ASHRAE Journal - June 2021 - 42

ENGINEER'S NOTEBOOK
COLUMN
Designing HVAC For
Humid vs. Arid Climates
BY ERIC DESPLINTER, P.E., ASSOCIATE MEMBER ASHRAE; JEFF BOLDT, P.E., HBDP, FELLOW ASHRAE
Most mechanical engineers design primarily in one climate zone. They know the
intricacies of that climate but can overlook important differences when designing in
a climate with which they are not familiar. Whether your project climate is humid or
arid, hot or cold impacts how you design an HVAC system.
Eric started his career in St. Louis, Mo., in ASHRAE
Climate Zone 4A (Mixed-Humid) with summer 0.4%
design temperatures of 96.2°F dry bulb/76.7°F wet
bulb (35.7°C DB/24.8°C WB) and a winter 99.6% design
temperature of 7.5°F (-13.6°C) DB. As the company
expanded across the nation, he transferred to our
Ontario, Calif., office. Ontario is a starkly different
climate, as it is in ASHRAE Climate Zone 3B (Dry) with
summer 0.4% design temperatures of 100.2°F DB/69.7°F
WB (37.9°C DB/20.9°C WB) and a winter 99.6% design
temperature of 38.4°F (3.6°C). How does designing an
HVAC system differ in these two climates?
When designing in a humid climate such as St. Louis,
it is critical to consistently dehumidify to keep occupiable
spaces from becoming too humid and minimize the
risk of mold. This is commonly done with a modulating
chilled water valve, but using a modulating chilled
water valve can be more difficult when designing a
direct expansion (DX) system. In DX systems, using a
modulating compressor is a good way to provide capacity
control and ensure your cooling coil stays active so
your system dehumidifies.
Cycling compressors off lets water on the cooling coil
evaporate into the airstream and increases humidity.
If variable-speed compressors are not an option,
42
ASHRAE JOURNAL ashrae.o rg
J U N E 2021
Eric DeSplinter, P.E., was an operations manager, and Jeff Boldt, P.E., is a managing
principal at IMEG Corp. Boldt is SSPC 90.1 mechanical subcommittee chair.
providing a multistage compressor is better than a
single-stage compressor. The on/off limited controllability
of nonmodulating DX systems can result in the
system satisfying space temperature and the compressors
turning off before enough moisture is removed
from the air. Therefore, with DX systems, it is very
important to not add a large safety factor to your load
calculation, which would oversize your system and
satisfy space temperature faster, turn off the compressor
and introduce air to spaces that have not been
dehumidified.
When a space is in a low-load condition, a singlestage
DX coil cycles off after quickly satisfying the space
demand. Once the compressor turns off, the ability
to dehumidify is lost. Water also reevaporates off the
cooling coil before it reaches the drain pan, which
then requires more dehumidification. And you cannot
just command the compressor to stay on, because
the coil will frost without enough load on the system.
Fortunately, you can use several methods beyond modulating
or multistage compressors that provide improved
dehumidification capabilities.
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ASHRAE Journal - June 2021

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