Start by defining "unexplained"
Usage that has risen is only unexplained once you have ruled out the ordinary. Before spending an evening on isolation tests, confirm that none of these applies: an extra occupant, someone newly at home during the day, a hot spell with a hosepipe, a new or failing appliance, a filled pool or hot tub, a softener regenerating more often, or building work drawing from your outside tap.
Also confirm the increase is real rather than a billing artefact. An estimated reading followed by an actual one, a longer billing period, or the tariff change on 1 April will all raise a bill without raising consumption. Checking the bill before the plumbing runs through that in order.
Step one: the overnight baseline
This is the measurement everything else builds on. Late at night, turn off every appliance that can draw water on a timer, including the dishwasher, washing machine, ice maker, irrigation controller and water softener. Take a full meter reading with the red digits, note the time, photograph it. Take another before anyone uses a tap in the morning.
Turn the difference into a rate: cubic metres lost, times 1,000 for litres, divided by hours elapsed. A steady figure over eight hours of genuine non-use is not usage. It is a loss, and the litres-per-hour number is its approximate size. Converting that rate into a daily cost takes about a minute and usually decides how urgently you act.
Zero over eight hours is equally informative. It means nothing is escaping, and the extra volume is genuinely being drawn by something during the day.
Step two: split the house from the supply pipe
Close the inside stop valve, normally under the kitchen sink, and repeat the reading and wait. Thames Water's guidance draws the line here: with the inside stop valve off and readings that still differ, the loss sits between the meter and that valve. CCW says the same, that a meter dial still spinning with the inside valve closed points to a leak on the outside supply pipe.
Two outcomes, two different jobs:
- Meter stops. The loss is inside, downstream of the valve. Continue to step three.
- Meter keeps turning. The loss is on the supply pipe between the boundary and the house. That pipe is yours: the water company owns the main, the communication pipe up to the boundary, the external stop tap in the footpath and the meter, while pipes on private property up to the boundary of the road are the homeowner's responsibility.
Step three: isolate inside, branch by branch
If the loss is inside, work down the branches. Most houses have isolation valves under basins, behind WCs, on washing machine tails and at the boiler. Close them in groups, take a reading, wait, read again. The group that stops the meter contains the fault.
What usually turns up, in the order we find it:
- A WC passing into the pan. Silent, invisible, continuous. Dry the porcelain behind the pan, hold a dry strip of toilet paper against it below the cistern outlet for a few minutes and look for a wet line. Usually a worn flush valve seal.
- A mixer or monobloc tap passing internally into the waste, which leaves nothing to see.
- A cold water storage tank float valve passing to the overflow, discharging outside where nobody looks.
- A water softener stuck in or repeating regeneration, running to drain.
- A boiler filling loop left slightly open, topping up a heating system that is itself losing water. If your boiler pressure drops as well, the boiler pressure guides deal with that pattern.
Every item on that list is a fix you can arrange with an ordinary plumber, or in some cases a washer and an hour. None of it needs leak detection, and it would be dishonest to sell you detection before these are ruled out.
When isolation does not settle it
Some situations defeat the simple test, and it is worth recognising them rather than repeating the test hopefully:
- No accessible stop valve, or one that is seized or passing. A stop valve that does not fully close makes every downstream result meaningless. It has to be replaced before the test means anything.
- A shared supply pipe. Where one pipe serves several properties, responsibility for maintaining it is shared between the owners it serves, and your meter may be recording a neighbour's loss or your own depending on where the meter sits on the shared run. These need tracing rather than arguing.
- Flats on a communal riser, where the meter bank is remote from the flat and the pipe between them runs through parts of the building you cannot access.
- A supply pipe under a drive, slab or extension, where the loss is confirmed but the position is not, and digging on a guess is the expensive option.
- Loss with no rise in the meter. If usage is normal but you have damp, the water is not coming from the incoming main. Look at heating pipework, wastes and rainwater instead.
What detection adds
At the point where you know water is being lost and not where, a visit does three things your own testing cannot. It confirms and sizes the loss by isolating and pressure testing the section, which separates a real leak from a passing valve. It traces the position using acoustic equipment, thermal imaging, moisture mapping and, where pipework runs under screed or slab, tracer gas, which is non-invasive. And it produces a written report with the location and the evidence, which is what a water company wants for an allowance claim and what an insurer wants for trace and access.
Our rate is £150 per hour, fixed before we attend, and if we do not find the leak you do not pay for the time spent looking. We repair the pipe and reinstate what we opened, rather than leaving you to find a builder.
Keep the paperwork from the start
Every dated reading and photograph you took while tracing the problem is evidence later. An allowance can only be claimed once the leak has been repaired, and the calculation compares historic usage against the metered volume during the leak, so the readings on both sides of the repair are what make the claim work. The evidence pack sets out what to keep. If the water has damaged the building as well, the insurance claims hub covers the separate route for that.
