HomeKit Weekly: Shelly’s Flood S Gen4 is a low cost Matter leak sensor for Apple

Shelly Flood S Gen4: Under $30 Matter-Certified Water Sensor for Apple Home

The Shelly Flood S Gen4 is a new leak sensor priced under $30 that offers Matter certification and native Apple Home support. It’s designed to detect water early and prevent costly damage to floors, cabinets, and appliances.

Background

Water damage is one of the most expensive and preventable issues in smart homes, where a single sensor can save thousands in repairs. Leak sensors have long been considered some of the best value devices in smart home setups due to their low cost and high impact.

The Shelly Flood S Gen4 improves on traditional designs by addressing a key limitation: most sensors rely on small metal pins that require water to pool in an exact spot to trigger. This often delays detection until damage has already begun.

Specifications

The Flood S Gen4 features three large stainless steel contact pads spread across the bottom of the unit, significantly increasing the detection area compared to conventional pin-based sensors. This design allows it to sense water faster and across a broader surface.

These pads are sensitive enough to detect clean or low-conductivity water, such as AC condensate, which many cheaper sensors miss due to low mineral content. The unit is also low-profile, enabling placement under appliances, behind toilets, and beneath sinks—ideal locations for early leak detection.

It supports both Wi-Fi and Zigbee connectivity and is powered by a single CR123A battery, which Shelly claims provides up to two years of operation. The device is Matter-certified, ensuring broad compatibility across major smart home platforms.

Features

Once integrated into Apple Home via Matter, the Flood S Gen4 enables a range of automations, such as cutting power to a washing machine or dishwasher through a smart outlet the moment water is detected. This can prevent further flooding and damage.

Users can also trigger all smart lights in the house to flash as a visual alert or receive instant notifications on their iPhone—especially useful when away from home. These features turn passive detection into active protection.

The sensor includes a built-in local alarm that sounds audibly in the home, ensuring someone nearby hears it even if their phone is silenced or in another room. This dual-alert system increases the chances of a timely response.

Shelly offers two operating modes: flood detection for sudden leaks and rain detection for slower, gradual moisture buildup. Users can choose from three alarm profiles—intense, normal, and economic—to balance detection speed with battery life based on their needs.

Performance

The larger contact area of the three stainless steel pads improves responsiveness and reliability compared to traditional pin-style sensors, reducing the chance of missed leaks due to uneven water spread. This is particularly valuable under appliances where water may not pool centrally.

Its ability to detect low-conductivity water like AC condensate addresses a common blind spot in leak detection, where slow drips from HVAC systems can go unnoticed for weeks, causing mold or structural damage. The Flood S Gen4 mitigates this risk effectively.

With a two-year battery life on a standard CR123A battery, maintenance is minimal, making it practical for long-term deployment in hard-to-reach spots like under water heaters or behind toilets. The combination of longevity and sensitivity enhances real-world usability.

Price and Availability

The Shelly Flood S Gen4 is priced under $30, making it affordable enough to deploy multiple units throughout the home without hesitation. This low cost encourages comprehensive coverage in high-risk zones.

Recommended placement includes in front of water heaters, dishwashers, washing machines, and under kitchen sinks—locations where leaks are most likely to occur and cause significant damage. Buying 3–4 units is presented as a no-brainer investment.

The sensor is available for purchase on Amazon or directly from Shelly’s official website. Its Matter certification ensures seamless integration with Apple Home, Google Home, Amazon Alexa, SmartThings, and Home Assistant out of the box.

Verdict

At under $30, the Shelly Flood S Gen4 delivers exceptional value as a Matter-certified, Apple Home-compatible leak sensor with advanced detection capabilities. Its large sensing pads, low-profile design, and dual alert system make it a standout choice.

There’s little to criticize: it detects a wide range of water types, works everywhere it’s needed, and integrates smoothly into automations. For anyone building or expanding a smart home, this is one of the smartest, most impactful accessories you can buy.