157650-UIM-B-1105
6
Unitary Products Group
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 Figure 9. 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 sec-
tions of this document.
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.
When the control locks out the compressor because of a pressure
switch lockout, it will energize the X/L output as shown in Table 2. The
control has a three second delay between fault code flashes.
DEFROST OPERATION
General
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.
•
Defrost is based on accumulated compressor run time, outdoor
coil termperature, and ourdoor 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.
FIGURE 9: Demand Defrost Control Module
TEST
AMBIENT
AMBG
COIL
G
COIL
1
2
3
4
COND
FA
N
HIGH
VOL
T
AGE
PRESSURE
M
SWITCH
REV
VALVE
X/L
R
C
Y
O
W
W1/66
X/L
R
C
Y
O
W
W1/66
DEMAND
DEFROST CURVE
SELECTION JUMPER
P
TABLE 1: TEST Input Functionality
Duration of connection (seconds)
Control behavior
Less than 2
No response
2-6
Bypass ASCD. If Y is present and pressure switch is
closed, contactors will be energized.
Clear lockout
More than 6
Initiate defrost cycle ignoring the liquid line and outdoor ambient temp.
Energize X/L with active defrost curve flash code
Connection removed
Terminate defrost as normal.
Connection not removed
Continue defrost cycle and X/L flash code until TEST connection removed.
TABLE 2: X/L Output Categories
Condition
X/L
Pressure Switch lockout - last mode of operation was heating 2 flashes
Pressure Switch lockout - last mode of operation was defrost 3 flashes
