351579-YTS-B-0608
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Johnson Controls Unitary Products
FAN DELAYS
There are separate Fan ON and OFF delay periods for heating
and cooling, to reduce the momentary change in SAT.
COMFORT VENTILATION MODE
Comfort Ventilation is a control mode that uses the economizer
to modulate SAT. Where possible, the economizer will modulate
the outside / return air mix to keep SAT within the upper and
lower Comfort Ventilation setpoints. The control will modulate
the economizer, and energize cooling or heating if necessary, to
keep SAT within the Comfort Ventilation setpoints, even though
space temperature may be satisfied.
For example, minimum economizer position can take SAT out
of the Comfort Ventilation range, requiring heat or cooling that
the thermostat isn’t calling for. The result of comfort ventilation
control is less variation of SAT, and fewer on-off cycles of
mechanical cooling or heating. If turning off a cooling stage
raises SAT above the upper setpoint, but leaving it running will
take SAT below the lower setpoint - then the compressor is left
on, and warmer outside air is brought in to raise SAT to the
lower setpoint to keep a compressor from cycling off. Similarly,
if leaving a heat stage on will raise SAT above the Upper
setpoint, the control will leave heat on and modulate cooler
outside air into the mix.
Figure 6: Comfort Ventilation Economizer Control
The Comfort Ventilation temperature-band minimum width is
five degrees. Comfort Ventilation high and low setpoints will be
within the Cooling Upper and Heating Lower setpoints.
Without using the Comfort Ventilation mode where available,
when the space temperature control loop is satisfied (zero
demand), all cooling and/or heating stages would be turned off
and the SAT would be allowed to float until the space
temperature control loop again generates a call for cooling or
heating. The supply fan may be kept on during the “satisfied”
periods, or may be turned off, depending on thermostat
settings, or “Fan ON mode with the Sensor” option setting.
If Comfort Ventilation is selected, it will take priority over supply
fan control and keep it running during the “satisfied” periods,
when there is no call for heating or cooling. The Comfort
Ventilation mode is used to optionally replace the uncontrolled,
floating SAT situation during the “satisfied” periods with a
“loose” SAT control in a fairly wide temperature band (between
specified Comfort Ventilation Upper Setpoint and Comfort
Ventilation Lower Setpoint). This may require some additional
energy, but improves space comfort (e.g. instead of bringing a
very hot and humid ventilation / outdoor air into space during
the “satisfied” periods, the ventilation air temperature is
“trimmed” to be within the specified SAT control band).
Comfort Ventilation mode terminates when there is a call for
heating or cooling from the space temperature control.
• Comfort Ventilation can be used only on units equipped
with an Economizer.
• Comfort Ventilation only operates in an Occupied mode.
• “Comfort Ventilation Mode” must be set to ON (default set-
ting is OFF).
The modulating range of the economizer dampers are limited
by a specified Economizer Minimum Position and by a specified
Comfort Ventilation Maximum Economizer Setpoints.
The Economizer capability to control SAT may be further limited in
case the Demand Ventilation Operation is enabled and overrides
the economizer to a more-open position in order to satisfy space
IAQ requirements. The values of “Comfort Ventilation Upper
Setpoint” and “Comfort Ventilation Lower Setpoint” would typically
be set such that they are centered around an expected return air
temperature. The band between the two setpoints should be set
wide enough so that SAT changes due to staging / destaging
compressors, or heating stages, can be compensated for by the
economizer control such that staging / destaging is minimized.
Also, a wider band minimizes use of additional energy during unit’s
“satisfied” periods.
ECONOMIZER CONTROL DURING COMFORT VENTILATION
Economizer control uses a Proportional-Integral control
algorithm that maintains SAT within the specified SAT band by
modulating the economizer dampers. The PI algorithm setpoint
is calculated as a midpoint between the programmed "Comfort
Ventilation Upper Setpoint" and "Comfort Ventilation Lower
Setpoint". As the controller uses outside air to maintain the SAT
at the setpoint, it must be capable of self-configuration for
Direct, or Reverse action, depending on the relationship of the
OAT to the specified SAT control band:
• If the OAT is below the specified SAT band low setpoint
("Comfort Ventilation Lower Setpoint"), the action is Direct
Acting. In this case, the economizer control can lower the
SAT temperature just by opening the economizer damper
and using more outdoor air. However, if the economizer
algorithm can not prevent the SAT from dropping below
the bottom control band limit by closing the economizer
damper to its programmed minimum position, one or more
heating stages may need to be turned on. Similarly, if the
economizer algorithm can not prevent the SAT from
increasing above the top control band limit, one or more
compressors may need to be turned on.
• If the OAT is above the specified SAT band high setpoint
("Comfort Ventilation Upper Setpoint"), the action is
Reverse Acting. In this case, the economizer control can
increase the SAT temperature just by opening the econo-
High Supply Air Setpoint
Economizer Control Setpoint
(Middle of SAT Control Band)
Low Supply Air Setpoint
SAT Control Band
(Gray Area)
