Arkansas HVACR NewsMagazine January 2026

HVACR NewsMagazine January 2026

Tech News

minute interval without opening the thermostat. The defrost cycle will terminate once the coil temperature sensor

Upgrading Time & Temperature Defrost with Demand Defrost for Heat Pumps Bryan Orr, HVAC School by Techs for Techs

Since heat pumps work by extracting heat from the outdoors to bring it inside, the outdoor coil will almost surely freeze up. To mitigate the freezing, heat pumps enter a defrost cycle in which they run in cooling mode to send hot, high-pressure refrigerant back through the outdoor coil and melt the ice. While the heat pump runs a defrost cycle, the backup heat will kick on to keep the customer from becoming cold while the unit doesn't actively move heat into the home. There are two different defrost methods: time & temperature (T/T or T-T) and demand defrost. Demand defrost is usually associated with greater energy savings and less wear on the compressor. Almost anytime you replace an OEM defrost board with a universal defrost control on a heat pump, you have an opportunity to choose how to configure the new control, and that could include upgrading the time & temperature defrost to demand defrost. We'll explore why and how to upgrade the heat pump defrost method in this tech tip, using the White-Rodgers Universal Defrost Control (47D01U-843) by Copeland as an example. This control replaces over 400 OEM part numbers, most single-stage heat pump defrost controls. Differences Between Defrost Types Time & temperature defrost requires the coil sensor (thermostat) to close when it detects a temperature below a specific set point, which then starts a timer. Defrost is initiated once the equipment has run for a 30, 60, or 90

reaches a set temperature or a certain amount of time has passed, whichever one comes first. Demand defrost checks the coil temperature sensor and outdoor air temperature sensors at all times. Instead of operating on a timer, it only goes into defrost when the coil and outdoor air sensors indicate a need for it. As such, the demand defrost strategy is usually better for energy savings and compressor longevity. Two things must be true for the defrost cycle to begin: the differential between the coil temperature and outdoor ambient temperature must be large enough, and the coil temperature must be below the set point. Even if you have a temperature swing between the coil sensor and outdoor air sensor, defrost will not begin until the coil temperature has dipped below the programmed setpoint. For OEMs and the White-Rodgers Universal Defrost Control (factory default), these setpoints are as follows:

Source: White-Rodgers by Copeland 47D01U 843 Installation Instructions, p. 5