The antifreeze valve allows the circuit medium to be drained when its temperature reaches an average value of 3 °C.
This prevents ice forming in the circuit of a system, generally with a heat pump, avoiding potential damage to the machine and to the pipes.
The version with the air sensor allows the system to work in cooling mode, even when the water temperature is approaching 3 °C.
In these conditions the air sensor cuts in to prevent water drainage.