035-20401-001 Rev. A (0904)
Unitary Products Group
17
Auxiliary Heat Defrost Operation – Air Handler Mode
The control will energize W1 OUT and W2 OUT 45 seconds prior to and
during defrost operation. If a call for heating (Y1 or Y1 + Y2) is still
present after the defrost cycle has terminated, the control will continue
to energize W1 OUT and W2 OUT for 180 seconds after the defrost
cycle has been terminated. The control will begin normal heat pump
heating mode operation upon termination of the defrost cycle.
Pipe Freeze Protection Timer – Air Handler Mode Operation
The control starts a sixty-minute timer when a call for compressor oper-
ation and auxiliary heat (Y1 + W or Y1 + Y2 + W) is received. If the call
for compressor operation and auxiliary heat is still present after the
timer expires, the control will energize W1 OUT and W2 OUT in addition
to the compressor output regardless of the balance point setting. If the
call for auxiliary heat (W) is removed but the call for compressor opera-
tion (Y1 or Y1 + Y2) remains, the control will de-energize auxiliary heat
(W1 OUT and W2 OUT) and reset and restart the timer. If the timer
expires again, the same functionality will be repeated indefinitely. The
purpose of this feature is to prevent the pipes in a home from freezing if
the balance point is set too low and the heat pump cannot heat the
home using compressor operation only. This will be a benefit if a home
is not occupied and a compressor problem occurs. The control shall
also store and display a fault flash code when the pipe freeze timer has
expired.
AUXILIARY HEAT - FOSSIL FUEL MODE
Standard Operation
The LTCO does not have any impact on the operation of auxiliary heat
when the control is in fossil fuel mode. Since the compressor is not
allowed to come on below the balance point and the balance point is
always above the LTCO, the balance point setting effectively becomes
the LTCO setting. This is done intentionally to allow the balance point to
function as an economic balance point based on the cost of heating
with electricity (above the balance point) versus heating with fossil fuel
(below the balance point).
If the measured outdoor ambient temperature is below the balance
point setting, the control will de-energize compressor outputs and ener-
gize W1 OUT immediately when the Y1 input is received and W2 OUT
immediately when a Y2 input is received.. If the outdoor ambient tem-
perature rises above the balance point setting before the thermostat is
satisfied, the control will continue to keep the compressor outputs de-
energized until the thermostat is satisfied. In other words, the control
will use furnace to satisfy the thermostat demand and not turn the com-
pressor back on even if the outdoor ambient temperature rises above
the LTCO setting during the call for heating.
If a W input is received with or without a Y1or Y1 + Y2 input, the control
will de-energize the compressor outputs and energize W1 OUT and W2
OUT immediately.
Table 11 describes the auxiliary heat operation for fossil fuel mode.
Bonnet Sensor – Fossil Fuel Mode
The heat pump may be equipped in the field with an optional indoor air
discharge temperature or bonnet sensor. The control does not allow the
heat pump and the furnace to operate simultaneously even with a bon-
net sensor installed except surrounding a defrost cycle. The control
cycles the fossil fuel furnace differently surrounding a defrost cycle
depending on whether a bonnet sensor is installed.
The bonnet sensor is installed in the indoor unit and is mounted so that
it measures the indoor air temperature after the air exits the furnace.
The bonnet sensor is connected to the heat pump control using the BS
and BSG terminals. Refer to the bonnet sensor accessory kit for com-
plete installation instructions.
The bonnet sensor only applies to fossil fuel furnace applications and
should not be installed with air handlers. If the bonnet sensor is present
and the control is set to air handler mode, the indoor unit might be a fur-
nace and the operation mode is incorrectly specified. The control will
operate in fossil fuel mode instead of air handler mode so that the fur-
nace and heat pump are not operated simultaneously.
Since the bonnet sensor is an optional accessory, the control cannot
detect a bonnet sensor that fails in the open position. If the control
senses that the bonnet sensor thermistor is open, it will assume that a
bonnet sensor thermistor is not connected and will function without
implementing the optional bonnet sensor features. If the control senses
that the bonnet sensor thermistor is shorted, it will cause a lockout con-
dition and store and display the appropriate error code.
If the control is in air handler mode and detects that a bonnet sensor
input is present, it will control W1 OUT and W2 OUT as required by the
fossil fuel mode. It will also cause a fault code to be displayed on the
thermostat via the X/L output to notify the homeowner that there is a
problem even though the system is operating.
TABLE 10:
Air Handler Auxiliary Heat Functionality
Outdoor
Ambient Temperature
CONTROL INPUTS
Y1
Y1 + Y2
Y1 + W OR Y1 + Y2 + W
OR
Ambient above BP
Heat Pump Operation
Heat Pump Operation (then + W1 OUT + W2 OUT
after pipe freeze protection timer expires)
W1 OUT + W2 OUT
Ambient below BP and
above BP-10F
Heat Pump Operation
Heat Pump Ope W1 OUT (immediate)
+ W2 OUT (after 15 minutes)
W1 OUT + W2 OUT
Ambient below BP-10F
and above LTCO
Heat Pump Operation
Heat Pump Ope W1 OUT
(immediate) + W2 OUT (immediate)
W1 OUT + W2 OUT
Ambient below LTCO
W1 OUT
W1 OUT + W2 OUT
W1 OUT + W2 OUT
W1 OUT + W2 OUT
TABLE 11:
Fossil Fuel Furnace Auxiliary Heat Functionality
Outdoor
Ambient Temperature
CONTROL INPUTS
Y1
Y1 + Y2
Y1 + W OR Y1 + Y2 + W
W
Ambient above BP
Heat Pump Operation
Heat Pump Operation (then W1 OUT + W2 OUT
after pipe freeze protection timer expires)
W1 OUT + W2 OUT
Ambient below BP
W1 OUT
W1 OUT + W2 OUT
W1 OUT + W2 OUT
W1 OUT + W2 OUT
