Central heating leak detection when boiler pressure keeps dropping

Why does a combi or system boiler keep needing to be topped up?

A combi or system boiler keeps losing pressure and needing to be topped up because water is escaping from its sealed circuit, most often at radiator valves and other system components, or because the pressure relief valve has a fault, according to Vaillant UK and Ideal.

Worcester Bosch describes combi and system boilers as sealed systems (the heating water circulates in a closed circuit), and they usually carry a pressure gauge on the front. Low pressure means the system is short of water, Vaillant UK says, usually through a leak, and bleeding the radiators lets some water out too.

How often you top up is the useful measure. Ideal says one or two top-ups a year are generally no cause for concern, yet frequent topping up, monthly or more for example, could mean the system has a small leak or the pressure relief valve has a fault. A falling gauge does not always mean a faulty boiler: Ideal points out that pressure loss can start elsewhere in the heating.

What reading on the boiler gauge counts as normal?

A normal reading on the boiler gauge generally sits between 1 and 1.5 bar when the heating is off and the system cold, on Vaillant UK's and Ideal's figures, and all four makers in the table count anything below 1 bar as low pressure; the markings on your own gauge and its manual set the exact range.

Read the table as four separate sets of guidance. The figures differ from maker to maker, and every one of them expects the needle to rise as the water warms.

The gauge also reads more than the boiler. Ideal notes that the pressure shown belongs to the whole heating circuit, so a drop can come from any radiator, pipe or loop on it.

Which signs point to a leak on radiators, heating pipes or underfloor loops?

The signs that point to a heating leak are a gauge that keeps falling, damp spots or rust marks around radiators, bubbling skirting boards, stains on floors or ceilings, a dripping pressure relief pipe outside, and a wet patch around underfloor heating.

• A boiler that keeps losing pressure can point to a leak on the heating: Vaillant UK puts leaks first among the causes of low boiler pressure, and Ideal counts frequent top-ups among the possible signs of a small leak.
• Rust marks or damp spots around radiators, bubbling skirting boards, flaking wallpaper, and water stains on ceilings or floors, the places Vaillant UK suggests looking.
• A dripping or wet discharge pipe from the pressure relief valve, which usually runs from the boiler to an outside wall; Worcester Bosch says that may mean the valve itself is faulty.
• A wet patch around underfloor heating laid under a finished floor, which Grant UK says can lead the search to its leak.

Some of these signs point away from a hidden leak. A dripping relief pipe can be a faulty valve rather than a leaking pipe, which is why the checks further down this page come before anyone lifts a floor.

How is a leak traced on a sealed heating circuit?

A leak on a sealed heating circuit is traced by pressure testing the system, or each underfloor loop in turn, to find the part that will not hold pressure, then following the warm pipe with thermal imaging or filling the drained circuit with tracer gas to find the exact point.

1. Pressure test the heating circuit to find the part losing water

   The system, or one section of it, is closed off, brought up to pressure and watched on a gauge, since a part that cannot hold pressure normally has a leak. For underfloor heating that loses pressure with no clear signs, Grant UK says testing one loop at a time may be the way to find the leak.

2. Follow the warm heating pipe with thermal imaging

   With the heating running, a thermal imager follows the pipes under a tiled or concrete floor, and in Testo's guide for heating contractors a hot spot with an irregular shape usually marks the leak.

3. Find the leak point in a drained circuit with tracer gas

   Where the pipe cannot be followed by heat, the drained circuit is filled with a mixture of 5% hydrogen and 95% nitrogen, and a detector swept over the floor finds where the gas comes up strongest. Sewerin lists central heating and underfloor heating among the pipes it suits.

Each result is checked against another before plaster or paving is broken, because a falling gauge and a warm patch can both mislead on their own, as the limits above show.

What can you check on the boiler and radiators before calling?

Before calling, you can compare the gauge with its markings and the manual, look for weeping valves and wet patches along visible pipework, and check whether the pressure relief pipe outside is dripping, but you should never open the boiler casing.

1. Hold the gauge reading against the markings on the gauge and the boiler's manual, as Vaillant UK and Ideal advise.
2. Run an eye along any pipework you can see for weeping valves or damp, Ideal's second check.
3. Have a look outside at the pressure relief pipe, which Worcester Bosch says usually leads from the boiler to an outside wall, to see if it drips or looks wet.

The casing stays on. Vaillant UK tells owners not to remove it or work on the boiler themselves, and the Health and Safety Executive says anyone employed to work on gas appliances in a home must be a Gas Safe registered engineer. Whether a leaking boiler can still be used is a question for its manual and that engineer.

Central heating leak sealers sit outside the makers' guidance used on this page, so it makes no judgement on them. What the makers do say is that pressure which keeps falling can point to a leak, in Vaillant UK's and Ideal's guidance.

What follows once the leaking heating pipe is pinpointed?

Once the leaking heating pipe is pinpointed, its position is marked on the floor, wall or boxing above it, so the repair can start at that point.

West Country Leak Detection marks the point and records the findings: where the leak is, what access reaching it will take, photographs, and the cause if it can be established. The repair is arranged separately, and anything inside the boiler stays with a Gas Safe registered engineer.

If the water has damaged floors or ceilings, photograph both the leak and the area around it, as MoneySuperMarket advises, and check whether your policy includes trace and access cover. You can also read how insurers pay to trace a leak before you claim.

Which water leaks can be traced besides the heating?

Besides the heating, leaks can be traced on the plumbing that carries hot and cold water round the home, on the buried supply pipe outside it, and for an insurance claim.

• Damp patches, stains or running water you can hear with every tap off point to the plumbing, covered by water leak detection for pipes in walls, floors and ceilings.
• If the meter dial still moves once the internal stop tap is shut, the leak most likely lies outside the house, on the buried pipe that underground water leak detection traces.
• An insurer asking for the source of escaping water to be found is asking for trace and access for escape of water claims.

Every investigation is set out side by side under leak detection across the South West.