York MILLENNIUM Y22, Installation Manual

Page: 79 / 140

102777-YIM-E-0206
Unitary Products Group 79
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 tem-
perature 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 set-
tings, or “Fan ON mode with the Sensor” option setting.
If Comfort Ventilation is selected, it will take priority over sup-
ply fan control and keep it running during the “satisfied” peri-
ods, when there is no call for heating or cooling. The Comfort
Ventilation mode is used to optionally replace the uncon-
trolled, 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 tem-
perature 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
setting is OFF).
The modulating range of the economizer dampers are limited
by a specified Economizer Minimum Position and by a speci-
fied Comfort Ventilation Maximum Economizer Setpoints.
The Economizer capability to control SAT may be further lim-
ited 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 Ven-
tilation Upper Setpoint” and “Comfort Ventilation Lower Set-
point” 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 VENTILA-
TION
Economizer control uses a Proportional-Integral control algo-
rithm that maintains SAT within the specified SAT band by
modulating the economizer dampers. The PI algorithm set-
point 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-configura-
tion for Direct, or Reverse action, depending on the relation-
ship 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 econo-
mizer 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 econ-
omizer damper and using more outdoor air. However, if
the economizer algorithm can not prevent the SAT from
increasing above the top control band limit by closing the
economizer damper to its programmed minimum posi-
tion, one or more compressors may need to be turned
on. Similarly, if the economizer algorithm can not prevent
the SAT from dropping below the bottom control band
limit by opening the economizer damper, one or more
heating stages may need to be turned on.
• If the OAT is within the SAT control band, i.e. between
the programmed "Comfort Ventilation Upper Setpoint"
and "Comfort Ventilation Lower Setpoint", the econo-
mizer damper is driven to the fully open position. In this
case, no other control action needs to be taken to main-
tain the SAT within the specified control band.
STAGING CONTROL DURING COMFORT VENTILATION
The economizer control alone may not be able to maintain
the SAT within the specified control band. A separate staging
control algorithm supplements the economizer control and
will stage heating, or mechanical cooling as necessary. If the
SAT increases above the "Comfort Ventilation Upper Set-
point" for more than 5 minutes, the control will destage a
heating stage (if any heating stages running), or add a com-
pressor. The control will repeat this process every 5 minutes
until the SAT gets back to within the control band.
If the SAT drops below the "Comfort Ventilation Low Supply
Air Setpoint" for more than 5 minutes, the control will destage
a compressor (if any compressors running), or add a heating
stage. The control will repeat this every 5 minutes until the
SAT gets back to within the control band. As the heating