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6.4 Supplemental Electric
Heat in Heating Mode
If the building load should exceed the heating
capacity in extreme outdoor conditions, the
EcoNet
TM
Control Center or 24 VAC 2-stage
thermostat will direct the RH3VZ or RHMVZ air-
handler to energize supplemental electric heat as
required to meet the building load. The indoor air-
flow will be forced to operate at the maximum level
for the system. For 13kW heaters and higher, the
supplemental electric heat will be energized in two
stages as required. The compressor and outdoor
fan will continue to operate at maximum capacity
while supplemental electric heat is energized.
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 Variable Speed Outdoor
Control (VSODC). (-)P16AZ heat pumps utilizes
“demand defrost” that initiates a defrost cycle
only when frosting is detected on the outdoor
coil during heating operation. The VSODC
continuously monitors the Outdoor Ambient
Temperature Thermistor (OAT) and Outdoor Coil
Temperature Thermistor (EVAPIN) 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 VSODC 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.
3. The outdoor EXV if is driven to the 100%
open position.
4. The outdoor fan motor is de-energized.
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.
5. 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.
6. 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 EXV if applicable will return to the
normal superheat control mode.
6. The compressor speed will ramp to a pre-
determined oil circulation speed for 6
minutes.
7. The system returns to normal heating
operation as directed by the EcoNet
TM
Control Center.
6.0 NORMAL SEQUENCE OF OPERATION
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