Abstract: A multi-zone HVAC system, including: (a) a hollow curb to be mounted to a building roof, (b) a hollow base mounted on or in the curb, the base having a paratition thereacross; (c) a hollow working unit mounted on the base, the working unit containing a cooling coil, a fan, and a heater so that return air from the building can pass through the curb and base to be heated or cooled in the working unit and then directed through the base and curb into the building, the portion of the base below the working unit having height of less than 24 inches.