Outdoor Airflow (Oa) Measurement And Control - Lennox Prodigy 2.0 Application Manual

18.3. Outdoor Airflow (OA) Measurement and Control

Target OA is calculated for all the modes (DCV, Free Cooling, Minimum Run) individually when there is a change in the
supply airflow target or minimum and maximum OA targets (Parameters 237, 241 and 242). DCV and minimum run modes
will be active only if occupied. Refer to section (7. Economizer) to understand the modes of operation of an economizer.
Target OA CFM is calculated as below.
A DCV Mode: Refer to section "7.11.4. Parameter Adjustments". Replace the "MaxOpen (Parameter 117)" with
Parameter 242 CFM to find out the Outdoor Airflow Target in DCV mode.
B Minimum Run Mode: OA Target is Parameter 237 CFM.
C Free Cooling Mode: Refer to section 7.7. Free Cooling Damper Maximum Position. Replace maximum damper
opening for free cooling (Parameter 131) with Parameter 241 to find the Outdoor Airflow Target in Free cooling
mode. If DCV target is greater than Free Cooling target then DCV will override.
For any damper opening during a blower demand, actual OA is measured using an empirical model created based on the
outdoor air differential pressure sensor input (PT5) specifically for the A box Economizers (3 ton – 5 ton).
Actual OA is calculated periodically (15 seconds) and compared with the target OA and the error is accumulated for a period
of 10 min. Target OA is then adjusted so as to minimize the accumulated error and a new damper position is calculated for
the adjusted target OA. Damper is moved to the new position and then the same process is continued every 10 min. Damper
control will be stopped once the difference between the new damper position and the current position is less than 2%, but the
error will be still accumulated and new damper position calculated every 10 min.
18.3.1. Ventilation Errors
•
Under Ventilation: If the actual outdoor air is less than the target outdoor air by a percentage set by Parameter 239
(default 25%) for a time period (seconds) set by Parameter 243 (default 1800 seconds), then Alarm Code 137 (Advance
Airflow Low Outdoor Airflow) is triggered. This is realized by calculating the average error for duration set by Parameter
243 from the accumulated error and then comparing it with Parameter 239 percent of the target outdoor air. Average
error will be negative during under ventilation.
•
Over Ventilation: If the actual outdoor air is greater than the target outdoor air percentage set by Parameter 240
(Advance Air Flow High Outdoor CFM Alarm Set Point) for a duration set by Parameter 244 (Advance Air Flow Low
Outdoor CFM Alarm Evaluation), then Alarm Code 139 (Advance Airflow Outdoor Airflow Too High) is triggered. This
is realized by calculating the average error for Parameter 244 seconds from the accumulated error and then comparing
it with the percentage set for Parameter 239 (Advance Airflow Outdoor CFM Alarm Set Point), the target outdoor air.
Average error will be positive during over ventilation.
18.3.2. Damper Diagnostics
Damper actuator takes control voltage as input (P262 VOT) and gives back the position feedback voltage (P262 DPOS).
The operating range of the damper is 2V to 10V corresponding to the 0 and 100 percent opening. Due to the slow nature of
the actuator when a new control voltage is set, it takes a predefined amount of time to move to the new position and set the
feedback voltage to a value corresponding to the new position. The current Siemens actuator (35 IN­LB for 3-ton and 4-ton A
box units) takes 30 seconds to move from 0 to 100 percent whereas the 62 IN­LB actuator for 5-ton A-box unit takes 90
seconds for the same function.
•
Damper Stuck Condition: When the control voltage is changed and the feedback voltage does not reach the control
voltage +/­ 0.15 V (+/­ 2%) within predefined times, then Alarm 136 (Advance Airflow Outside Air Damper/ECON
Mechanical Fault) is triggered. Damper travel time is a prorated value of the total travel time (fixed as 90 seconds)
considering the current position and the target position of the actuator. For example if damper is to be moved from 40
to 60 percent then the damper travel time is 18 seconds.
•
Damper Error: Once the damper has reached the target position, differential pressure across the damper (PT5) is
measured and the measured value is diagnosed. Because of the empirical model of outdoor air control, the M3 Unit
Controller knows if the value measured for a given supply airflow and damper opening is a legitimate value or not. In
a condition where the differential pressure is found to be not legitimate (either very high or very low than normal) for
a period set by Parameter 245 (factory default is 1800 seconds), then Alarm Code 143 (ADVANCE AIRFLOW
DAMPER PRESSURE ERROR) is triggered. Valid range for Parameter 245 is 0 to 3600 seconds.
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