OK, you’ve exhausted all of the existing air from an interior space; now how do you replace it? You bring in makeup air from another source. For the purpose of an HVAC discussion, your source will be mechanical equipment — makeup air units.
“Applications where the air within a building or room cannot be recirculated, or where an HVAC system does not have the means for introducing fresh outside air, are good candidates for makeup air units,” said Mary Milmoe, vice president of marketing, Carrier North America Commercial, Carrier Corp., Syracuse, NY. “The decision to use makeup air units (over packaged rooftop units) to heat, ventilate, cool, humidify, dehumidify, and filter is mostly application driven.