An unoccupied 1972 mobile home was instrumented and equipped with a unitary air-to-air heat pump. After making measurements of air flow rates and air temperatures, a semi-empirical, semi-analytical model was developed to describe the thermal performance of the air distribution system. The model accounted for air leakage from the supply air duct, conductive heat loss from the supply duct, air infiltration to the return air system, and conductive heat loss from the return air system. The experimental data revealed that mass and heat exchange between the supply and return air was large. It also revealed that the heat exchange between the return air and the outdoor ambient air was excessive and increased as the indoor-outdoor temperature difference increased. As a result, the computer-predicted performance of the heating system for an entire heating season in west Lafayette, Indiana was extremely poor. The seasonal Coefficient of Performance for the heat pump-air distribution system was predicted to be less than 1.0. One conclusion from this study, then, was that the home with the duct work as designed would be more efficiently heated with baseboard electric resistance heaters than with the unitary heat pump and air distribution system. It was also concluded that the subfloor return air system and leaks in the duct work were mostly responsible for the extremely poor operating performance of the heating system. The use of direct return air systems, instead of systems with subfloor cavity return is also noted to be preferable.