1088917-YIM-B-1014
36 Johnson Controls Unitary Products
Duct Static Pressure Transducer
A 0-5" WC pressure transducer, located in the control box
compartment, is used to sense static (gauge) pressure in the
supply air duct and convert this pressure measurement to a
proportional 0-5 VDC electrical output.
Pressure-transmitting plastic tubing (1/4" diameter) must be
field supplied and installed from the transducer to both the
ductwork and to the atmosphere. Connect the tubing from the
'HIGH' pressure tap of the transducer to a static pressure tap
(field supplied) in the supply duct located at a point where
constant pressure is expected. To prevent an unstable signal
due to air turbulence, there should be no obstructions, turns or
VAV terminal boxes up- or down-stream of the sensing tube
location for at least a distance of 6-10 times the duct diameter.
Tubing must also be run between the 'LOW' pressure tap of the
transducer and atmospheric pressure (outside of the unit).
Factory-installed VFD
The factory-installed VFD is mounted in the Blower Access
Compartment above the blower assembly. The drive comes
wired from the factory to include both 3-phase power and
control connections (run permit signal, speed reference signal &
fault signal).
All required drive parameters are pre-programmed at the factory,
except in the case of 208-volt applications, in which the
parameter that defines motor nameplate voltage must be
changed to a value of 208.00 and the parameter that defines
motor-rated current must be changed to the appropriate value
appearing on the motor's nameplate. Refer to the enclosed drive
material or access the UPGnet Commercial Product Catalog
website for instructions on changing parameter settings.
For units also equipped with gas/electric heat, a terminal block
located in the unit's control box and connected to the VAV
board's "VAV BOX" terminal, must be field wired to the
building's VAV boxes to ensure fully open dampers during
heating operation.
Manual Bypass
An optional, factory-installed manual bypass switch available
with factory-installed VFD can be found in the Blower Motor
Access compartment and has the following three positions:
• DRIVE - routes power through the VFD for modulating
control of the indoor blower motor.
• LINE (or BYPASS) - routes power directly to the motor
which provides full-speed motor operation and complete
electrical isolation of the drive.
• TEST - routes power to the VFD but not to the motor to
allow for drive programming and/or diagnostics.
If a drive failure occurs, the unit does not automatically switch to
bypass mode. The LINE/DRIVE/TEST switch must be manually
switched to the LINE (BYPASS) position. If there is a call for the
fan, the indoor blower motor will run at full-speed while in the
bypass mode.
BAS-Ready VFD
Factory-installed VFD is also available with 'BAS-ready'
models. Terminal blocks are provided in the control box (in
place of the VAV control board) for field wiring of a customer-
installed BAS to receive 24 VAC power and to connect to the
following control signals:
• a duct static pressure transducer input signal (0-5 VDC)
• an economizer actuator input signal (2-10 VDC)
• an economizer actuator output signal (2-10 VDC)
• a VFD speed reference output signal (2-10 VDC)
The use of shielded cable is recommended for the above
control wiring connections.
NOTE: Factory-installed VFD is not available with factory-
installed BAS options due to space limitations in the
control box.
A solid-state, lock-out relay (LR) and 100-F, 50 VDC capacitor
must be field-supplied and installed to provide a means to
transmit a potential fault signal back to the BAS controller. The
specific relay part number required will depend upon the need
for either AC-output or DC-output. See price pages for further
details.
Do not run plastic tubing in the supply or return air ducts
as air movement could cause erroneous pressure
measurements. If the tubing penetrates through the
bottom of the unit be sure openings are sealed to
prevent air and water leakage.
If the unit is operated with the manual bypass switch in
the LINE (BYPASS) position and there are VAV boxes
present in the duct system, then boxes must be driven to
the full-open position using a customer-supplied power
source to prevent over-pressurizing and possible
damage to the ductwork.
Before beginning any service, disconnect all power to
the drive. Be aware that high voltages are present in the
drive even after power has been disconnected.
Capacitors within the drive must be allowed to discharge
before beginning service.
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