ACC-APG001A-EN
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Heat Pump Support Sequences
Heat Pump Switchover Valve
The Switchover Valve function is only applicable to Heat Pump units. Depending on the
refrigeration system configuration for a unit, it may have one or two switchover valves.
Additionally, some units with two switchover valves could control each valve independently
while others will control in tandem.
In normal unit operation, the Unit Mode will determine the operation of the switchover valve.
Unit Mode COOL will turn the switchover valve ON while Unit Mode HEAT will turn the
switchover valve OFF.
If the unit is in active Heat Pump Heating (switchover valve is OFF) and then enters defrost, the
switchover valve will be turned ON for the duration of defrost. When leaving active defrost, the
switchover valve transition to OFF is delayed 5 seconds.
Demand Defrost Control
There are two schemes in common usage for heat pump outdoor coil defrosting: Demand
Defrost and time temperature defrost. Demand Defrost is more efficient because defrost cycles
are initiated only when necessary, compared with initiation based on operating time below the
threshold temperature.
Outdoor coil defrosting occurs only when operating in heating mode with outdoor ambient
temperature below 52° F and the outdoor coil temperature below 33° F. The first defrost cycle
after power-up is initiated based on operating time at the required conditions. Shortly after
completion of the defrost cycle, the temperature difference between the outdoor coil and outdoor
air is calculated and is used as an indicator of unit performance at dry coil conditions.
Over time, as moisture and frost accumulate on the coil, the coil temperature will drop,
increasing the temperature difference. When the temperature difference reaches 1.8 times the
dry coil temperature differential (ΔT), a defrost cycle is initiated. While defrosting, the reversing
valve is in the cooling position, outdoor fans are off, and the compressors continue to operate.
The defrost cycle is terminated when the coil temperature rises high enough to indicate that the
frost has been eliminated. Termination of the defrost cycle includes a soft start delay. At the end
of each defrost cycle, the outdoor fan comes on 5 seconds before the reversing valve is de-
energized. This reduces stress on the compressor and makes for a quieter defrost.
Figure 8. Typical Demand Defrost cycle
During the defrost cycle, the Switchover Valve is turned ON, the Condenser Fan is turned OFF,
and auxiliary heat is turned ON regardless of their prior operating status while maintaining
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