ASHRAE Journal - ACMA Supplement - Fall 2013 - 16

By Wade W. Smith, P.e.
ExEcutivE DirEctor,
AMcA intErnAtionAl

The New
“new most efficient thing”
in Commercial HVAC Systems

he year i joined this industry (1973),
america suffered under an arab oil

embargo that led to gas lines and a

shared belief that we needed to save energy.
I owned a 4-year-old Toyota Corolla at the time that doubled in
value. In commercial buildings, focus turned to an innovative
alternative to terminal reheat: the variable air volume (VAV)
system. I was a young engineer in the Trane commercial group
at the time. We scrambled to make a licensing agreement with
Tempmaster to fill the gap until we could develop our own
product line. In a single year, VAV went from an obscure newwave system on the fringes of the industry to the mainstream
high-efficiency system of choice—and it has maintained that
position ever since.
But like a pair of comfortable old shoes, traditional VAV has
become boring to those who want to push the envelope of higher
efficiency in their design decisions. “New” approaches capture
our imagination, using geothermal, variable refrigerant volume,
and an old idea (low pressure induction) in a new package called
the chilled beam. With all the hype surrounding these new system
alternatives, many have overlooked the exciting efficiency results
that are available from VAV, when it is enhanced with highefficiency design choices.
Of course, it is popular to crow about how much more efficient
new system alternatives are when compared to a straw man
version of VAV left over from the 1970s. It’s not a fair comparison, their best against VAV’s most basic. These comparisons are
neither objective nor realistic, as Jeff Stein and Steve Taylor
pointed out in their May 2013 ASHRAE Journal article (see
http://bit.ly/13xmXjf). The truth is that high-performance VAV
systems cost less initially, and consume less energy than more
exotic alternatives that are gaining popularity as high-efficiency
systems. Air systems have evolved in North America because
our construction practices provide space for ductwork. Liquidbased systems (either water or refrigerant) have evolved in
Europe and Asia because their construction practices often use
16

Fa l l 2 0 1 3

solid walls and ceilings with no ceiling plenums. So, more
expensive “wet” systems evolved overseas to their current best
practice, which we now see in America. Here, the VAV system
has also evolved to an advanced state, which turns out to be
more efficient—and lower cost when applied in North American
buildings that conform to local norms of construction.
Free cooling

There is no lower energy use than “0.” When outside air is sufficiently cool to satisfy interior cooling loads, the compressor
remains off in high-performance air systems, and fans alone can
deliver cooling capacity. Yes, of course, wet systems can take
advantage of ambient relief to deliver wet economizer capability.
But water side economizers consume more energy (three pumping systems and a cooling tower fan must operate) and they are
not as effective. They cannot function above nominally 60 F
ambient, because cooling tower and wet heat exchanger approach
temperatures must be overcome before the system can deliver
any cooling effect to the occupied space. In the case of variable
refrigerant flow systems, compressors must operate to cool the
space. Air systems with 100%+ outside air capability, low leak
dampers, and variable speed fan capacity control cannot be beat
when outside ambient enthalpy is below indoor enthalpy
levels.
Some might argue that fans consume more transport energy than
pumps, to deliver the same cooling (or heating) effect. Of course,
this is true. But fans must operate in all systems to satisfy ventilation and space heating/cooling requirements. So, wet systems
operate system level pumps and local fans, plus ventilation fans.
Refrigerant systems operate compressors, local fans, and ventilation fans. These new-wave systems use less transport energy,
but they are simply not more efficient, largely because they
cannot deliver the same capacity of recovered “free” cooling
effect that is available from 100% outside air economizers with
low leak dampers at relatively higher ambient enthalpy levels.
Today’s economizer controls are significantly more reliable and
sophisticated than those of the 1970s and 1980s. Modern economizer
controls measure airflow rates, and compare return air enthalpy to
outside air enthalpy, using free cooling even when mechanical
cooling is needed to supplement capacity. Additionally, modern
economizers vary airflows to meet loads. Separate zoning of interior

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inmotion

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ASHRAE Journal - ACMA Supplement - Fall 2013

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