ASHRAE Journal - September 2013 - 78

Column HVAC AppliCAtions
Dan Int-Hout

Air Balancing Area Factors
By DAn int-Hout, Fellow AsHRAe

Balancing a zone’s air-distribution outlets is a critical requirement for a
complex system to be able to meet the design loads in all spaces. While relatively simple for constant volume systems, which often includes series fanpowered VAV systems, an air balancer forces a VAV system (or a section of a
system) to go to full design airflow, typically by setting zone thermostats to call
for full cooling in that zone. The balancer then must adjust balancing dampers in each diffuser (or better, the damper in the duct supplying the diffuser)
to provide designed airflow at each outlet.
Measurement of the air supplied by an outlet can
be accomplished by a traverse of the duct velocity (complex and time consuming), using a “capture hood” (a
device that collects the airflow from a device and passes
it through a measurement grid) or use a velometer to
measure the discharge velocity at each outlet and convert that velocity to an airflow rate. The conversion from
velocity to airflow rate requires a known “area factor,”
or Ak, which when multiplied by the velocity converts
velocity to flow, or from fpm to cubic fpm. This value is
not the measured open area of the outlet, but the “effective” free area, which is the ratio of the discharge velocity to the actual airflow rate. The challenge with the discharge velocity method is to come up with a discharge
velocity measurement method that yields a consistent
value, an issue compounded in complexity by the inlet
conditions of the air outlet that seldom result in equal
airflow across all sides of the device.
Air-distribution equipment manufacturers have long
published “area factors” for balancing purposes, that is,
until recent years. These values, however, are no longer
being presented for a number of reasons. So, when a
request is received, the first question asked is “What are
you going to do with the value?” The second question is
“What device are you intending to use?”
The Ak value is reported as a constant, reflecting the
ratio of sensed discharge velocity to the measured air
quantity. It is not based on any physical dimensional
relationships. Rather, it is dependent on the type of
device used to measure the discharge velocity along with
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ASHRAE JouRnAl

ashrae.org

Septem ber 2013

the location and orientation of the device relative to
airflows. For the Ak to be accurately applied, the sensor
must be held in a proscribed manner and the data averaged in a specified array of measuring points.
The device used has to be clearly specified, as
the geometry of the sensor greatly affects the value
observed. In other words, two different devices may
yield differing results when applied in an identical
manner. Early area factors specified the use of an Alnor
2000 Series device with a 2220A tip on the sensor; later,
it changed to the newer Alnor 6000 Series device with a
6070 tip. The 2000 Series used a single tube to transmit
a pressure reading to a mechanical vane that was then
connected to a gauge.
The 6000 Series units used two tubes and were typically more accurate and repeatable. The 6000 Series
also had the ability to measure velocities in a duct using
a different type of probe tip. Both were available with
extensions that allowed the measurement of ceiling diffusers while standing under the outlet/inlet without a
ladder. In the past, these devices were a single sourced
“industry standard” for balancing systems, but times
have changed. The 2000 Series device hasn’t been sold
for many years and the 6000 Series has seen limited
sales in recent years. In fact, neither of these devices
is commonly used at present, having been replaced

Dan Int-Hout is a chief engineer at Krueger in Richardson, Texas. He is a member of
SSPC 55, SPC 129 and consultant to SSPC 62.1.

ASHRAE Journal - September 2013

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