AMCA InMotion - September 2019 - 28

in coordination with a VAV damper. Airﬂow rates for the

the primary (VAV) airﬂow, a series fan box with an ECM

fan and VAV damper are independent of each other.

can be turned down, at which point it will use signiﬁcantly

A series box induces plenum air, which mixes with
primary air from a VAV damper before passing through

less energy. The extent to which it can be turned down is
largely dependent on diffuser choice and spacing.

the fan. The fan must never deliver less air than the VAV
valve to prevent backﬂow into the ceiling plenum.

Summary

With a parallel box, the central-system fan delivers air

When designing a VAV system for a large building, it is

to a diffuser through the box. With a series box, the central

imperative to understand the role each component plays.

system needs only to get air to a VAV inlet, resulting in a

Air handlers are used to supply air to zones. They track

lower system fan static-pressure requirement. The series

demand and determine the gross amount of air to provide.

fan, however, must run at all times during occupancy, but

Terminal units are used to ﬁ ne-tune the amount of air

with constant airﬂow from the series terminal, diffuser

supplied to a zone and control noise. Diffusers are used to

turndown no longer is an issue. Which-series or parallel-

mix air within a space, ensuring comfort without drafts

then, is the better choice?

or stagnation. Each of these devices is equally important.

More often than not, the answer has more to do with
local practice than actual data. Many building codes require

All function together in a well-designed HPAS to provide
an acceptable environment for occupants.

summation of motor horsepower or other measures, while
available building-load programs do not accurately reﬂect

Note and References

the savings that result from new technologies. ASHRAE and

1) Assuming the ANSI/ASHRAE Standard 62.1, Ventilation for
Acceptable Indoor Air Quality, default occupancy of 150 sq ft
(13.9 m2) per person, an annual employee cost (salary, taxes,
beneﬁts, etc.) of $70,000 per person, and an annual energy cost of
$2 per square foot.

the Air-Conditioning, Heating, and Refrigeration Institute
(AHRI) established a joint research program to look at the
system energy use of series boxes vs. that of parallel boxes
and of permanent-split-capacitor (PSC) motors vs. that of
electronically commutated motors (ECM).

Validation of energy use. The purpose of the
ASHRAE and AHRI joint research program was to produce
data enabling accurate and validated energy-use calculations. Research Project 1292 was conducted at Texas A&M
University.
More than two dozen technical papers on the results
of the study have been presented. Additionally, ASHRAE
Journal published a three-part series of articles describing
the purpose of the research,6 summarizing the ﬁndings,7
and discussing shortcomings of EnergyPlus and other
energy models.8
Data show that, in most situations, a series box with an
ECM will use less energy than a parallel box with either an

2) Bade, J.E., Faris, G., Int-Hout, D., & Terzigni, M. (2018). Highperformance air systems for improved comfort, energy
efﬁciency, IAQ. AMCA inmotion, pp. 2-8. Available at http://bit.ly/
AMCAinmotion_2018
3) ASHRAE. (2017). ASHRAE handbook-fundamentals. Atlanta,
GA: ASHRAE.
4) Arens, E., et al. (2015). RP-1515-thermal and air quality
acceptability in buildings that reduce energy by reducing minimum
airﬂow from overhead diffusers. Atlanta, GA: ASHRAE.
5) Paliaga, G., Zhang, H., Hoyt, T., & Arens, E. (2019, April).
Eliminating overcooling discomfort while saving energy. ASHRAE
Journal, pp. 14-28.
6) Faris, G., Int-Hout, D., O'Neal, D., & Yin, P. (2017, October). Fanpowered VAV terminal units. ASHRAE Journal, pp. 18-24.
7) Faris, G., Int-Hout, D., O'Neal, D., & Yin, P. (2017, November). Fanpowered VAV terminal units, part 2. ASHRAE Journal, pp. 20-28.
8) Faris, G., Int-Hout, D., O'Neal, D., & Yin, P. (2017, December). Fanpowered VAV terminal units, part 3. ASHRAE Journal, pp. 42-45.

ECM or a PSC motor. Additionally, if fan airﬂow is reduced
at less than design load, rather than held constant, energy
savings with a series box can be signiﬁcant.

Minimum recommended airﬂow. When close to

20 19 A M CA i n m o t i o n

About the Author
Dan Int-Hout is chief engineer for Krueger.

w w w. a m c a .o r g

http://www.bit.ly/AMCAinmotion_2018 http://www.bit.ly/AMCAinmotion_2018 http://www.amca.org

AMCA InMotion - September 2019

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