Bypass Dampers (Not Applicable for 100% OA
Units)
The bypass dampers are driven closed (Bypass Damper
Closed output is energized) whenever the OA Damper position
is less than or equal to the Minimum OA Position
The Bypass dampers are driven open (Bypass Damper Open
output is energized) whenever the OA Damper Position
exceeds the Minimum OA Damper Position by more that 3%
Outside Air Damper Control
Minimum Outside Air Damper Control
Control of the dampers in the Economizer state is described
in the Economizer Control section. The outdoor air dampers
are driven open in the cooling operating state if economizer
operation is enabled and to the Minimum OA Position if
economizer operation is disabled. For all other operating
conditions, the outdoor air dampers are set to the Minimum
OA Position. The Minimum OA Position is set to zero or
closed position when the supply fan is off, the unit is in the
Recirculation state, Occupancy is set to Unocc, or the fan has
been on for less than the Zero OA Time.
As a result, the OA dampers are driven closed in night
setback, night setup, morning warm-up, and morning cool
down situations unless economizer operation is required. In all
other conditions the Minimum OA Position is equal to or below
a Ventilation Limit and equal to or above a Demand Control
Ventilation limit. For CAV units, the Ventilation Limit equals the
keypad editable Vent Limit and the Demand Control Ventilation
Limit equals the keypad editable DCV Limit. For VAV units, the
Ventilation Limit varies with VFD speed between the editable
Vent Limit at 100% Supply Fan speed and the editable LoFlo
Vent Limit at the Min Clg Spd, as shown in Figure 14. For
VAV units, the Demand Control Ventilation Limit varies as
the Ventilation Limit value changes so that the ratio between
them remains constant. In the example shown in Figure 14
the Demand Control Ventilation Limit would always be 1/2 the
Ventilation Limit since the DCV Limit= parameter (10%) is half
of the Vent Limit= parameter (20%).
The editable parameters are to be determined when the airow
for the unit is balanced and are located in the Min OA Set-up
menu.
NOTE: The MinClgSpd is prevented from being set equal
to the Design Cooling Speed. The DCV Limit is
prevented from being set greater than the Vent Limit.
If the VentLimit or the LoFloVent Limit is set to 0, the
Ventilation Limit is overridden to 0.
Figure 14: Damper Position versus Fan Speed Chart
Cold Start Operation
A special “cold start” sequence will slow the opening of the
dampers when it is cold outdoors and the unit is equipped
with either Hot water/Steam or F&BP heating. This is to try
to prevent nuisance freezestat trips associated with dampers
opening up rapidly to minimum position before the heat has a
chance to ramp up. This “cold start” sequence is initiated if the
following conditions are all true:
• OAT is below the current LoDATLimit
• The unit is equipped with an Airside Economizer
• The current Unit State is beyond the Recirc
• The current Minimum Outdoor Damper Position set point
is greater than 0%
• The unit is equipped with Hot water/Steam or F&BP
heating
When the sequence is active the dampers will move more
slowly the colder it is outdoors. The minimum and maximum
ramp rates are adjustable via the keypad by navigating to the
Commission Unit/Min OA Set-up menu. The effective ramp rate
will vary from the minimum 40% 100% (Minimum) Airow rate
at OAT equal to -30°F to the maximum at OAT equal to 100°F.
Once the damper position reaches a point 1% below the actual
effective minimum position normal operation will begin. If the
unit enters the Economizer operating state before the damper
regulation begins, the regulation will begin from the current
economizer position.
OM 920-6 • MICROTECH UNIT CONTROLLER 102 www.DaikinApplied.com
operaTor’s guIde
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