I am just learning the residential hvac trade. when new homes are built where i live, a central exhaust fan collects all stale air from house and vents it. code dictates that when the central exhaust unit is running, it must be tied into the furnace so that the furnace fan can run so as to draw in fresh air through a central intake to prevent negative pressure in the home. however, i have heard that the central exhaust being wired into the furnace incorrectly can cause the central air to come on when the central exhaust comes on. so i understand a relay can be installed into the furnace to isolate the air cond and central exhaust and still allow heat, fan, a/c and central exhaust to all work.
i know how a relay works, but how these systems can be tied together confuses me. would this relay control the24v or the 120v or both
a schematic with a good explanation would be greatly appreciated
thanks in advance