Same Problem Different Issues

In the last
 we looked at how climate can vary across the country
(and the world for that matter) and how these climate variations
might impact the operation of an economizer equipped HVAC system.
In this post we will look at the impact of an economizer that is
stuck in the 100% outdoor air position on two identical systems,
one of which is located in San Deigo, California, and the other of
which is located in Key West, Florida.

The following plots paint a picture of the two climates similar
to the picture in the previous post. The Mean Coincident Wet Bulb
plot allows us to contrast the humidity of the two locations.

Plotting the data from these two illustrations on a psychometric
chart allows us to paint a picture that takes both
humidity and temperature into account, as illustrated below.

The green constant enthalpy line on the psych chart is the
demising line between outdoor conditions that make the outdoor air
suitable vs. not suitable for cooling in an integrated economizer
cycle, based on a space condition of 75°F/50% RH. Stated
another way, if the outdoor air conditions lie to the left of the
line, then, on a statistical basis, it will take less energy to
cool a supply flow that is made up of 100% outdoor air when
contrasted with cooling return air mixed with the minimum outdoor
air required for ventilation purposes. If conditions lie to the
right of the line, then the opposite is true and energy consumption
will be minimized by terminating the economizer process. This
analysis implies that, on a statistical basis, an economizer
process should be terminated in San Diego when ambient dry bulb
temperatures exceed 68°F (the point where the constant green
constant enthalpy line crosses the red statistical San Diego
climate data line). In Key West, economizer processes should be
terminated when the ambient temperature exceeds 67°F based on
this analysis.

Now, let’s return to the plots of dry bulb temperature vs.
hours of occurrence for the two climates and superimpose the hours
during which economizer operation is possible on them. The result
is depicted below.

As can be seen from the upper plot, there are a significant
number of hours when an integrated economizer cycle equipped air
handling system in San Diego would be able to use outdoor air as a
source of free cooling approximately 7,400 hours per year or 84% of
the time). Many of those hours (approximately 5,700 or 65% of the
annual hours) would be spent on 100% outdoor air for a system that
was operating round the clock and had a discharge temperature
requirement in the 55°F range.

In contrast, a similar system located in Key West would only be
able to benefit from an integrated economizer cycle for
approximately 1,500 hours per year or 16% of the time and virtually
all of those hours would be spent operating at or near 100% outdoor

As a result, a commissioning provider or building operator who
encountered a system with the economizer dampers stuck in a
position that delivered 100% outdoor air in San Diego would have
encountered an opportunity to reduce the load and perhaps improve
performance on a peak day by returning the economizer to service.
But, the energy savings would be modest since the system spends
most of the hours of the year operating at or near 100% outdoor

On the other hand, a commissioning provider encountering the
same problem in Key West would not only reduce the peak load and
improve performance by returning the economizer to service. They
would also generate significant energy savings because in Key West
the economizer only provides benefit for a fraction of the

The bottom line is that the same problem in identical systems
located in two very different climates yields a very different
energy savings benefit when corrected due to the nature of the
climate relative to the requirements of the HVAC process associated
with the system.

In the next post, we’ll take a look at the test
from the PEC AHU
economizer in the context of the San Francisco climate.

This entry was posted in Weather and Climate Interactions with Buildings and Systems. Bookmark the permalink.

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