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835961-UIM-B-0412
SECTION V: SYSTEM OPERATION
ANTI-SHORT CYCLE DELAY
The control includes a five-minute anti-short cycle delay (ASCD) timer
to prevent the compressor from short cycling after a power or thermo-
stat signal interruption. The ASCD timer is applied when the control is
first powered from the indoor unit thermostat and immediately following
the completion of a compressor run cycle. The compressor and the out-
door fan will not operate during the five minutes that the timer is active.
The ASCD timer can be bypassed by connecting the TEST terminals
for three seconds while the thermostat is calling for compressor opera-
tion (Y input signal energized).
LOW VOLTAGE DETECTION
The control monitors the transformer secondary (24 VAC) voltage and
provides low voltage protection for the heat pump and its components.
In particular, the control prevents contactor chatter during low voltage
conditions. If the voltage drops below approximately 19 VAC, the con-
trol will continue to energize any relays that are already energized but
will not energize any additional relays until the voltage level increases.
If the voltage drops below approximately 16 VAC, the control will imme-
diately de-energize the relay outputs and will not energize any relays
until the voltage level increases.
TEST INPUT
The control includes a TEST input connector that can be used for vari-
ous testing functions during installation and service. The TEST input
connector is shown in Figures 11 & 12. The following table summarizes
the behavior of the control when the two TEST pins are connected.
More detailed descriptions of the various functions are included in other
sections of this document.
TABLE 2: TEST Input Functionality
Duration of Connection (seconds)
Time-Temp
Demand Defrost
Less than 2
Less than 2
2-9
2-6
More than 9
More than 6
Connection removed
Connection not removed
FAULT CODE DISPLAY
X/L Output
The X/L terminal of the heat pump control is typically connected to the
X/L input of the room thermostat. The thermostat uses this signal to
notify the homeowner of a problem with the heat pump using an LED or
LCD display. When the control energizes the X/L terminal, the thermo-
stat displays the flash code so the homeowner can see it.
TABLE 3: X/L Output Categories
Condition
Pressure Switch lockout - last mode of operation was heating 2 flashes
Pressure Switch lockout - last mode of operation was defrost 3 flashes
Incorrect Defrost Curve - jumper selection
When the control locks out the compressor because of a pressure
switch lockout, it will energize the X/L output as shown in Table 3. The
control has a three second delay between fault code flashes.
If the conditions above exist during cooling mode, the system could be
in the loss of charge condition.
10
Control Behavior
No response
Bypass ASCD. If Y is present
and pressure switch is closed,
contactors will be energized.
Clear lockout
Initiate defrost cycle. (Demand
only: Energize X/L with active
defrost curve flash code)
Terminate defrost as normal
Continue defrost cycle (Demand
only: and X/L flash code) until
TEST connection removed.
X/L
On
DEFROST OPERATION
Time/Temperature Defrost (13 Seer 2-5 Ton)
The defrost control is a time/temp. control which includes a field-select-
able (tap located at board edge) time period between defrost cycle (30,
60, and 90 minutes). The jumper is factory set at 60 minutes. See Fig-
ure 13.
The electronic timer and the defrost cycle will start only when the con-
tactor is energized and the defrost thermostat is closed. The defrost
thermostat is closed when the liquid temperature falls below approxi-
mately 31° F.
The defrost mode is identical to the cooling mode except that the out-
door fan motor stops and the first stage of heat is turned on through W1
/ 66 to continue warming the conditioned space.
The defrost cycle will be terminated when the defrost thermostat is
opened at 55° F or 10 minutes of compressor accumulated run time,
whichever comes first.
NOTICE
The defrost thermostat delay will make the coil temperature about
75-80° F. Please note that the timer will stop the circuit when R to Y
is disconnected.
Demand Defrost (13 Seer 1.5 ton, 14.5 Seer 1.5-4 Ton)
The control maintains proper airflow through the outdoor coil during
heating operation by melting frost and ice that may form on the coil.
Frost may accumulate unevenly in different sections of the coil because
of the arrangement of the refrigeration circuit within the coil. The control
may initiate a defrost cycle even when the coil is not completely cov-
ered with frost. This is normal operation.
The control regulates the defrost operation of the heat pump based on
accumulated compressor run time, outdoor coil temperature, and out-
door ambient temperature. The control will cause the unit to operate in
the normal heating mode until it determines that a defrost cycle is
needed.
All defrost timings are based on accumulated compressor run time.
Operation
The defrost mode is equivalent to the cooling mode except that the out-
door fan motor is de-energized. The control shall do the following to ini-
tiate a defrost cycle.
• De-energize the outdoor fan.
• Energize the reversing valve.
• Energize the auxiliary heat output through the W1/66 terminal.
• Begin the maximum defrost cycle length timer.
If the call for heating (Y) is removed from the control during the defrost
cycle, it will terminate the defrost cycle and de-energize the compres-
sor. The control will also stop the defrost cycle length timer but not reset
it. When the control receives another call for heating, it will restart the
defrost cycle and the timer at the point at which the call for heating was
removed. This will happen only if the liquid line temperature conditions
allow defrost to occur.
Defrost Curves
The control uses a set of defrost curve parameters that are selected
using the defrost curve selection jumper. The location of the defrost
curve selection jumper is shown in Figure 13. Table 4 shows the jumper
position that is appropriate for each heat pump model. Jumper position
4 is not used and the control will not allow the compressor to operate
when the jumper is in this position.
Johnson Controls Unitary Products
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