4-14
Sequence of Operation
µ
P-D Controller
CSR-20 & CSR-40, September 1999
Modulation Cool
• Controller calls for Modulation Cool when the Calculated
Temperature Differential is between 2.5 C (4.5 F) above
setpoint and setpoint (on temperature pull-down).
• Controller opens and closes the modulation valve to reg-
ulate the flow of refrigerant to the compressor. The
position of the modulation valve balances the unit cool-
ing capacity against the actual load requirements.
• The warm gas bypass valve pulses open and closed
(energized and de-energized) when the Calculated
Temperature Differential is less than 0.6 C (1.0 F) above
setpoint (on temperature pull-down). The amount of ON
(open) time increases as the modulation valve closes.
The bypass valve is ON (energized) continuously when
the modulation valve is closed 100%.
• After the initial temperature pull-down, the In-range
LED turns OFF if the supply air temperature increases
more than 2.5 C (4.5 F) above setpoint. The supply air
temperature must decrease to 1.7 C (3.0 F) above setpoint
to turn the In-range LED ON again.
NOTE: If the supply air sensor temperature
stays out-of-range high for 75 minutes after the
unit is In-range, the controller turns ON the
Alarm LED. Alarm code 12 (a check alarm) is
also recorded in the controller’s memory.
Null
• The controller calls for Null when the Calculated
Temperature Differential is between setpoint and 0.6 C
(1.0 F) below setpoint (on temperature pull-down).
• The controller de-energizes the compressor contactor to
stop the compressor.
• If the condenser fan was ON, it will operate for 30 sec-
onds and then stop. If the condenser fan was OFF, it
will remain OFF.
• The evaporator fans continue to operate.
• The controller de-energizes (opens) the modulation
valve and de-energizes (closes) the bypass valve.
Heat
• The electric heaters pulse ON and OFF (energized and
de-energized) when the Calculated Temperature
Differential decreases to 0.6 C (1.0 F) below setpoint
(on temperature pull-down). The amount of ON time
increases as the Calculated Temperature Differential
decreases below setpoint. The electric heaters remain
energized continuously (100% of time) when the
Calculated Temperature Differential decreases to 3.1 C
(5.5 F) below setpoint.
On temperature pull-up (and initial start-up), the con-
troller pulses the electric heaters ON and OFF until the
Calculated Temperature Differential increases to setpoint.
• The evaporator fans continue to operate.
• The In-range LED turns OFF if the supply air tempera-
ture decreases more than 2.5 C (4.5 F) below setpoint.
The supply air temperature must increase to 1.7 C (3.0 F)
below setpoint to turn the In-range LED ON again.
NOTE: If the supply air sensor temperature
stays out-of-range low for 75 minutes after the
unit is In-range, the controller turns ON the
Alarm LED. Alarm code 13 (a check alarm) is
also recorded in the controller’s memory.
Chill Load Control Sequence (Setpoints at -9.9 C
[14.1 F] and Above)
A. Cool
B. Modulation Cool
C. Null
D. Heat
E. In-Range
1. Decreasing Temperature
2. Setpoint
3. Increasing Temperature
4. Calculated Temperature Differential
5. Supply Air Temperature
