"Putting a Damper on Cold Air"

Why does my A/C come on whenever I turn on my designated Ventilation switch?

If you have recently installed your ADP-1101-05A Relay adaptor and find that your Air Conditioning is now on unexpectedly, that may be due to the fact that your GF and GT wires have been installed in reverse. the GT wire should be connected to your Thermostat terminal G and your  GF wire should be connected to your Furnace terminal G.

This is why your ADP-1101-05A is necessary. If a simple 24v relay were to be used to control your damper with a ventilation switch then the 24v would risk feeding the voltage signal back through the Furnace and Thermostat "G" terminals back to the "Y" terminal which would in turn signal your Air conditioner to come on.