While an integrated system is the recommended choice for fresh air ventilation, low-cost, secondary options include an exhaust system and/or a make-up air system. Exhaust ventilation systems remove air from a particular location, often leading to depressurization in the home. Replacement or make-up air will infiltrate through leaks in the building shell and other uncontrolled sources. A make-up air damper can be added to the central system when exhaust ventilation is employed. The MAS provides replacement air as needed from a controlled source into the return air plenum where it is tempered and distributed throughout the home.

The Make-Up Air System is a pressure activated activated damper that opens whenever the central fan is on or when the home becomes depressurized. Functions with or without the HHSC+.