JOHNSON CONTROLS
139
SECTION 5 – SEQUENCE OF OPERATION
FORM 145.05-NOM7
ISSUE DATE: 10/31/2019
5
• Six Foot Cooling Zone: When designing a Flex-
Sys system, only the rst 6 feet from the oor up
is to be conditioned. Above 6 feet, the air needs
to be mixed with heat loads. This ensures that the
RAT is at least 78.0°F. It has been determined that
RATs cooler than 78.0°F cannot properly raise
the temperature of the air leaving the evaporator
coil to the recommended MX SAT of 62–64.0°F.
It has been determined than MX SATs lower than
62.0°F are uncomfortable to occupants of a space.
• Multiple Plenums/One Unit: When designing a
system that serves multiple plenums, it is highly
recommended that each plenum have its own vol-
ume damper controlled by an actuator. Each ple-
num should also have its own pressure transducer
that controls the actuator driven volume damper.
Each plenum should be maintained at 0.05 ”W.C.
The above issues should have been taken into consid-
eration during the design and engineering phase of the
project.
Since a FlexSys unit delivers air through an underfloor
plenum, some of the setpoints differ from a typical
VAV unit. Below are recommended setpoints provided
by the Johnson Controls engineering team. There has
been a lot of testing and research completed on these
systems to arrive at these setpoints. Please be advised
that these are recommendations only, and job specif-
ic setpoints could be different. We recommend using
these setpoints at least as a starting point.
Factory Recommended Setpoints
• RAT Cooling SP: 78.0°F
• MX SAT SP: 62–64.0°F (see note below)
• Evap Leaving Air Temp High SP: 58.0°F
• Evap Leaving Air Temp Low SP: 55.0°F
• Duct Static SP: 0.05 ”W.C.
• Heating SAT SP: 80.0°F
On a FlexSys unit, the MX SAT is the tem-
perature of the supply air off the evapora-
tor coil mixed with the warmer return air
that is bypassed around the evaporator
coil. The bypassed return air is introduced
directly under the supply air fan.
FlexSys: Current Operating Mode
(Occupied)
The “CURRENT OPER MODE” for a FlexSys config-
ured unit is decided the same as for a VAV configured
unit: by the Return Air Temperature.
• If the RAT is ≥ the COOLING RAT SP by 0.5°F,
the unit enters the COOLING mode
• If the RAT is ≤ the HEATING RAT SP by 0.5°F,
the unit enters the HEATING mode
• If the RAT is between the COOLING RAT SP and
the HEATING RAT SP, the unit remains in the
STANDBY mode
FlexSys: Fan Operation
The same as a VAV configured unit, the supply fan is
controlled by a VFD. The VFD controls the speed of
the supply fan up/down to achieve and maintain the
ACTIVE DUCT STATIC SP.
The same as a VAV configured unit, the supply fan is
on whenever the unit is in the OCCUPIED mode and
cycles on/off in the UNOCCUPIED mode with a de-
mand for heating or cooling.
FlexSys: Cooling
Occupied
Whenever a FlexSys unit enters an OCC COOLING
mode, it always starts in the OCC COOLING W/O
BYPASS mode for the first 30 seconds. After this time
delay has expired, the unit controller determines which
mode it needs to be in: OCC COOLING W/O BYPASS
or OCC COOLING W/ BYPASS.
Occ Cooling w/o Bypass:
The unit controller cycles the compressors or modulate
the O/A damper to achieve and maintain the MX SAT
SP.
• If the economizer is active, the unit remains in the
OCC COOLING W/O BYPASS mode
OR
• If the RAT ≥ RAT SP plus 0.5°F but RAT < MX
SAT SP + the RA Diff SP (user adjustable between
2–10.0°F)
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