Vaillant UK	Generally 1 to 1.5 bar with the heating off and 1 to 2 bar with the boiler on; 3 bar is the upper limit	Treated as a boiler that has lost pressure
Ideal	1.0 to 1.5 bar cold, the band its gauges usually show in green	Topping up may be needed, and the boiler may stop with a low pressure fault
Baxi	1.0 to 2.0 bar across boiler and heating system, and no higher than about 2.5 bar while heating	Needs repressurising

Ideal points out that the gauge on the boiler shows the pressure across the whole heating circuit. For any one boiler, its manual and the markings on its gauge set the range, so those two come before any figure in this table.

Where does a central heating system lose water?

A central heating system loses water at radiator valves and joints, along pipework under the floor, in underfloor heating loops, inside the boiler, or through the pressure relief valve's outside pipe.

The boiler makers and Grant UK point owners to these places, listed from the radiators to the boiler.

Radiator valves, where damp spots or rust marks around a radiator are the sign Vaillant UK gives.
Visible joints, which Worcester Bosch tells owners to check along with the radiator valves.
Pipework under the floor, which Worcester Bosch asks owners to consider when the system keeps losing pressure.
Underfloor heating loops, where Grant UK advises testing each loop or circuit on its own to find exactly where the leak is.
The boiler, which Vaillant UK says may itself be leaking where no other leak shows.
The pressure relief valve's outside pipe, usually led from the boiler to an outside wall; Worcester Bosch says a pipe that is dripping or wet may mean a faulty valve.

The hot and cold water pipes are a separate system from the heating circuit, and a leak on them is traced through pipe leak detection instead.

What can you check before booking a heating leak investigation?

The checks you can make before booking need no tools: compare the gauge with its markings, look along visible pipework and radiator valves for weeping or wet patches, and see whether the pressure relief valve's outside pipe is dripping, all without opening the boiler.

Read the gauge and compare it with the markings on its face and with the boiler's manual, the first check Vaillant UK and Ideal both give.
Look along the pipework for a visible leak, a weeping valve or a wet patch, as Ideal suggests.
Find the pressure relief valve's outside pipe on the outside wall and see whether it is dripping or wet, the check Worcester Bosch describes.
Write down how often the boiler needs topping up, because Ideal reads once or twice a year differently from once a month or more.

The boiler casing stays on: Vaillant UK tells owners never to take it off or work on the boiler themselves.

How is a central heating leak traced under a floor?

A central heating leak under a floor is traced by pressure testing the circuit, or each underfloor loop separately, then reading the floor with a thermal imaging camera for the warm patch the escaping hot water leaves, with tracer gas where the leak makes no heat or noise.

Testo's practical guide for heating firms shows a thermal imager following heating pipes under tiled or concrete floors, where an irregular hot spot usually marks the leak; in its worked example, a hot spot under a tiled walkway turned out to be a central heating leak.

Red Rose Leak Detection puts the four methods below together to prove the leak and place it before a floorboard comes up. Each one has a limit, and the limit decides which method follows it.

Pressure testing

On the heating, the circuit or a single underfloor loop is sealed off and brought up to pressure; the part that loses pressure on the gauge normally holds the leak.

What it detects: A fall in pressure on a sealed system or section.
Its limit: A drop on the gauge is not proof by itself: Northumbrian Water notes that plastic pipe expands when pressurised, and Wunda that trapped air must be purged for an accurate test.

Thermal imaging

Water escaping from a heating or underfloor pipe warms the surface above it, and the camera shows that warm area on the floor or wall.

What it detects: Warmer or cooler areas on a wall, floor or ceiling.
Its limit: A wall or a concrete floor blocks its view, since it reads only the surface and needs a difference in temperature there; a cool area can be a draught, and Fluke notes that the camera narrows down the likely area while an investigation may still need some invasive work.

Tracer gas

For a heating leak that gives off no heat or sound, the emptied circuit is filled with 5% hydrogen in 95% nitrogen, and the detector finds where the gas comes up through the floor.

What it detects: Hydrogen reaching the surface above the point where the gas escapes.
Its limit: Its timing cannot be promised: INFICON says how long the gas takes to surface varies considerably with the ground material above the pipe.

Acoustic listening

Sewerin makes a compact receiver for heating installations: on a pressurised circuit it hears the leak, loudest over the point where the water escapes.

What it detects: The sound that escaping water makes in a pipe under pressure.
Its limit: Without pressure in the pipe, or with a leak too quiet to hear, it has nothing to pick up, and Sewerin recommends tracer gas for those cases.

Is topping up the boiler pressure enough?

Topping up the boiler pressure is not enough when it has to be done often: it puts back the water the system has lost, and Ideal says topping up once or twice a year is generally no concern, while once a month or more could point to a small leak or to a pressure relief valve at fault.

Where that leak is hidden, Admiral says trace and access cover pays for tracking it down so that it can be repaired, and the wording of each home insurance policy decides whether it applies.

What if the leak is not on the heating system?

The leak is on a water pipe or on the underground supply pipe when the boiler holds its pressure but water still appears, and Red Rose Leak Detection traces both.

Damp patches, brown stains on a ceiling or the sound of running water with every tap off, while the boiler gauge stays where it should, point to the hot and cold pipes inside the building and to pipe leak detection inside walls, floors and ceilings.

A water meter that keeps turning with the internal stop tap closed points outside, to the supply pipe buried under the drive or garden, and underground water leak detection follows that length.

All four investigations sit side by side on the page for leak detection across the North West.