I appreciate the feedback.
Have another idea... What about a simple disconnect on the load side of the supply fan's contactor? The exhaust and supply control circuit's are interlocked which will continue to operate as designed with exception of the supply fan not being energized while the disconnect is open.
Ghost307, I did not provide much information on this matter. The room is at a hospital and is used for isolation. I believe they need the option to run only the exhaust fan while testing the room or something of that nature.
