Air Proving Switch; Fan Delays; Comfort Ventilation Mode; Comfort Ventilation Economizer Control - York MILLENNIUM Y22 Installation Manual

use this chapter as a reference whenever a more detailed
understanding of a particular mode is needed.
You should keep in mind that, especially with digital controls,
interlocking between control modes is common and easily
achieved. Also, by the very nature of digital control program-
ming, many simultaneous rules can be implemented because
they generally require only software programming and very
little peripheral hardware. This reduces the costs associated
with manufacturing and servicing equipment with this level of
interacting features - but you can't see the status of inter-
locked rules as you can with discreet relays.
Several control modes will override other modes of operation.
For example, The Demand Ventilation (or, Indoor Air Quality)
control may override Comfort Ventilation / Economizer control
and drive the OA dampers above the established minimum
position. The Excessive SAT control will override all tempera-
ture control modes, Economizer mode, and compressor oper-
ation as well. If you suspect that a problem exists with the
economizer, or the compressors are locked out when no
alarms are set, verify that one of the control modes is not
overriding the normal mode of operation or the operation you
might expect to see. Another important example is the Air
Proving Switch interlocking mode. If the Air Proving Switch
alarms on a VAV unit, the unit will refer to the duct pressure
sensor; if duct pressure is present, the control will assume
the fan is working even though the APS is not.
If the fan status [APS] fails, and the duct pressure switch of a
VAV unit sees no duct pressure, all controlling algorithms and
operations, and outputs (except hydronic heat) will cease and
the control will shut down all outputs.

AIR PROVING SWITCH

When the control starts the supply fan, it waits 90 seconds to
check for closure of the Air Proving Switch (APS). If the APS
does not close, the control will turn off all outputs except Fan
and the VFD speed signal, flag an alarm, and flash an alarm
on the display. If it is a VAV unit, the control will look to the
duct pressure signal; if it is above .05", the control will flag a
bad APS and continue operation.
It will retry the Fan output every 30 minutes for three retries. If
after three retries it still cannot qualify the Fan, it will Alarm
and lock all heating and cooling operation out. If the switch
closes after an alarm has been flagged, the control will
resume normal operation and clear the active alarm.
After the control has turned off the fan, it will wait 90 seconds
and verify that the switch opens. If the switch does not open
after 90 seconds, the control will flag a failed switch and flash
the alarm. On the next startup, the control will stage up equip-
ment normally.
This switch-closed failure mode of the Air Proving Switch can
only be detected with the Supply Fan off. It is important to
78
detect because it effectively disables the fan failure alarm
checking while fan is running, described in the paragraphs
above. Those checks would always pass as the switch would
remain closed. In networked applications, the error flag is
readable by the network. The alarm will automatically reset
after the problem that caused it has been corrected.
When the control is running the fan and the APS has already
been proven, and then it opens, the control will wait 2 sec-
onds before shutting down heating, cooling, and locking out.
It will alarm and retry as if it happened during start-up.

FAN DELAYS:

There are separate Fan ON and OFF delay periods for heat-
ing and cooling, to reduce the momentary change in SAT.

COMFORT VENTILATION MODE

Comfort Ventilation is a control mode that uses the econo-
mizer 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 heat-
ing 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 ven-
tilation 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.
High Supply Air Setpoint
Economizer Control Setpoint
(Middle of SAT Control Band)
Low Supply Air Setpoint
FIGURE 29 - COMFORT VENTILATION ECONO-
MIZER CONTROL
102777-YIM-E-0206
SAT Control Band
(Gray Area)
Unitary Products Group