Note See Microprocessor Controller chapter for complete
sequence of operation.
A sequence start of the required loads occurs during initial start-up
of the unit and when a control mode shift requires the compressors
to start. As the controller relays and unit loads energize, the controller
LCD display shows the setpoint temperature. The controller LED display
shows the controlling (return) air sensor temperature.
Temperature control by the controller is based on the return air
sensor temperature. The evaporator fans operate continuously on low
speed (except during defrost).
Cooling until return air temperature decreases
to 1 C (1.8 F) below setpoint. Minimum 6 minute compressor ON (running)
time and 6 minute compressor OFF time prevents rapid cycling between
Cool and Null modes.
Null until return air temperature increases to 1 C (1.8 F)
above setpoint. Both compressors and the condenser fan stop while
the evaporator fans operate on low speed during the null mode.
Defrost: Resistance heaters turn ON during defrost while the
evaporator fans stop.
A Demand Defrost can be initiated by the controller when the temperature
difference between the return air sensor and setpoint increases to
a pre-set value and a minimum of 6 hours of compressor ON (running)
time have elapsed since the previous defrost.
A Defrost Timer also initiates defrost every 12 hours. During extended
unit operation, timed defrost intervals increase 6 hours each time
a timed defrost occurs without a demand defrost in between. Maximum
time interval is 36 hours. The Defrost Timer resets if the unit is
OFF more than 12 hours or the setpoint is changed more than 5 C (9
F).
