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Auxiliary Heat & Alternative Heat
• The relay output at terminal AUXH at connector CN51
is set up at DIP switch SW1 to control the optional 2nd
heat source (besides the primary heat pump heating)
in two operation modes: auxiliary heating (see Figure
1) and alternative heating (see Figure 2).
• In auxiliary heating mode, when room return air
temperature T1 drops to 3°F below setpoint Ts
(default, options 2, 3, 4, 5, 6 or 7°F), the 2nd heat
source (e.g. electric heater, hot water or steam coil,
etc.) is turned on in addition to the primary heat pump.
• In alternative heating mode, when ambient
temperature T4 drops below the compressor lockout
setpoint Ta (See instructions of ODU LCD module and
ODU main PCB for both VRF and Mini-VRF), heat
pump is turned o and then the 2nd heat source (e.g.
gas furnace, or oil burner, etc.) is turned on.
• Some heat sources (e.g. electric heater, hot water or
steam coil, etc.) may be set up as either auxiliary heat
or alternative heat. The others (e.g. gas furnace, or oil
burner, etc.) shall be set up as alternative heat only as
they are not intended to work at the same time with
refrigerant coil under code restrictions.
Table 4. Auxiliary Heat & Alternative Heat
Auxiliary Heat Alternative Heat
Type of 2nd heat source Electric heater, water or steam, etc. Gas furnace, oil burner or any in aux-
iliary heat sources
Relay output AUXH (CN51) AUXH (CN51)
Illustration Figure 1 Figure 2
SW1 1 0 (default)
Heating mode HP with auxiliary heat HP or alternative heat
Heating control T4>Ta+2°F, AUXH closed or open by
thermostat in addition to HP
T4>Ta+4°F, AUXH open, only HP on
or o by thermostat
T4<Ta, switch to alternative heat T4<Ta, AUXH closed or open by ther-
mostat, HP o
When Ta=”1 FREE”, AUXH closed or
open by thermostat in addition to HP
When Ta=”1 FREE”, AUXH closed or
open by thermostat, HP never on
IDU fan speed control T4>Ta+2°F, speed is determined by
kit
T4>Ta+4°F, speed is determined by
kit
T4<Ta, switch to alternative heat T4<Ta, speed is determined by gas
furnace/AHU controller
See the SW1 details in the Congure DIP Switches section for details.
An example for alternative heat is a gas furnace in a
heat pump system. When the ambient temperature T4 is
lower than the compressor lockout temperature setpoint
Ta (e.g. 30°F, set at outdoor unit), the heat pump heating
is turned o. Then the relay output AUXH is closed and
the gas furnace is turned on (when room temperature T1
is lower than thermostat setpoint Ts). When ambient is
higher than 34°F, AUXH is open and the gas furnace is
turned o. Then the heat pump is turned on (when room
temperature T1 is lower than thermostat setpoint Ts).
An example for auxiliary heat is an electric heater in a
heat pump system. Room return air temperature setpoint
Ts is 70°F (set at the controller) and auxiliary setpoint is
67°F set at controller). When the room temperature is
lower than 70°F, the heat pump is turned on. When the
room drops further below 67°F, AUXH is closed and the
electric heater is turned on while the heat pump keeps
on. When the room temperature is higher than 70°F, both
the heat pump and the electric heater are turned o.
When T4 is lower than Ta, the electric heater is turned on
or o the same as the gas furnace in alternative heating.
By default, Ta is set to ”1 FREE” (LCD module) or “1” (no
LCD).
When SW1=0, the kit is set in alternative mode. Relay
AUXH is open or closed by thermostat Ts to turn o or on
gas furnace and HP is never turned on for heating;
When SW1=1, the kit is set in auxiliary mode. Relay
AUXH is open or closed by thermostat Ts in addition to
HP to turn o or on electric heater.
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