5
Airflow Selection for Variable Speed Furnaces
The variable speed furnaces provide blower operation to match the
capacities of the compressor during high and low stage cooling
operation. The furnace control board allows the installing
technician to select the proper airflows for each stage of cooling.
Below is a summary of required adjustments. See furnace
installation instructions for more details:
1. Turn SW1- - 5 ON for 400 CFM/ton airflow or OFF for 350
CFM/ton airflow. Factory default is OFF.
2. The A/C DIP switch setting determines airflow during high
stage cooling operation. Select the A/C DIP switch setting
corresponding to the available airflow shown in the furnace
Installation Instructions that most closely matches the
required airflow shown in the air conditioning Product Data
for HIGH speed.
3. The CF DIP switch setting determines airflow during low
stage cooling operation. Select the CF DIP switch setting
corresponding to the available airflow shown in the furnace
installation instructions that most closely matches the
required airflow shown in the air conditioning Product Data
for LOW speed. If a higher or lower continuous fan speed is
desired, the continuous fan speed can be changed using the
fan switch on the thermostat. Refer to the furnace
Installation Instructions for details of how to use this
feature.
Airflow Selection for FVM4 Fan Coils
The FVM4 provides high - and low- stage blower operation to
match the capacities of the compressor at high- and low- stage.
To select recommended airflow, refer to the FVM4 Installation
Instructions. The FVM4 utilizes a control board that allows the
installing technician to select proper airflows. This fan coil has an
adjustable blower- off delay factory set at 90 sec. for high- and
low- stage blower operation.
Install Electrical Accessories
Refer to the individual instructions packaged with kits or
accessories when installing.
Start- Up
CAUTION
!
UNIT OPERATION AND SAFETY HAZARD
Failure to follow this caution may result in personal injury,
equipment damage or improper operation.
S Do not overcharge system with refrigerant.
S Do not operate unit in a vacuum or at negative
pressure.
S Compressor dome temperatures may be hot.
CAUTION
!
PERSONAL INJURY HAZARD
Failure to follow this caution may result in personal injury.
Wear safety glasses, protective clothing, and gloves when
handling refrigerant and observe the following:
S Front seating service valves are equipped with
Schrader valves.
Follow these steps to properly start up the system:
1. After system is evacuated, fully back seat (open) liquid and
vapor service valves.
2. Unit is shipped with valve stem(s) front seated (closed) and
caps installed. Replace stem caps after system is opened to
refrigerant flow (back seated). Replace caps finger-tight and
tighten with wrench an additional 1/12 turn.
3. Close electrical disconnects to energize system.
4. Set room thermostat at desired temperature. Ensure that set
point is below indoor ambient temperature and is set low
enough to energize desired stage. In order to start the system
in low stage, refer to the literature for the installed
thermostat and adjust the set point so that the differential
between the room temperature and the set temperature
energizes Y1.
5. Set room thermostat to COOL and fan control to ON or
AUTO mode, as desired.
6. Operate unit for 15 minutes. Verify system refrigerant
charge by measuring the sub-cooling for low stage and
comparing with the published sub-cool on the rating plate.
7. In order to switch the the system to high stage, refer to the
literature for the installed thermostat and adjust the set point
to a lower value so that the differential between the room
temperature and the set temperature energizes Y2. Verify
system refrigerant charge by measuring the sub-cooling for
high stage and comparing with the published sub-cool on
the rating plate.
SYSTEM FUNCTIONS AND SEQUENCE
OF OPERATION
The N4A7 models utilize a 2-stage cooling indoor thermostat.
With a call for first stage cooling, the outdoor fan and low-stage
compressor are energized. If low-stage cannot satisfy cooling
demand, high-stage is energized by the second stage of indoor
thermostat. After second stage is satisfied, the unit returns to
low-stage operation until first stage is satisfied or until second stage
is required again.
When both first stage and second stage cooling are satisfied, the
compressor will shut off. Therefore, with first stage of cooling Y1
is powered on; and with second stage of cooling Y1 and Y2 are
powered on. When a 2-stage unit is operating at low-stage, system
vapor (suction) pressure will be higher than a standard single-stage
system or high-stage operation.
Compr essor Operation
The basic scroll design has been modified with the addition of an
internal unloading mechanism that opens a by-pass port in the first
compression pocket, effectively reducing the displacement of the
scroll.
The opening and closing of the by-pass port is controlled by an
internal electrically operated solenoid. The modulated scroll uses a
single step of unloading to go from full capacity to approximately
67% capacity. A single speed, high efficiency motor continues to
run while the scroll modulates between the two capacity steps.
Modulation is achieved by venting a portion of the gas in the first
suction pocket back to the low side of the compressor, thereby
reducing the effective displacement of the compressor. Full
capacity is achieved by blocking these vents, thus increasing the
displacement to 100%.
A DC solenoid in the compressor controlled by a rectified 24 volt
AC signal in the external solenoid plug moves the slider ring that
covers and uncovers these vents. The vent covers are arranged in
such a manner that the compressor operates at approximately 67%
capacity when the solenoid is not energized and 100% capacity
when the solenoid is energized. The loading and unloading of the
two step scroll is done “on the fly” without shutting off the motor
between steps.
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