DCN: 141-00940-09 11/02/07
9
HEAT PUMP HVAC SYSTEM OPERATION
The TXB16 has Heat Pump operating modes of Heating, Cooling, Auto changeover and Emergency Heat.
The Heat Pump HVAC system operation is the same as for Standard systems for normal first and second
stage operations. In addition, Heat Pumps may have a third stage of heating, which is for heat strips. The
third stage comes at 5 degrees from setpoint and stays on until the stage 1 setpoint is satisfied.
Emergency Heat Mode.
When the Control Unit is selected as a
Heat Pump system, there is an
Emergency Heat Mode that can be selected from the WDU mode menu. In the event of a compressor
failure, the “EH” mode can be selected. When in this mode, the heat strips (W1 output) will be used for
stage one heat calls. This mode is for temporary use until the compressor can be repaired.
Heat Pump systems have the same Minimum Run Time and Short Cycle Protection /Minimum Off Time
delays as Standard systems.
Heat Pump systems have a different HVAC system setup, which is described below.
Heat Pump HVAC System Setup
Heat Pump Mode Selection.
For Heat Pump operation, set SW1-1 to the HP SYS position (ON).
Fan Mode Selection.
Set the Fan selection, SW1-2, to the STD FAN position (OFF).
Change Over Selection.
Heat pump systems change from heating to cooling by reversing the direction of
Freon flow in the system. This change over is controlled by the CHANGEOVER (CO) output from the
Control Unit (sometimes this is referred to as the reversing valve).
Most heat pump systems are designed to work normally in the heating mode and require a change over
output for cooling. Set dipswitch SW1 position 3 to the CO/CL position (SW1-3 = OFF) for this type
system. Check your HVAC system requirements for correct settings.
If your system requires changeover with heating, set SW1 position 3 to the CO/HT position (SW1-3 = ON).
Changeover Relay Operation:
The changeover relay output stays ON after a heating/cooling call for the
duration of the minimum off delay period. If another call occurs before the delay period expires, the CO
relay will already be on and this avoids continuously cycling the CO valve on/off for each call.
TXB16 Control Unit HVAC System Outputs for Heat Pump Systems
Operating Mode
Changeover with Cool
Changeover with Heat
Manual Fan
G
G
Heating Stage 1
Y1, G
Y1, G, O
Heating Stage 1 + 2
Y1, Y2, G
Y1, Y2, G, O
Heating Stage 1 + 2 + 3
Y1, Y2, G, W1
Y1, Y2, G, O, W1
Cooling Stage 1
Y1, G, O
Y1, G
Cooling Stage 1 + 2
Y1, Y2, G, O
Y1, Y2, G
Emergency Heat
W1, G
W1, G
