Detecting a leak from six hundred kilometres: what sensors can and cannot do
Fifteen-euro pads, motorised valves, smart meters: the market is rich and the promises generous. Device by device, here is what works in an empty house and what does not.
Let me start with the conclusion, because it is counter-intuitive: in a second home, the value of a sensor is not measured by its ability to detect, but by what happens in the hour that follows.
A sensor that raises an alert nobody can act on protects nothing. It timestamps the claim.
I. The four families of device
The contact sensor
A pad placed on the floor, battery powered, closing a circuit when two electrodes get wet. Fifteen to forty euros each.
What it does well: report water on the floor at the precise spot where it sits. Reliable, proven, and battery life often exceeds two years.
Its limits: it detects late, since water has to reach the floor and spread as far as it. A leak inside a wall, a screed or a ceiling can bypass it entirely. And it only detects where it is: under each basin, behind each WC, under the washing machine, under the cylinder, in the plant room. A properly equipped villa needs eight to fifteen.
The motorised valve
An actuator on the main supply, driven by the sensors, that closes the water automatically. Three hundred to a thousand euros including installation.
It is the only device that turns an alert into an action. In an unoccupied house it is the keystone: without it, the chain stops at the notification.
Points to watch: installation needs a plumber, the valve should be cycled periodically so it does not seize, and it needs power. Provide a backup battery or a fail-closed model.
Flow analysis
A module fitted to the main pipe, ultrasonic or turbine, measuring flow continuously and learning the house's profile. It triggers on deviation: a continuous flow lasting longer than any normal use, an abnormal overnight volume, a permanent trickle.
It is the only technology that detects before water reaches the floor. It finds leaks in a wall, under a screed, in a buried run. Four hundred to twelve hundred euros.
Its limits: learning takes a few weeks, and an unoccupied house has no consumption profile to learn — which is in fact an advantage: in an empty house, any flow is abnormal by definition. The threshold can then be set to a few litres.
The utility's smart meter
Many communes around the Bay have rolled out remotely read meters. The customer portal often allows an over-consumption alert to be set.
It is the cheapest device in existence — it is already installed and the alert is free — and almost nobody enables it. Detection is slower, often twenty-four to forty-eight hours, but against six weeks the difference is decisive. If you do only one thing after reading this, do that one.
II. The architecture we install
For a 200 to 400 m² villa unoccupied for most of the year, this is the arrangement we consider complete.
- Supply closed by default. Between stays, the water is off. That is absolute protection; everything else covers the periods when the house is live.
- Motorised valve on the main supply, with automatic closure on alert and remote control.
- Flow analysis upstream, with the threshold set low during unoccupied periods.
- Contact sensors at the critical points: under each basin, behind each WC, under the washing machine and dishwasher, under the cylinder, in the plant room and pool room, at the low point of the basement.
- Temperature probes in unheated volumes — roof space, garage, plant room — to anticipate frost rather than record it.
- Supervision of power supply and connectivity. A silent sensor is indistinguishable from a sensor detecting nothing. You need to know the system is alive.
That last point is the most often forgotten. An installation that does not report its own failure gives false security, which is worse than no security at all.
III. What technology will never do
It does not go up on the roof. It does not smell damp in the dressing room. It does not see the crack slowly opening in the render, the green line at the base of the north wall, or the tile displaced on the west verge.
It does not shut off a gas supply, sheet a roof, let a plumber in, or make a decision.
I have seen a remarkable home automation installation — sensors everywhere, a tidy dashboard, well-tuned alerts — in a house whose north façade render had been delaminating over three square metres for two years. No sensor measures that. A glance sees it in five seconds.
The right way to frame the question is not "sensors or visits". It is: sensors cover the instantaneous and the invisible, visits cover the slow and the visible. Neither covers the other's ground.
IV. Do you need all of it?
No. There is an order of priority, and it takes three lines.
If you do one thing: enable the over-consumption alert on your smart meter and get into the habit of shutting the water off when you leave. Cost: nothing.
If you do two: add a motorised valve with a few contact sensors at the critical points. That covers the periods when the house is live.
If you want full cover: add flow analysis, temperature probes and supervision of the system itself — and above all, name someone who can be on site within the hour. Without that last line, the three before it lose most of their value.
Frequently asked questions
Is a connected leak detector enough for a second home?
Not on its own. A sensor detects and notifies but does not act. In an empty house it needs a motorised valve to shut off the supply automatically, and someone able to attend within the hour.
How many sensors does a villa need?
Between eight and fifteen for a 200 to 400 m² house: one under each basin, behind each WC, under the washing machine and dishwasher, under the hot water cylinder, in the plant room, in the pool room and at the low point of the basement.
What is the difference between a contact sensor and a flow analyser?
A contact sensor detects water once it is on the floor, so after the event. A flow analyser measures consumption continuously on the main pipe and flags any abnormal movement, including a leak inside a wall, a screed or a buried run, before anything is visible.
What if the house has poor mobile coverage?
Prefer low-power wide-area technologies such as LoRa or Sigfox, which work where mobile signal is weak. A Wi-Fi hub remains dependent on the router, and therefore on mains power and the line — two further points of failure.
Is the water utility's smart meter alert really useful?
Yes, and it offers the best ratio of usefulness to cost, since it is generally free. Detection at twenty-four to forty-eight hours is slow next to a sensor, but very fast next to the weeks that often pass between visits to a second home.
Updated June 4, 2026