I've just looked back at your diagram again after printing it out. Two more points.
First, aren't the NO and NC markings the wrong way round on both relays?
Second, I've just realized that one of your relays is completely redundant and you could do it with one DPDT unit.
Scratch the left-hand relay and wire the hot/neutral from vent fan #1 directly to the NC contacts of the right-hand relay.
Now, with the relay in its released position the aux. fan will just go on and off with vent fan #1. When vent fan #2 kicks in the relay energizes and connects the aux. fan to that supply instead.
[This message has been edited by pauluk (edited 05-25-2002).]
