Central Heating Leak Detection

A sealed heating system does not consume water. If the gauge on your boiler read 1.5 bar in October and reads 1.0 bar in November, that water went somewhere. This page is about finding out where, in an order that stops you buying a part you did not need.

There are only three destinations. The water is inside the building, having left a pipe or a joint. It is outside on the ground, having gone through the pressure relief valve. Or it never left at all, and the gauge is misleading you because the expansion vessel has stopped doing its job. Each has a different test, and the first three tests cost nothing but a night of waiting.

What a falling gauge is actually measuring

The gauge is not a water meter. It reads the pressure of a gas cushion inside the expansion vessel, and that cushion is the only reason pressure tracks volume at all. Take a Worcester Bosch Greenstar 25Si as a worked example, because its installation manual publishes the numbers: the integral vessel is pre-charged to 0.75 bar, equal to a static head of 7.5 metres, commissioning sets the system to 1 bar cold, and the replacement section says any substitute vessel must be at least 7 litres.

Apply Boyle's law to that vessel and the arithmetic is sobering. At 1.5 bar cold, the vessel is holding roughly 2.1 litres of system water against its gas cushion. At 1.0 bar cold it is holding roughly 0.9 litres. The half a bar you watched disappear is about 1.2 litres of water. Top that up weekly and you are putting something like 60 litres a year into your floors, your ceilings or your garden.

Two things follow, and both explain why this fault drags on for months. First, a leak big enough to move the needle is far too small to make a puddle, so people look under the boiler, see nothing and conclude the boiler is at fault. Second, once system pressure falls to the pre-charge figure the vessel has no water left in it, so the gauge collapses, because water itself barely compresses. A gauge that held 1.2 bar for a year and then fell to zero in a fortnight has not sprung a bigger leak. It has crossed the pre-charge line.

Four tests, in this order

1. Read the gauge cold and hot on the same day

Read it before the heating fires, then again with every radiator fully hot. A sound sealed system rises a few tenths of a bar and comes back to where it started as it cools. A system that has lost its vessel pre-charge behaves completely differently. The expanding water has nowhere to go, so pressure climbs steeply and reaches the relief valve, which on domestic sealed heating is set at 3 bar. The valve opens, discharges water outside, and once the system cools the gauge sits lower than it did that morning. That is water genuinely gone from the system with nothing leaking inside the house at all.

2. Go outside and look at the discharge pipe

The relief valve discharge is a 15mm copper pipe run downwards to an external drain or soakaway. Worcester's own instructions warn that it may discharge boiling water and steam, so do not put a hand under it. Look instead for limescale streaking, a damp patch below the outlet on a dry day, or staining on the wall. If that pipe has been weeping, you have found where the pressure went, and the fault is a passing relief valve or a dead vessel rather than anything under your floor. This is the commonest reason a system reads as leaking when no leak can be found.

3. Shut the flow and return valves under the boiler overnight

Repressurise to 1 bar, close both isolating valves, wait eight to twelve hours and read the gauge again. If pressure holds with the valves shut but falls with them open, the loss is out in your pipework and radiators, and that is a detection job. If it falls with the valves shut, the loss is inside the appliance, and that is a boiler engineer's work rather than ours. This test decides which trade you call, and it costs nothing.

4. Test the vessel pre-charge with the system drained

The Schrader valve on the vessel can only be read honestly when there is no water pressure on the other side of the diaphragm, which means draining the system down first. A tyre gauge on the Schrader should give the figure printed in the boiler manual. Water coming out of the Schrader means the diaphragm has failed. A reading well under the published pre-charge usually means it needs recharging rather than replacing. Anyone who condemns a vessel without having drained the system has not tested it.

When the loss really is in the pipework

If the isolation test points outward, the job changes from component checks to locating one point on one circuit. We do that by narrowing, not by scanning.

First we split the system. Most London houses have at least two heating zones, and every radiator has a lockshield that will isolate it. Every section that holds pressure is a section nobody has to open a floor to look at. Then we pressurise what is left and watch the rate of fall, because the rate is diagnostic in itself. A circuit that drops half a bar in ten minutes and one that takes three days to do it are different faults, usually in different places.

Only then do the instruments come out, and they come out in a fixed order. Thermal imaging goes first, because heating is the one case where the escaping water is hot and the structure around it is not, so with the system running under load a buried leak shows as a bloom spreading away from the pipe route rather than a straight warm line along it. That is also what people are seeing when they report a warm patch on the floor. Acoustic listening then confirms the point where water is passing through a hole. Tracer gas goes last, because it means draining the circuit, and because it is the method that still works under screed and solid floors where the others do not. Gas adds an £80 bottle on the jobs that need it, and we say so before we attend.

Why underfloor loops change the job

Underfloor heating removes the two things that usually help. The pipe is continuous, so there are no joints to inspect, and it is buried in screed, so there are no boards to lift. A 16mm underfloor circuit is normally limited to around 100 metres including the tails back to the manifold, so a London ground floor or extension will have several loops, each with its own flow and return valve on the manifold.

The manifold is the whole advantage. We cap the loops, pressurise them one at a time, and find the one that will not hold. On a five loop floor that cuts the search area to about a fifth before anything is disturbed. Tracer gas into the failing loop then pinpoints the escape, and the access is usually one tile or one small square of screed rather than a room. That is the method behind our underfloor heating leak detection work.

The loop that fails also tells you something. A floor laid properly is pressure tested before the screed goes down and held under pressure through pouring and curing, so damage from a trowel or a fixing shows up immediately rather than years later. When a well commissioned floor leaks later, it is far more often at the manifold connections or where the pipe rises out of the screed than in the middle of a run. Worth knowing before anyone quotes you to lift the lot.

Three things this survey will not do

We do not work inside your boiler. If the overnight isolation test puts the loss in the appliance, the answer is a boiler engineer, and we will tell you that on the phone rather than attend and charge you to confirm it.

We do not add leak sealer. On a system that has been dosed with it, detection gets harder, not easier: sealer can partly close the hole we are trying to find and then let it reopen. If a leak has already been sealed once and has come back, say so when you call, because it changes what tracer gas will do.

We cannot survey a heating system cold. Thermal imaging on heating pipework needs the system hot and running, which in July means an hour of heating in a warm house. It is uncomfortable, and it is not optional.

What it costs and what protects it

Heating leak detection is charged at £200 an hour. A typical domestic survey takes two to four hours, so £400 to £800 in total, plus the £80 tracer gas bottle on the jobs that need gas. The rate is agreed before we travel and it does not rise on the day. Behind it sits no find, no fee: if we attend a confirmed live leak and cannot locate it, the detection time is not charged, however long we spent looking. Every rate is set out on our pricing page.

The one number we can give you from our own work: across our last 400 leak detection surveys, the cause was found in 95% of them. In 2% the water was coming from a neighbouring property. In 3% it was not a plumbing leak at all, but gutters, roofs and other structural defects letting water in.

If the leak has already damaged floors, ceilings or walls, the trace and access clause on a buildings policy normally reimburses the cost of finding it and opening up to reach it, but not the cost of repairing the pipe. Our report sets out cause, origin, method and photographs for the loss adjuster; the wording that matters is explained in our note on trace and access cover.

Call 020 7123 8560 with your postcode and two gauge readings, one cold and one hot. We will tell you on the phone whether this is a detection job or a boiler engineer's job, and confirm the £200 hourly rate, how long we expect to need and the arrival window, before you commit to anything.