34
6.5 Dual Fuel Applications
–
Heating Mode
If the building load should exceed the heat pump
heating capacity in extreme outdoor conditions,
the EcoNet
TM
Control Center will direct the outdoor
unit to shut down and heating responsibility will be
transferred to the gas furnace. An outdoor ambient
temperature can be set in the EcoNet
TM
Control
Center to direct the system to automatically switch
to gas heat when the outdoor ambient temperature
drops below that set-point. If the EcoNet
TM
Control
Center is set to the EMERGENCY HEAT (Furnace
Only) mode, heating responsibility will be transferred
to the gas furnace.
6.6 Demand Defrost
Defrosting of the outdoor coil in the heating mode is
controlled by the Universal Outdoor Control (UODC).
RD17 heat pumps utilizes “demand defrost” that
initiates a defrost cycle only when frosting is detected
on the outdoor coil during heating operation. The
UODC continuously monitors the Outdoor Ambient
Temperature Thermistor (OAT) and Outdoor Coil
Temperature Thermistor to determine when a defrost
cycle is required. The following sequence is followed
for defrost cycles.
• Defrost Initiation: A defrost cycle is initiated
when the following conditions are met.
1. The outdoor coil temperature is below 35°F
[1.7°C].
2. The compressor has operated for at least
34 minutes with the outdoor coil
temperature below 35°F [1.7°C].
3. The UODC determines a defrost cycle
is required based on the OAT and EVAPIN
temperatures.
If the above conditions are met, the following
action will be taken.
1. The compressor speed is ramped to the
predetermined reversing valve switching
speed for 30 seconds before the reversing
valve is de-energized.
2. The reversing valve is de-energized which
will cause it to shift to the cooling position.
Note: If the demand for defrost is initiated
while on the drive the unit will remain on the
drive until the defrost cycle is complete. If the
demand for the defrost is initiated while on
line voltage then the unit will remain on line
voltage until the defrost cycle is complete.
4. The indoor air-handler or gas furnace
is directed to supply supplemental heat
to prevent cold air from being discharged
from the supply registers during the
defrost cycle.
5. The compressor speed is ramped to the
pre-determined defrost speed for the
duration of the defrost cycle.
• Defrost Termination: The defrost cycle will
continue until the coil temperature has reached
the termination temperature or 14 minutes
have elapsed, whichever comes first. The
factory default termination temperature is 60°F
[15.6°C]. When the coil temperature reaches
the termination temperature or 14 minutes have
elapsed, the following action is taken.
1. The compressor speed is ramped to the
reversing valve switching speed for 30
seconds before the reversing valve is
energized.
2. The reversing valve will be energized,
switching it back to the heating position.
3. The outdoor fan motor is energized.
4. Auxiliary heat is de-energized.
5. The compressor speed will ramp to a pre-
determined oil circulation speed for 6
minutes.
6. The system returns to normal heating
operation as directed by the EcoNet
TM
Control Center.
7. The defrost termination temp can be adjusted
using the Contractor App.
NOTE: Should the outdoor ambient
temperature thermistor fail, the UODC will
initiate a defrost every 34 minutes of operation
when the coil temperature is below 35°F.
6.0 NORMAL SEQUENCE OF OPERATION
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