ASHRAE Journal - August 2014 - 46

Column EnginEEr's notEbook
Daniel H. Nall

Waterside Economizers & 90.1
BY DANIEl H. NAll, P.E., BEMP, HBDP, FAIA, lIFE MEMBEr ASHrAE

Most engineers probably feel that they are safely code compliant with their waterside economizers if their system is configured per Steve Taylor's June column:1 as an
integrated economizer with the heat exchanger in series with, and upstream of, the
chillers, if their cooling tower is intelligently selected for their peak load condition,
and if they provide the appropriate controls to initiate economizer mode during the
appropriate exterior conditions. But, they would likely be wrong. It's not only about
the cooling tower.
A number of factors determine whether ASHRAE/IES
Standard 90.1-2013's requirements can be met,* some
of which are completely out of the engineer's control.
The fraction of room sensible and total design load
experienced by the airside and the waterside systems of
the building during those conditions is one of the most
important variables that determines whether or not the
waterside economizer can meet the entire cooling load
at the 50°F dry bulb/45°F wet bulb (10°C/7.2°C) condition. Even in a new, fully code compliant building, these
fractions can vary dramatically.

Supply Air Temperature reset
Re-resetting the supply air temperature upwards
during the 50°F/45°F (10°C/7.2°C) condition is almost a
prerequisite for achieving waterside economizer code
compliance for a variable air volume system. With an air
temperature setpoint off the coil of 53°F (11.7°C), necessary to control space humidity for design weather conditions in many locations, there is an inadequate system
approach temperature, given that the cooling tower,
heat exchanger and cooling coil each has approach
limitations.
Resetting the supply air temperature requires that it
be able to meet the sensible load in all zones, including
interior zones not affected by the exterior temperature.
* Standard 90.1-2013 states, "Water economizer systems shall be capable of cooling supply air by
indirect evaporation and providing up to 100% of the expected system cooling load at outdoor air
temperatures of 50°F dry bulb/45°F wet bulb and below."
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Therefore, terminals in all interior zones must be
upsized so loads can be met with the higher supply air
temperature, as required by Standard 90.1-2013: "Zones
that are expected to experience relatively constant loads,
such as electronic equipment rooms, shall be designed
for the fully reset supply temperature."2
Of course, systems and components are usually
sized with a safety factor, but it only takes one fully
loaded interior zone to prevent successful supply air
temperature reset. Upsizing terminals must be done
carefully because the minimum supply air fractions
on today's pressure independent terminals may cause
overcooling or reheat activation in underloaded interior zones.
Having dealt with the troublesome interior zones,
the engineer seeking compliance for his/her waterside
economizer has to deal with another factor, the coincident internal sensible load fraction for the entire
building. While internal loads do not vary with external
temperature, and solar loads will vary with the changing
relationship of the building geometry to seasonal solar
geometry, conduction loads across the building skin will
vary with the temperature difference between inside
and outside.
In Standard 90.1-2013, however, envelope performance requirements change significantly from
Climate Zone 3 to Climate Zone 4, markedly changing
Daniel H. Nall, P.E., FAIA, is vice president at Syska Hennessy Group, New York.
ASHRAE Journal - August 2014

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