Johnson Controls 40 25 TON, Installation Operation & Maintenance

Page: 148 / 251

148
JOHNSON CONTROLS
5276178-JIM-C-0118
Sequence of Operation
The
“HEAT CONTROL OFFSET”
will be rounded up
to the nearest half degree. If the “HEAT CONTROL
OFFSET” is calculated to be less than 2°F, it will be
set at 2.0°F.
When the UNIT MODE is
“COMFORT VENT
HEATING” and active, the “HEATING CONTROL
OFFSET” will be fixed at 5.0°F and the heat will be
staged up or down as required.
TABLE 76 - CFM
CV UNITS CFM UNIT SIZE
VAV UNITS
FLEXSYS UNITS
CFM
15000 25 – 40 TON 7500
TABLE 77 – HEAT STAGES
“GAS HEAT CAPACITY”
(MBH)
MAXIMUM STAGES
266 1
532 2
798 3
Active SP
See Tables 60 and 61
to determine what the
“SUPPLY
AIR TEMP ACTIVE SP”
value is for CONSTANT
VOLUME units.
On “VARIABLE AIR VOLUM” and “FLEXSYS”
units, the
“SUPPLY AIR TEMP ACTIVE SP”
is the
“HEATING SAT”.
Heating Control
The Unit Control uses the “S U P P LY A I R
TEMPERATURE” as described below, to determine
when to cycle stages ON and OFF.
•
If the “SUPPLY AIR TEMP” is less than the
“SUPPLY AIR TEMP ACTIVE SP”
minus the
“HEATING CONTROL OFFSET” additional
stages will be brought ON after an Interstage
Delay timer has expired.
•
If the “SUPPLY AIR TEMP” is greater than the
“SUPPLY AIR TEMP ACTIVE SP” plus the
“HEATING CONTROL OFFSET” additional
stages will be turned OFF after an Interstage
Delay timer has expired.
•
If the
“SUPPLY AIR TEMP”
plus 2 times the
“HEATING CONTROL OFFSET” is more than
or equal to the “HEAT LIMIT TEMPERATURE”
the Unit Control will prevent any heat stages from
being energized.
During the initial call the Unit Control uses the
following formula to determine how many stages of gas
heat to bring ON. After which the Unit Control uses
the procedure above for staging ON or OFF additional
capacity.
N =
SAT - SATSP
2 x HCO
Where:
•
N = Number of Heat Stages Required (Rounded
Down to the Nearest Integer).
•
SAT = “ SUPPLY AIR TEMP”.
•
SATSP = “SUPPLY AIR TEMP ACTIVE SP”.
•
HCO =
“HEATING CONTROL OFFSET”.
If the Number of Heat Stages required calculates to less
than 1, the Number of Stages is set to 1.
If the Number of Heat Stages required calculates to
be greater than the derived “HEAT STAGES” value
the Number of Heat Stages required will be set to the
“HEAT STAGES” derived value.
When the UNIT MODE is
“COMFORT VENT
HEATING” or “SUP AIR TEMPERING” is active, the
initial number of heating stages will not be estimated.
Instead, one stage of heat will be brought ON at a time.
When UNIT MODE is
“STANDBY”
and
“UNDER
FLR TEMP OVRD” is active, one stage of heat is
turned on.
Compressor Staged Gas Heat Mode of
Operation When Economizer Status
The Unit Control monitors the time it takes to go
between stages of operation (prepurge, ignition, ON,
purge, etc) and the specified operating state (OFF, low
fire, high fire) to monitor and display the current mode
of operation for each gas heat sections
The following information can be viewed for each