Defrost System
This section addresses:
SEmergency Heat
SDefrost System Overview
SDefrost Control Connections, Jumper Settings and Features
SOperational Mode Overview (Calibration, Normal and Defrost)
SDefrost Cycle Actuation
EMERGENCY HEAT (AMBER LIGHT)
An emergency heat function is designed into some room thermostats. This feature is applicable when isolation of the outdoor unit is required, or when auxiliary electric heat is staged by outdoor thermostats. When the room thermostat is placed in the emergency heat position, the outdoor unit control circuit is isolated from power and
Emergency heat is usually used during an outdoor unit shutdown, but it should also be used following a power outage if power has been off for over an hour and the outdoor temperature is below 50°F (10°C). System should be left in the emergency heat mode at least six hours to allow the crankcase heater sufficient time to prevent compressor slugging.
DEFROST SYSTEM OVERVIEW
The control monitors ambient temperature, outdoor coil temperature, and total run time to determine when a defrost cycle is required. The coil temperature probe is designed with a spring clip to allow mounting to the outside coil tubing. The location of the coil sensor is important for proper defrost operation.
NOTE − The demand defrost control accurately measures the performance of the system as frost accumulates on the outdoor coil. This typically will translate into longer running time between defrost cycles as more frost accumulates on the outdoor coil before the demand defrost control initiates defrost cycles.
DEFROST CONTROL CONNECTIONS, JUMPERS SETTINGS AND FEATURES
Pressure Switch 5−Strike Lockout
The internal control logic of the demand defrost control counts the pressure switch trips only while the Y1 (Input) line is active. If a pressure switch opens and closes four times during a Y1 (Input), the control logic will reset the pressure switch trip counter to zero at the end of the Y1 (Input). If the pressure switch opens for a fifth time during the current Y1 (Input), the control will enter a lockout condition.
The 5−strike pressure switch lockout condition can be reset by cycling OFF the 24−volt power to the demand defrost control or by shorting the TEST pins between 1 to 2 seconds. All timer functions (run times) will also be reset.
If a pressure switch opens while the Y1 Out line is engaged, a 5−minute short cycle will occur after the switch closes.
Demand Defrost Control Pressure Switch
Connections
The unit’s automatic reset pressure switches (LO PS − S87 and HI PS − S4) are factory−wired into the demand defrost control on the LO−PS and HI−PS terminals, respectively.
Demand Defrost Control Sensors
Sensors connect to the demand defrost control through a
Testing Demand Defrost Control Sensors
Sensor resistance values can be checked by ohming across pins shown in table 11.
Table 11. Sensor Temperature /Resistance Range
|
|
| Pins/W |
| Temperature | Resistance values | ire |
Sensor | Range °F (°C) | range (ohms) | Color |
|
|
|
|
Outdoor | −35 (−37) to 120 (48) | 280,000 to 3750 | 3 & 4 |
|
|
| (Black) |
|
|
|
|
Coil | −35 (−37) to 120 (48) | 280,000 to 3750 | 5 & 6 |
|
|
| (Brown) |
|
|
|
|
Discharge (if | 24 (−4) to 350 (176) | 41,000 to 103 | 1 & 2 |
applicable) |
|
| (Yellow) |
|
|
|
|
Note: Sensor resistance increases as sensed temperature decreases.
Tables 12 and 13 shows how the resistance varies as the temperature changes for various types of sensors.
NOTE − When checking the ohms across a sensor, be aware that a sensor showing a resistance value that is not within the range shown in table 11, may be performing as designed. However, if a shorted or open circuit is detected, then the sensor may be faulty and the sensor harness will need to be replaced.
Defrost Temperature Termination Jumper
Settings (P1)
The demand defrost control selections are: 50, 70, 90, and 100°F (10, 21, 32 and 38°C). The shunt termination pin is factory set at 50°F (10°C). If temperature shunt is not installed, default termination temperature is 90°F (32°C).
Test Pins (P1) Function
Placing the jumper on the field test pins (P1) allows the technician to:
SClear short cycle lockout
SClear five−strike fault lockout
SCycle the unit in and out of defrost mode
SPlace the unit in defrost mode to clear the coil See figure 27 for flowchart of test pin (P1) operations.
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