JOHNSON CONTROLS
104
FORM 145.05-NOM7
ISSUE DATE: 10/31/2019
SECTION 5 – SEQUENCE OF OPERATION
Supply Fan VFD Bypass
This feature allows the unit to be placed in the VFD
BYPASS mode through the Unit Interface the follow-
ing parameters must be set through the User Interface:
• “SUPPLY FAN VFD BYPASS” – OPTIONS key,
SUPPLY SYSTEM subsection. This is set to IN-
STALLED or NOT INSTALLED.
• “SF VFD BYPASS MODE” – PROGRAM key,
SUPPLY SYSTEM subsection. This is set to BY-
PASSED.
Sequence of Operation
When “SF VFD BYPASS” is set to“INSTALLED and
“SF VFD BYPASS MODE” is set to BYPASSED, the
Unit Controller closes contact at connector J17 – ter-
minal 6 to switch the VFD bypass into BYPASS mode.
The bypass VFD is a standard Variable
Frequency Drive packaged with an ad-
ditional set of contactors. When BYPASS
mode is activated, contactors route power
around the VFD, connecting the indoor
fan motor directly to the supply voltage.
At this point, the motor goes to full RPM
regardless of the duct pressure signal
because the VFD is out of the loop, and
there is a potential for over pressuring the
ducts. The supply fan proving switch is
active when VFD BYPASS is ENABLED.
The air balancer must set maximum duct
static / CFM to stay within a static pres-
sure that the ductwork of that installation
can tolerate when the motor is at full
RPM, considering that the VAV boxes,
if they are part of the system, may not be
fully open. If the duct system includes
VAV boxes, they must be driven open in
BYPASS mode. Failure to do so could
result in damage to the ductwork and the
building structure.
VAV Supply Fan Speed Control
Set point Determination
The following parameters must be set through the User
Interface on the display of the unit:
• “DUCT PRESS TRANSDUCER SPAN” –
SETPOINTS key, SUPPLY SYSTEM subsection.
This can be set for 0.25 "W.C., 1.25 "W.C., 2.50 "W.C.,
or 5.00 "W.C. based on the span of duct static
pressure transducer installed; a technician needs
to verify this in the field.
• SETPOINTS key, SUPPLY SYSTEMS subsec-
tion:
• “DUCT STATIC RESET LOW SETP”
• “DUCT STATIC RESET HIGH SETP”
• OPTIONS key, SUPPLY SYSTEMS subsection:
“DP SENSOR 2" set to INSTALLED or NOT IN-
STALLED
• PROGRAM key, SUPPLY SYSTEMS subsec-
tion: “DUCT PRESS CNTRL” set to AVERAGE,
LOWEST, HIGHEST, or DUCT PRESS 1 ONLY
Sequence of Operation
The “DUCT STATIC PRESS ACTIVE SP” are varied
through the communicated input, DSP_RST_BAS,
AV05.
If DSP_RST_BAS is set to 0%, “DUCT STATIC
PRESS ACTIVE SP” is the “DUCT STATIC RESET
HIGH SETP.”
If the DSP_RST_BAS is set to 100%, the “DUCT
STATIC PRESS ACTIVE SP” is the “DUCT STATIC
RESET LOW SETP.”
When DSP_RST_BAS is between 0% and 100%, the
“DUCT STATIC PRESS ACTIVE SP” is linearly in-
terpolated between “DUCT STATIC RESET HIGH
SETP” and the “DUCT STATIC RESET LOW SETP.”
DUCT STATIC RESET
HIGH SETP
Active DSP
Setpoint
DUCT STATIC RESET
LOW SETP
Duct Static Pres Rst %
0%
100%
LD10309B
FIGURE 63 -
ACTIVE DSP SETPOINT VS. DUCT
STATIC PRES RST VOLTAGE
The “DUCT STATIC HIGH SETP” can never be
greater than the programmed “DUCT PRESS TRANS-
DUCER SPAN” (0.25 "
W.C., 1.25 "W.C., 2.50 "W.C.,
or 5.00 "W.C.
).