Shelly's Flood S Gen4 is a new compact water leak sensor announced in August 2026, priced at $27.99 in the US. It's a slimmer, cheaper sibling to the existing cable-based Shelly Flood Gen4, built to sit flush under a washing machine, dishwasher, or sink where a bulkier sensor won't fit. The headline feature for Home Assistant users is protocol flexibility: it ships running Matter over Wi-Fi by default, but five rapid presses of its button switch it to Zigbee 3.0 instead — useful if you'd rather keep it off your Wi-Fi network entirely and run it locally through ZHA or Zigbee2MQTT. It also exposes Shelly's own local HTTP, MQTT, and UDP API, so Home Assistant users aren't required to use either wireless standard at all. As of publication it hadn't appeared on Shelly's UK/EU store, so UK buyers will need to check stock before ordering.
What is the Shelly Flood S Gen4?
The Flood S Gen4 is a point-detection leak sensor — a small puck that sits directly on the floor rather than using a trailing cable. It measures 23×77mm and weighs 70g, runs on a single CR123A battery rated for roughly two years, and carries an IP67 rating, so it can handle sitting in a shallow puddle without damage. Detection happens through large-surface stainless steel contact pads on its underside, designed to pick up shallow or low-conductivity water — including air-conditioning condensation — that smaller pin-style sensors can miss. It has two operating modes: Flood, which triggers a full alarm, and Rain, which only reports a state change without sounding an alert, useful for outdoor or condensation-prone spots where you want data but not a 2am notification.
Does the Shelly Flood S Gen4 work with Home Assistant?
Yes — the Flood S Gen4 works with Home Assistant in three different ways, and which one suits you depends on how much you want it tied to Wi-Fi. First, Shelly's own local API (HTTP, MQTT, and UDP, plus ten configurable webhooks) lets the sensor report leak events directly to Home Assistant without going through Matter or Zigbee pairing at all, and without touching Shelly's cloud. Second, switching the sensor to Zigbee mode brings it onto your existing Zigbee mesh via Zigbee2MQTT or ZHA, though at the time of writing official Zigbee2MQTT device support was still an open request on the project's GitHub tracker rather than a confirmed, merged integration — worth checking before you buy if Zigbee is your priority. Third, its default Matter-over-Wi-Fi firmware lets you commission it straight into Home Assistant's built-in Matter integration alongside Alexa, Google Home, Apple Home, or SmartThings.
How does it compare to the standard Shelly Flood Gen4 and other sensors?
Shelly already sells a corded Flood Gen4 with a 2-metre leak sensor cable (extendable to 150m), aimed at monitoring a wider area such as a whole utility room floor. The Flood S Gen4 trades that reach for a compact, cable-free design that fits in tighter spots. It's also a useful point of comparison against the Aqara Water Leak Sensor T1, one of the picks in our best smart flood detectors UK guide.
| Model | Connectivity | Battery | IP rating | UK price |
|---|---|---|---|---|
| Shelly Flood S Gen4 | Matter (Wi-Fi) or Zigbee 3.0 (switchable) | CR123A, ~2 years | IP67 | Not yet listed on Shelly UK at time of writing; $27.99 in the US |
| Shelly Flood Gen4 (cable) | Wi-Fi, Bluetooth 5 LE, Zigbee, Matter | 4× AA, ~2 years | IP44 | £33.99 |
| Aqara Water Leak Sensor T1 | Zigbee 3.0 | CR2032, ~2 years | IP67 | ~£20 |
The corded standard Flood Gen4 is rated only IP44 — fine since its cable, not the body, sits in the water — while the compact Flood S Gen4 and the Aqara T1 are both IP67 and designed to be submerged briefly. Shelly's advantage over the Aqara is protocol flexibility: the Aqara T1 is Zigbee-only, while the Flood S Gen4 can run Matter or Zigbee and exposes a local API either way.
How do you switch between Matter and Zigbee on the Flood S Gen4?
Press the sensor's physical button five times in quick succession to toggle it between Matter firmware, which is the factory default, and Zigbee mode — and the same five-press action switches it back again. According to Shelly's own knowledge base, moving into Zigbee's pairing/inclusion mode after the switch requires a few more presses, so it's worth having the sensor within reach of both your router and your Zigbee coordinator during setup rather than trying to reconfigure it once installed under an appliance.
Is the Shelly Flood S Gen4 worth buying?
For anyone already inside Shelly's ecosystem, or who wants a leak sensor that can run entirely locally without committing to Zigbee or Matter, the Flood S Gen4 is a sensible, keenly priced option — its $27.99 US price undercuts Shelly's own corded Flood Gen4 and sits close to budget Zigbee sensors like the Aqara T1. The catch for UK buyers right now is availability: it wasn't listed on Shelly's UK or EU store at the time of writing, so check current stock and confirm UK pricing directly with Shelly or an authorised UK reseller before ordering, and if you need it immediately, the corded Flood Gen4 or an established Zigbee option such as the Aqara T1 remain available today. Shelly Plug S owners already familiar with the brand's local-first approach to Home Assistant will recognise the same philosophy here: cloud is optional, not required.
