Ive ordered 2 carrier rooftop 7.5 ton package gas units to be installed on top of a hotel..model 48TMD008---5. These units will have a horizontal supply duct. There is no existing return duct and is open 100 percent to outside ambient. The units im replacing are carrier 7.5 ton units and they are ducted this way and have worked fine for many years. My question is this. I was going thru my carrier book the salesgirl gave me and noticed that it says that when this unit is ducted horizontal that the accessory barometric relief or power exhaust MUST be installed on the return ductwork. I was not told of this before I ordered the unit and salespeople were fully aware of the setup i have. Its gonna piss me off if i have to send for this fan accessory as i did not quote that in part of my bid price. How important is it that this accessory be installed in this particular application. My thinking is that it will help with head pressure. Any info you can give me will be greatly appreciated. The existing units appear to have been altered in slowing the fans down by altering the pulleys and I will be doing the same with these units.