All available information on heat pumps was read and the articles pertaining directly to design, operation, and the overall performance of air-to-air heat pumps were included in the Review of Literature. The advantages and disadvantages of the basic types of systems and their heat sources were explained. A few of the outstanding units as well as small commercial units were discussed, but the data published on these units were very limited as to design and operating values. One of the major problems encountered with air-to-air heat pumps is the defrosting of the outside coil. Several methods were outlined but the best solution for defrosting the coil would depend upon the conditions pertaining to each unit. Several suggestions were outlined where research is needed for the proper utilization of air as a heat source and sink. arter several trial and error solutions the system was designed to give a conditioned air temperature of 100 degrees F for an outside temperature of 30 degrees F and a heat de= Livery of 436.3 Btu/min. The assumed room temperature was 75 degrees F which gave a 25 degree differential across the inside coil, With an 18 degree differential across the outside coil a refrigerating effect of 338.0 Btu/min could be picked up from the outside air. These values appeared to give the best operating conditions, The tubing was sized accordingly and considerations were given to the fact that the vapor lines would be used for both cycles. The stands were constructed and the component parts of the system were mounted on their respected stands. The piping, duct, and electrical work were completed. ‘The design and construction of each of these phases was done with emphasis on clarity, simplicity, and ease in operation and maintenance. The valves numbered 1 through 12 are the control valves. The system is operated on the heating cycle with the even-numbered valves opened and the odd-numbered valves closed. All the control valves, except the expansion valves and their bypasses, are located on the control board. A dehydrator and oil separator were installed in the pining circuits. A two-wattmeter method of power measurement was installed on the unit. This method enables the operators to determine the power supplied to each motor and instantaneous power when the load on the unit is changing. Each of the motors is protected by fusible disconnect boxes. The procedure for starting, operating, and shut down was explained with emphasis on the factors that should be watched for safe and satisfactory operation. The calculations were based upon the conditions of the air, the heat picked up from the outside air, the work supplied to the motors, and the heat gained by the room air. / Master of Science