The demand for year round comfort systems in heavy duty trucks and off-highway equipment, dictated to some extent by OSHA requirements, is fast approaching that of the passenger car. This trend, as has been the history of automotive applications, is leading toward the O.E.M. adaptation of combination units wherein heater and evaporator are encased in a common assembly with air directional capabilities for either operating mode. It offers the further potential of the reheat cycle as a temperature modulation means during air conditioning operation.Heating principles are conventional in nature, utilizing engine coolant as the heat source. Cooling, while following basic automotive concepts, is more complex, particularly with respect to condenser placement and air filtration.Performance sufficient to warrant customer satisfaction is an obvious requirement; durability, strength of materials and operating life of functional components, is an even more critical criterion. Continued development in this field is mandatory.