Traditional float switches use a mechanical float that rises with the water level until it physically trips a switch contact. Water leak switches (also called wet switches or puck switches) work differently. They use metallic probes and a microcontroller to detect water electronically.

Water leak switches are electronic devices. Within all of these devices there is a small circuit board with a microcontroller, a relay, and at least two metallic sensing probes exposed on the bottom of the housing. The entire circuit is powered by 24V AC or DC, typically sourced from the HVAC equipment's control transformer.

That 24V power supply does two things: it powers the microcontroller and relay, and it allows the controller to apply a very small sensing voltage across the probes. The controller sends a low voltage signal to one probe and monitors the second probe for a return signal. When the probes are dry, there is no conductive path between them. Open circuit, no return signal, no alarm. When water bridges both probes, it acts as a conductor and completes the circuit. The microcontroller detects this return signal, confirms it is sustained, and triggers the relay to switch state, shutting down the connected equipment and activating any alarm outputs.

The relay is what actually interrupts the HVAC control circuit (typically the 24V thermostat R wire) to lock out the system. Relay ratings matter. A 2A relay limits what you can switch directly, while a 5A relay gives more flexibility for interlocking with larger loads or valve actuators without needing an intermediate contactor.

Many modern water leak switches come with a felt or hydrophilic pad mounted underneath the housing around the probes. The pad wicks water toward the sensor array using capillary action, increasing sensitivity so that even a few drops can trigger detection. The tradeoff is reset time. Once that pad absorbs water, it holds moisture long after the leak is gone, delaying the dry-out needed to reset the device. Some pads take 30 minutes or more to dry, leaving the HVAC system locked out the entire time.

The other issue with felt pads is metal drain pans. A wet pad sitting on a conductive metal surface can create an unintended path between the probes through the pan itself, causing false alarms. This is why some manufacturers include small rubber feet to lift the pad off the pan floor. It is a workaround, not a fix.

Probe-only designs (no felt pad) avoid both problems. The probes must make direct contact with standing water to complete the circuit. When the water recedes, the probes dry almost immediately and the device is ready to reset. Because the probes are electrically isolated from one another by the housing material, a metal pan cannot bridge them, eliminating that class of false alarm entirely.

In summary, all water leak pucks work on the same principle: use a small voltage across two probes, detect when water completes the circuit, and fire a relay to shut down equipment. The differences between products come down to probe design (felt pad vs. bare probe), housing material, relay capacity, and whether the controller adds features like WiFi alerts, audible alarms, or time delays.