359513-UIM-A-0408
Johnson Controls Unitary Products
11
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 11 "Demand Defrost Control Module".
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.
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 3 "X/L
Output Categories". 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 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 11 "Demand Defrost Control
Module". Table 4 "Defrost Initiate Curves" 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.
Defrost Curve Selection
The factory activates the correct defrost curve during production. They
will place the defrost curve selection jumper in the P position or in a
numbered position appropriate for the specific heat pump model. You
should not have to change the defrost curve selection jumper during ini-
tial installation.
If the jumper is inadvertently moved, it should be placed in the appropri-
ate numbered location based on the model number and Table 4
"Defrost Initiate Curves". If the factory has activated the curve using the
P position, the jumper may also be returned to that position. If, however,
the factory has not activated the curve in the P position and the jumper
is placed in the P position, the control will not energize the compressor.
The control will also not energize the compressor if the defrost curve
selection jumper is in a numbered position that is not described in
Table 4 "Defrost Initiate Curves" or if the defrost curve selection jumper
is missing. The control will display the proper fault code when a defrost
curve jumper error is present. If the jumper is missing, the control will
behave as if the jumper was in the P position. If the jumper is placed in
a numbered position, the defrost curve selected by the jumper will over-
ride the defrost curve activated at the factory until the jumper is returned
to the P position. The control will display the active defrost curve using
the X/L terminal when the heat pump is operating in a defrost cycle that
has been forced using the TEST inputs.
TABLE 2:
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 3:
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
