the compressor starts, the VSODC will direct the
variable speed ECM outdoor motor to operate at
the appropriate speed to deliver the necessary
outdoor air-flow for that capacity percentage.
Upon a call for heating, the Digital Control
Center accesses the indoor air-flow parameters
found in the model data residing in the VSODC
memory card and directs the communicating air-
handler or gas furnace control board to deliver
an appropriate indoor air-flow level based on the
capacity percentage required at that point in time.
The communicating air-handler or gas furnace
control board will then direct the variable speed
ECM indoor blower motor to start and operate
at the requested heating air-flow level. As the
building load changes throughout the call for
heating operation, the compressor speed, outdoor
fan speed, and indoor air-flow level will respond
by ramping up or down as required to meet the
building load exactly rather than simply cycling
on and off as conventional systems do. Since the
noise level of the compressor increases as speed
increases, the maximum allowable compressor
speed in the Overdrive Mode can be lowered
using the Digital Control Center Installer Menu
(RPM Blocking) should the homeowner find the
compressor noise objectionable in the Overdrive
Mode. The current compressor RPM can also
be viewed in the Digital Control Center Service
Menu. When the call for heating has ended, the
Digital Control Center will direct the outdoor and
indoor units to shut down. The compressor and
outdoor fan motor will ramp down to zero RPM
and the indoor blower motor will ramp down to
the minimum air-flow level for 30-45 seconds to
extract the residual heating from the cold indoor
coil before it stops.
6.5 Supplemental Electric
Heat in Heating Mode
If the building load should exceed the heating
capacity in extreme outdoor conditions, the
Digital Control Center will direct the 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.
6.0 NORMAL SEQUENCE OF OPERATION
6.6 Dual Fuel Applications
– Heating Mode
If the building load should exceed the heat pump
heating capacity in extreme outdoor conditions,
the Digital 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
Digital Control Center to direct the system to
automatically switch to gas heat when the outdoor
ambient temperature drops below that set-
point. If the Digital Control Center is set to the
EMERGENCY HEAT (Furnace Only) mode, heating
responsibility will be transferred to the gas furnace.
6.7 Demand Defrost
Defrosting of the outdoor coil in the heating mode
is controlled by the Variable Speed Outdoor
Control (VSODC). 20 SEER 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
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
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.
The outdoor EXV is driven to the 100%
The outdoor fan motor is de-energized.