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Defrost types
The A5xx Controller provides defrost control for off-cycle defrost systems on medium temperature refrigeration
applications of 2°C to 7°C or 35°F to 45°F. The controller also provides defrost control for electric heat or hot gas
defrost systems on low-temperature refrigeration applications of less than 2°C or 35°F.
Off-cycle defrost
Off-cycle (passive) defrost does not use the defrost relay. Off-cycle defrost is the defrost type commonly used on
medium temperature refrigeration applications. Low-temperature applications do not use off-cycle defrost. During
off-cycle defrost, refrigerant flow to the evaporator is interrupted. The evaporator fans remain on to move air over
the evaporator coil and melt any accumulated frost or ice. During off-cycle defrost, the compressor relay remains
off for the defined defrost duration. Off-cycle defrost uses timed defrost termination only. Off-cycle defrost does not
use an evaporator sensor or switch, and you cannot set up temperature defrost termination.
Electric defrost
Electric defrost uses electric resistive heating elements to melt the ice that accumulates on the evaporator coil
during normal cooling operation. The defrost relay controls these electric resistive heating elements. Typically, you
can find the electric resistive heating elements in or near the evaporator coil and drain pan. Electric defrost is often
used on low-temperature refrigeration applications. During electric defrost, the compressor relay is off for the
defrost duration, the defrost relay is on, and the evaporator fan relay or relays are typically off. Electric defrost
cycles use temperature-based defrost termination or time-based defrost termination.
Hot gas defrost
Hot gas defrost systems use a bypass valve and extra refrigerant piping to temporarily reroute the hot gas
discharge from the compressor through to the evaporator coil. The defrost relay controls the bypass valve. The re-
routed hot gas melts the accumulated ice on the evaporator coil. Hot gas defrost is used on low and very low-
temperature refrigeration applications. During the defrost duration, the compressor relay is on, the defrost relay is
on, and the evaporator fan relay or relays are typically off. Typically, hot gas defrost cycles use temperature-based
defrost termination. You can also set up time-based defrost termination.
Defrost termination types
The A5xx Controller can terminate defrost durations based on time or temperature. Temperature termination
applications use the Sn2 temperature sensor or a temperature switch mounted on the evaporator coil. On
applications that use temperature-based termination by sensor or switch, you can define the maximum defrost
duration value in minutes. The defrost duration value terminates any defrost durations that do not reach the defined
defrost termination temperature.
Time-based defrost termination
Time-based defrost control terminates the defrost duration when the user-defined maximum defrost duration
elapses. You must set the defrost duration time at a sufficient length to remove all of the ice that accumulates on
the evaporator during the previous defrost interval. Time-based defrost termination does not require a defrost
termination sensor or switch.
Temperature sensor defrost termination
Temperature sensor defrost termination uses a defrost termination sensor (Sn2) mounted on the evaporator coil to
sense the coil temperature and terminate defrost at the user-selected termination temperature. The defrost
termination temperature value is the temperature at which the evaporator coil is clear of ice. Set the termination
temperature sufficiently high to ensure that all of the accumulated ice has melted off the evaporator coil at the
termination temperature.
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