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Normal Sequence Of Operation - Fujitsu FO20R Series Installation Instructions Manual

Variable speed r-410a heat pump outdoor units
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6.0 NORMAL SEQUENCE OF OPERATION

The following sections provide details on how
the 20 SEER heat pump systems are designed
to operate under normal conditions. Under
abnormal conditions, Active Compressor and
Inverter Protection features built into the controls
may alter how the system operates to protect
the compressor and inverter from damage. The
sequence of operation is based on the system
being set up for digital communications using a
communicating air-handler or gas furnace and an
Digital Control Center. A section is provided at the
end that describes how the system will operate if a
conventional 24VAC 2-stage thermostat is used in
emergency situations when communication issues
are encountered to allow the system to operate
until the communication issues are resolved.
NOTE: The use of a conventional 24VAC
thermostat eliminates many of the advanced
operating and diagnostic features the customer
has paid for and is strongly discouraged
except in temporary emergency situations until
communication issues are resolved.
6.1 Cooling Mode
The Digital Control Center will determine when
cooling is necessary and the percentage of
capacity required and will then communicate this
information to the Variable Speed Outdoor Control
(VSODC) which will direct the Power Inverter
to ramp the compressor up to the appropriate
speed to meet the required capacity percentage
being requested. The percentage of capacity
required is based on the difference between the
temperature set-point and the room temperature
measured at the Control Center. One second after
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 cooling, 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 air-handler or gas furnace control board
will then direct the variable speed ECM indoor
blower motor to start and operate at the requested
cooling air-flow level. As the building load
changes throughout the call for cooling 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. Should the building load
exceed the capacity of the system, the inverter will
direct the compressor to speed up and operate
at greater than 100% capacity (Overdrive Mode)
30
in an attempt to meet the building load until the
maximum compressor speed is reached. 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. When the call for cooling 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 cooling from the cold indoor
coil before it stops.
6.2 On-Demand Cooling
Dehumidification
The Digital Control Center can be configured
for On-Demand Dehumidification. This feature
allows the system to automatically increase the
level of dehumidification in the cooling mode by
decreasing the indoor air-flow by 15% when the
indoor relative humidity measured at the Digital
Control Center rises above the set-point. Once
the relative humidity drops to or below the set-
point, the indoor air-flow level returns to normal.
6.3 Low Ambient
Cooling Mode
Some applications require for space cooling when
the outdoor ambient temperature is below normal
(<55°F [13ºC]) such as a computer room or an
interior space with a heat source that must be
cooled. 20 SEER heat pumps are equipped with
a special low ambient cooling mode that allows
the system to provide cooling when the outdoor
temperature is below 55°F [13ºC] by cycling the
outdoor fan to maintain an adequate condensing
temperature and subcooling level based on an
input from the coil temperature thermistor.
6.4 Heating Mode
The Digital Control Center will determine when
heating is necessary and the percentage of
capacity required and will then communicate this
information to the Variable Speed Outdoor Control
(VSODC) which will direct the Power Inverter to
ramp the compressor up to the appropriate speed
to meet the required capacity percentage being
requested. The percentage of capacity required is
based on the difference between the temperature
set-point and the room temperature measured
at the Digital Control Center. One second after

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