Filter Status Switches; Airflow Monitoring - Trane Horizon OABD Series Installation, Operation And Maintenance Manual

General Information
Figure 9. Top view of OAN (rev 5) 30 to 60 tons
Figure 10. Top view of OANG (rev 6) 40 to 50 tons
Figure 11. Top View of OANG (rev 6) 55 to 80 tons

Filter Status Switches

Each unit is equipped with up to three filter status switches
based on ordered options. These switches are differential
pressure measured across the filters that triggers a binary
input into the controller.
The final filter status is located on the filter bank just before the
indoor cooling coil (or just upstream of the supply fan for no
cooling units). When equipped with dual filters (such as Merv-
14 and Merv-8) in the same filter bank, the switch measures
the pressure drop across both filter sets, and not each
individual filter type.
Each unit with an ERV has a filter status switch on the exhaust
path (before ERV) and on the outdoor air path, but the outdoor
air filter bank is removed on units with pre-heat. The two filter
switches are located near the filter banks and are labelled
Energy Recovery Wheel Outdoor and Exhaust Air Filter Status.

Airflow Monitoring

Supply Airflow Monitoring
When equipped with airflow monitoring on the supply fan(s),
the airflow is measured using a piezo reading on the cone of
the supply fan and should not be confused with a static
pressure reading. Each unit has a local and global airflow
reading, with the local value being critical for unit operation,
and the global value having an available k-factor to align the
unit with BAS readings for VAV's or other purposes.
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Piezo pressure is measured with supply fan piezo pressure
local, and a multiplier is applied using the supply fan wheel size
to generate a supply fan airflow local. The local value is used
within the program as a safety measure. To utilize the supply
airflow reading for fan control, refer to the supply fan section.
The supply airflow active is calculated by multiplying the supply
fan airflow local by the supply fan airflow k-factor. The k-factor
should be the only adjusted value.
Exhaust Airflow Monitoring
When equipped with airflow monitoring on the exhaust fan(s),
the airflow is measured using a piezo reading on the cone of
the exhaust fan and should not be confused with a static
pressure reading. Each unit has a local and global airflow
reading, with the local value being critical for unit operation,
and the global value having an available k-factor to align the
unit with BAS readings for VAV's or other purposes.
Piezo pressure is measured with exhaust fan piezo pressure
local, and a multiplier is applied using the exhaust fan wheel
size to generate an exhaust fan airflow local. The local value
is used within the program as a safety measure. To utilize the
exhaust airflow reading for fan control, refer to the exhaust fan
section.
The exhaust airflow active is calculated by multiplying the
exhaust fan airflow local by the exhaust fan airflow k-factor.
The k-factor should be the only adjusted value.
Outdoor Airflow Monitoring
Outdoor airflow is measured using averaging probes mounted
in front of the outdoor air damper, measuring velocity pressure.
The pressure measurement is fed back into the controller to
calculate an airflow reading.
Units equipped with outdoor airflow monitoring and modulating
dampers have the capability to control the dampers to an
airflow setpoint. To set the unit up for outdoor airflow control
using the outdoor air dampers, set the outdoor airflow setpoint
to the requested value. The unit will modulate the outdoor air
damper position command to maintain the outdoor airflow
active to the setpoint. Damper position is restricted between
the outdoor air damper minimum position setpoint and outdoor
air damper maximum position setpoint.
On VAV units, the supply airflow minimum setpoint active is
limited to not fall below the outdoor air minimum flow setpoint,
so that the minimum amount of outdoor air is sustained.
Note on Accuracy: The airflow measurement device is
designed for 0-100% of the full range of each cabinet. As such,
it's not possible to design a measurement device that works
across the wide ranges of operating conditions.
At full flow, the readings are accurate within ±5%, but
measurement accuracy falls off at low outdoor airflow ranges.
At about 25% of the full cabinet airflow, accuracy may fall to
±10%, and continues to decline from there.
For example, the OAB has a range up to 3,000 cfm. At 200 cfm
of outside air, the accuracy may only be 20%.
OAU-SVX007A-EN