The idea behind having an air ventilation switch is that today’s houses are more air tight than ever and not having a proper air ventilation, especially during the cold winter times can cause problems.
This issue can be solved by having a system in place which can supply new air and suck the old air out at the same time.
Fortunately we normally have both devices in our houses installed: Bathroom fan can take care of the old air and forced air furnace can bring new air ( by turning on it’s fan )
We wire this system with an regular line voltage switch ( located beside the thermostat ) interlocked with bath fan and furnace.
Power (14/2) comes in to the bathroom device box, from there 14/3 wire goes to the ventilation switch device box. A 14/2 wire will connect the ventilation switch to the 24VAC transformer (chime transformer attached to an octagon box ) in the mechanical room. From here we start our class 2 wiring.
18/2 FAS cable will connect the transformer to the relay coil.
And finally terminals 2 and 4 on the relay will be connected to the terminals G and R on the furnace control board.