A gas heat sub-base for use in conjunction with a PTAC or similar HVAC terminal unit, the sub-base having a housing with a front side, back side, top and bottom all defining an interior space. The top is configured to mate with the bottom of the PTAC unit. The sub-base also includes a plurality of air intake louvers on the front side of the housing and a heat exchanger located within the interior space of the housing. The gas heat sub-base is separate from, but is integrated with, the PTAC unit. A heating and cooling unit includes a PTAC unit with the gas heat sub-base.