I have a client that has a very old lab type building. He is wanting to put new outside air units in the building and interlock them with some existing fume hood exhaust fans. The equipment is old so there is no type of control system. Basically when the lab hood exhaust fan turns on the outside air unit needs to turn on via an interlocking mechanism. What would be the best way to acheive this (contactor, relay, other)? Thanks for the help.