Condensate water cannot overflow or back up into the fau heat exchanger with this set up. Equipment is set in fau closet with a raised platform for return air. Home has very nice hardwood flooring all around this interior fau closet. With this mechanical switch I don't anticipate water damage from this installation. Without these safety switches a condensate back up on a call for cooling would fill the heat exchanger, overflow the fau vent/trap assembly and make a mess.
3/4" Cross tee allows for the installation of a safety switch to break the "Red".
Photo taken from the back of closet looking out at drain assembly and me.
Drawing of this drain set up.