Where to put water leak sensors

The principle: sensors go where water sits, not where it starts

A point sensor is a pair of contacts that close when water bridges them. It does not see a pinhole inside a wall or a running toilet; a flow monitor on the main does that, and our smart shutoff valve guide covers those. What a sensor does is catch the appliance failures that cause most of the water losses we dry out, and it only catches them if it is sitting in the first puddle.

So the rule for every spot below is the same: find the lowest point of the floor within a foot of the appliance, where water will pool before it spreads, and put the contacts there. On a level slab that is usually directly under the fitting that fails. On a floor with any slope, it is the downhill side. Pour half a cup of water where you think the leak would start and watch where it goes; that is where the sensor goes.

Order matters because budgets are finite. Homeowners put the first sensor under the kitchen sink, where they would smell a leak within a day anyway. Start where the losses are big and silent.

The first six, in order

1. Water heater drain pan

A tank water heater fails at the bottom seam and then keeps refilling itself from the cold inlet, so a 50-gallon tank can put several hundred gallons on the floor overnight. The sensor goes in the pan, contacts down on the pan floor, on the side away from the burner access. If there is no pan, put it on the floor at the base of the tank on the downhill side. If the pan has a drain line, pour a cup of water in and confirm the line actually goes somewhere; half the pans we see drain to a capped stub. A probe-cable sensor is ideal here: the body sits on the tank's top or a shelf, the probe lies in the pan. Add a second sensor under the temperature-and-pressure relief discharge pipe if it ends over the floor rather than in the pan. Our water heater emergency guide covers what to do when this one fires.

2. Washing machine

Behind or under the washer, on the floor directly below the two supply valves and the drain standpipe. Supply hoses burst under constant pressure, the drain hose jumps out of the standpipe during a spin cycle, and the pump seal weeps. A probe cable earns its keep: the sensor body sits on the shelf above the machine where you can reach the battery, the probe snakes down behind the machine to the floor. If the washer is in a pan, the probe goes in the pan. On a second floor, this sensor should be the one that closes the main (see pairing with a shutoff, below), because the ceiling below is the expensive part.

3. Sump pit, above the high-water line

This is the sensor that tells you the pump has failed before the floor does. Mount the sensor on the pit wall or hang a probe disc on a cord from the lid so the contacts sit 1 to 2 in. above the level at which the primary pump normally turns on, and below the lid. Too low and it alarms every cycle; too high and the floor is wet first. If there is a battery backup, set the sensor between the primary's turn-on level and the backup's; then an alert means the primary quit and the backup is carrying the house. The pit is a Wi-Fi dead zone under a slab, which is why this is the spot that justifies a long-range hub radio. The battery backup roundup covers the dedicated high-water alarms that come with backup systems.

4. Under the kitchen sink

At the back of the cabinet, under the trap, where a weeping compression nut or a dishwasher supply drip lands. Kitchen first, because the dishwasher supply, the disposal and the filter housing all share that cabinet; bathrooms after. Set the puck flat on the cabinet floor, not on top of the bottles. If the cabinet floor is particleboard, it will swell before the sensor sees anything if the puck is in the wrong corner; pour the half cup and find the low spot.

5. Behind the refrigerator

The icemaker and dispenser line is 1/4 in. plastic or copper, and it fails quietly behind an appliance nobody moves for years. The sensor goes on the floor behind the fridge, near the wall where the line comes out of the saddle valve or the box. A probe cable lets you fish it in without pulling the fridge; a puck can go in when you clean the coils. If the fridge sits on hardwood, this is the sensor that saves the floor.

6. Attic or closet air handler pan

A clogged condensate line overflows the secondary drain pan under the air handler, and by the time the ceiling below stains, the drywall is finished. A probe disc in the pan beats the float switch that nobody has tested since the install. Put the sensor body where you can reach it from the attic hatch and run the probe to the low corner of the pan. If the air handler is in a closet on a slab, the puck goes on the floor beside the unit on the downhill side.

The second tier

Once the six are covered, add sensors here in roughly this order, depending on your house:

• Dishwasher kick plate. On the floor under the front edge of the dishwasher; a probe cable slides under the toe kick. Pump seals and door gaskets leak slowly for weeks.
• Toilets. On the floor behind the bowl, under the supply valve and the fill-line connection. The plastic nut on a braided supply line and the wax ring are the failures; a second-floor toilet supply is a top-three loss on our books.
• Bathroom vanities. Same as the kitchen sink, lower priority because you are in there daily.
• Main shutoff and pressure regulator. A puck on the floor under the main valve catches a weeping PRV or a failed union before it stains the slab. If the main is in a crawl space, this is where the crawl space sensor goes.
• Boiler, water softener, whole-house filter. All have housings and relief valves that let go. A puck at the base of each.
• Basement floor drain. A sensor beside the drain catches a sewer backup coming up before it crosses the floor. Not in the drain; beside it, on the uphill side so it is not the first thing fouled.
• Crawl space low point. One sensor at the lowest spot of the vapor barrier, and one near any plumbing manifold. Long-range radio required.
• Window wells and walkout thresholds. On the floor inside, where the water comes in. These are the only spots where a sensor is watching for outside water.

A typical house ends up with six to ten sensors. Count the appliances and fixtures above that you actually have; that is your number.

How many sensors, and which system

Buy for the six spots first, then the second tier that applies to your house. Multi-packs make that cheap: the Govee 1s gateway kit and the YoLink starter kit each come with three sensors and a hub, and both report five-year battery life with add-on sensors available singly.

Pick the system by the worst radio location in the house:

• Sump pit, crawl space, detached garage or outbuilding: YoLink. Its LoRa sensors are rated to a quarter mile open-air to the hub and we have seen them report from pits under slabs. The same hub runs YoLink's valve controller, which closes the main.
• A large house with a basement laundry and no outbuildings: Govee's 1s kit; 1,804 ft rated range to its gateway, a 105 dB siren on each sensor, three sensors in the box.
• Already on Ring Alarm: Ring's Flood and Freeze Sensor rides the Z-Wave mesh you already have and adds freeze alerts.
• A Moen Flo on the main: Moen's Smart Water Detector, a no-hub Wi-Fi sensor with a probe cable, temperature and humidity, that can tell the Flo to close the valve.
• Apple Home: Eve Water Guard, a plug-in sensing cable for a long run along a basement wall or around a mechanical room.
• No apps, a rental, an occupied room: the Zircon Leak Alert, a 90 dB local alarm on a 9 V battery. It will not text you, but it will be heard.

The leak detector roundup scores all six on battery life, radio, probe cable, siren and freeze alerts.

Pairing sensors with a shutoff valve

A sensor that can close the main turns a 3 a.m. alert into a 3 a.m. puddle instead of a 7 a.m. lake. Three ways to do it: Moen's detector triggers a Moen Flo; YoLink's sensors trigger the YoLink valve controller, which bolts onto your existing quarter-turn ball valve and runs on batteries through an outage; and Phyn's and Resideo's systems take their own sensors. Set the shutoff rule on the sensors that guard unattended, high-volume failures (water heater, washer, main shutoff) and leave it off for a bathroom vanity where a wet puck from a mopped floor would shut the house down. Our shutoff valve roundup and the Moen Flo vs Phyn comparison cover the valve side.

Freeze sensors are a placement question too

Burst pipes from freezing are the single most expensive residential water loss we see, and they happen where nobody is standing: garages, crawl spaces, cabins, the kitchen wall on the north side. A sensor with a temperature alert (Ring's, Moen's) goes where the pipe is coldest, not where it is convenient: at the hose bib's interior shutoff, in the crawl space near the supply manifold, in the garage by the utility sink. Set the alert at 38°F so you have time to open cabinet doors and start a drip before the pipe reaches 32°F.

When sensors are not enough, and when to call

Sensors catch puddles. They do not catch a pinhole in a copper line inside a wall, a slab leak, a toilet flapper running all night, or a supply line in the ceiling; for those you need a flow- or pressure-monitoring valve on the main. If your water bill climbed with no change in habits, or a wall or floor is warm or damp with no appliance near it, skip the sensors and start with a moisture meter and a plumber.

And if a sensor has already fired and you are standing in water: shut the main, kill power to the wet area if it is safe, and start drying within the first day. Our 72-hour dry-out guide is the sequence; the form below reaches a local restoration contractor if the water has reached finished walls or flooring.