The Alberta Building Code requires the principal exhaust fan to be interlocked with the furnace fan. An average size home usually has 2 bathroom fans upstairs, 1 downstairs. How do I determine what is the principal exhaust fan? Do I have to interlock all the bathroom fans with the furnace. On some newer homes I've seen there is a switch in the hallway, that somehow is wired in with the fan and furnace. What is the correct way to wire the fan and furnace in a new home. Thanks for any replies.