Much of the seasonal cooling operation of a heat pump occurs at part-load conditions when the unit must cycle on and off to meet the cooling load. The loss in efficiency due to cycling is estimated using the degradation coefficient, CD. CD is estimated from cycling measurements made at one condition in the laboratory. A series of tests were designed to quantify the part-load performance of a three-ton heat pump operating in the cooling mode. The tests include cycling rates from 0.8 to 10 cycles per hour (cph) and percent on-times of 20%, 50%, and 80%. The indoor and outdoor conditions were constant for all test runs. All experiments were performed in psychrometric chambers. Results indicated that the part-load factor (PLF) can be as low as 0.65 at low percent on-times and high cycling rates. It took between 6 to 15 minutes for the system to reach steady state, depending on the operating conditions. The dehumidification process started between 60 and 150 seconds after start-up, depending on the test conditions. A single-time-constant model developed in a previous study was used to compare results from the experimental measurements. Results from the model showed the same trends as the experimental data. The model overpredicted losses at 4 cph and underpredicted losses at 1.2 cph.

KEYWORDS: performance, efficiency, heat pumps, speed, calculating, measuring, testing, cooling, cyclic, cooling load, factors, part load, comparing, operations, indoor, outdoor, experiment, psychrometry