1
U.S.A • CANADA • U.K.
ENGINEERING BULLETIN
"Complete Air Control and Distribution Solutions"
DATE:
April 2018
PRODUCT:
Terminal Units – Fan Powered Terminal Units
SUBJECT:
Efficiency of Fan Powered Air Terminal Units – PART I
ISSUED BY:
Gus Faris
Fan powered air terminal units (ATUs) came into being here in Houston, Texas. John McCabe with the
EMDE Company and Charlie Chenault with Chenault Engineering Company designed the first ones in
1973. Their idea was to make office buildings more efficient, an admirable cause while we were
suffering from an oil embargo from the Middle East. Mr. Chenault put their sketches on his plans. Dick
Graves bought the parts and assembled the first ones in the field. By the third job, the local Honeywell
rep was supplying the dampers and actuators, and Magna-Flow Industries was supplying the fans and
heaters. Magna-Flow was supplying complete units with the dampers and fans integrated into one
casing shortly after this. Initially, these terminal units were parallel arrangements. All controls were
pneumatic and pressure dependent. Duct systems were designed for static regain or constant velocity
with a large number of reducers in the trunk ducts. Series units came along in the late 1970s. The new
devices were very popular and were soon being manufactured by several companies.
In the 1980s, pressure independent pneumatic controls became available. This made duct designs
simpler with fewer reducers. Variable volume air handlers were also being used. Dick Graves presented
an ASHRAE paper in 1989 claiming that this combination of fan powered terminal units and VAV air
handlers was saving about 20% of the building energy at that time. Of course, this was not solely due
to the terminal units, but they were definitely instrumental to the system. The fan powered options
were energy savers for buildings from the very start.
The parallel ATUs and series ATUs have different operating characteristics. The parallel units run the
fan only in the deadband and heating modes. During the cooling mode, they depend on the air handler
to drive the cool air down the duct, through the VAV damper in the ATU, through the ATU and down
the runout ducts to the occupied space. The fan cycling can be irritating. The series unit places the VAV
damper and fan in series, separated by a mixing chamber that is open to the return air plenum or ducted
to the occupied space. The fan runs in all modes during all occupied hours. The sound from the fan is
background noise and generally not noticed. The air handler only drives the cool air through the VAV
damper. Air handler energy requirements is lower for the series unit.
Since the 1980s, a lot has changed. Glass has gotten much better at limiting radiant heat into the
occupied space. Thermally broken mullions have been designed limiting much convection heat into the
occupied space. It is not unusual today to find new construction with u factors of 0.03 or lower. The
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U.S.A • CANADA • U.K.
ENGINEERING BULLETIN
"Complete Air Control and Distribution Solutions"
building enclosures are tighter today than in the past. When this tightening first started, we went
through the sick building syndrome until we learned to better control the amount of outdoor air needed
and supplied into the occupied space. Controls are better with much more capability than in the past.
Motor technology is dramatically better. ECMs use significantly less energy and are capable of
regulating airflow with digital controls that reset airflow based on the instantaneous demand in the
occupied space. Air handlers have changed to smaller units with direct drive fans or banks of fans which
can also be controlled based on instantaneous demand in the occupied space and static pressure
control. Duct leakage limits have been decreased. Ceiling diffusers have improved with higher induction
rates and better throw patterns at low flows. ATUs have also improved with damper assemblies, flow
measurement devices and modulating fan motors allowing the entire ATU to regulate the air volume
to the occupied spaces based solely on the actual instantaneous demand in the space.
Engineers generally gravitated toward one unit or the other. It was natural as we began to talk about
energy use due to rising oil prices again, that there would be differences of opinions about whether a
building would be more efficient with one type over the other. The first ASHRAE research started in
2003. It was followed by two more research projects, all of which have generated over 25 ASHRAE
research papers describing the process, findings and conclusions.
I was lucky enough to be chosen to chair all three research projects investigating fan powered terminal
units: ASHRAE RP 1292, which compared series and parallel fan powered terminal units
(ATUs); The VAV CONSORTIUM AT TEXAS A&M UNIVERSITY, which compared AC induction motors
(PSC) VS. Electronically Commutated Motors (ECMs); and AHRI
RESEARCH PROJECT 8012, which was to rewrite and verify all the equations from the previous 2 projects
into heat and mass balance equations to make them fit easier into modeling programs like Energy Plus.
I was also chosen as co-chair of the committee to write the new ASHRAE DESIGN GUIDE for Air Terminal
Units where much of this research is described and used to evaluate these systems. We will look at
more current issues in Part II
