ASHRAE Journal - March 2020 - 58

TECHNICAL FEATURE

FIGURE 6 Terminal unit tube connections.

FIGURE 7 Typical terminal unit calibration chart.

Red (HI)
Thermostat
Connection

Open to
Atmosphere

To Static
Pressure Tap

Note: Other manufacturer specific connections not shown here.

the desired static pressure value on the Y-axis, across to
the intersection point with the box diameter line. Once
at this intersection, read down the X-axis to the air volume flow value needed to program into the controller.
For example, in the test case the controller was seeking
~0.12 in. w.g. (30 Pa) of static pressure, so a value of 500
cfm (236 L/s) was programmed into the terminal unit
controller. Wait for the terminal unit controller to reach
steady state and confirm readings with a diaphragm
gauge before continuing.

Fan Filter Unit Start-Up Procedure
With the terminal unit now seeking constant static
pressure, the fan filter unit may be turned on. Increase
the fan filter unit airflow setpoint to its maximum and
measure the airflow using a hood or otherwise suitable
airflow measuring device.
As the fan filter unit should have a higher air volume
flow capacity than the design airflow, the reading will be
higher than the desired airflow. The motor speed has a
linear correlation with air volume flow (not accounting
for filter loading), so the following equation should be
used to fine-tune the motor to the correct airflow:
Control Percentage design =

Qdesign
Qreading

(1)

where
Q = Air volume flow, in cfm (ft3/minute)
For example: After programming the terminal unit
and setting the fan filter unit to 100%, the airflow measurement equipment is reading 670 cfm (316 L/s), while
only 500 cfm (236 L/s) is desired. Using Equation 1, the
control percentage should be tuned to 75%. Please note
that while in most cases a single balance is sufficient to
58

ASHRAE JOURNAL

ashrae.org

MARCH 2020

Inlet Sensor Differential (in. w.g.)

Inlet Diameter (in.)

cfm

be within airflow tolerance, occasionally a second iteration must be performed to ensure accuracy.

Constant Air Pressure Balancing Methodology
1. Balance the terminal unit to the air volume flow
corresponding to the desired static pressure, using a
static pressure probe connected to the high side and
having the low side open to atmosphere.
2. Turn the fan filter unit to 100% motor speed and allow it to reach steady state.
3. Take an airflow reading with a hood or approved
measurement device to find the maximum air volume
flow output by the fan filter unit.
4. Use Equation 1 to decrease fan filter unit motor speed
to match desired air volume flow.
5. Take a second reading, and if airflow is not within
tolerance, repeat step 4 until within tolerance.

Conclusion
Problems seen in the field using terminal units and fan
filter units with EC motors in series have given industry
professionals hesitation for good reason. While the implementation of the EC motor has undoubtedly been an asset
for fan filter units, too many points of control produced a
system operating at less than maximum efficiency. Using
a static pressure reading to control the terminal unit
corrects this problem, and has been verified by an AABCcertified balancer. This should give engineers the peace
of mind to specify terminal units in series with fan filter
units, and once again take advantage of the numerous
benefits offered by fan filter units with EC motors.
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ASHRAE Journal - March 2020

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