CI-2663 -- An Energy Conservation Evaluation of Two Variable Interval Time/Temperature Heat Pump Defrost Control Strategies

Residential heat pumps historically have utilized one of two methods of defrost control: demand or time/temperature.

Demand defrost control systems typically measure either the difference between outdoor air temperature and outdoor coil temperature, or the air pressure drop across the,outdoor coil, and initiate a defrost when the measured value exceeds a preset threshold. Demand defrost control systems are thereby directly responsive to the formation of ice on the outdoor coil.

The historical time/temperature defrost control systems cause defrosts to occur at fixed intervals. Because the intervals between defrosts are a fixed value, the systems are referred to here as fixed interval time/temperature (FITT) defrost control systems.

Several defrost control systems have been developed recently that utilize the time/temperature approach, but vary the interval between defrosts depending on the outdoor air dry-bulb temperature . Such systems are generically referred to here as variable-interval time/temperature (VITT) defrost control systems. VITT systems monitor outdoor air dry-bulb temperature and use the measured data to alter the interval between defrosts. However, for any fixed outdoor temperature, the VITT systems defrost at constant intervals of time.