I want to add a relay to my furnace so the attic fan can't run while the furnace burner is firing. Am I correct that the 24V side of the relay should be connected to the C and W terminals on the furnace?
I'm not sure why you would want to do that, but paralleling the 24V relay coil to the C and W wire and connecting the attic fan to the normally closed contacts on the relay would achieve what you want to do. I would use a RIB (relay in a box) and make sure all wiring above 50V is protected per NEC. Also make sure your control voltage transformer has enough VA to handle the addition.