In the last
post, I presented the results of the economizer damper test I
discussing. The findings related to the test results fall into four
The leakage through the outdoor air damper when it is fully
closed meets or exceeds the system’s
worst-case minimum outdoor air flow requirement.
There is significant leakage through the return air dampers
when they are fully closed and the system is on a 100% outdoor air
The damper performance is linear in the mid-flow range,
despite the predictions I made based on the published damper
The initial damper motion from 0% outdoor air towards 100%
outdoor air has little impact on the outdoor air flow.
In subsequent posts, I will focus on each of these findings and
discuss some of their implications in terms of performance and
energy consumption. But first, I think it’s important to
recognize that for these findings in particular and for many HVAC
system issues in general, the implications of the issues are very
much a function of the local climate and the application served. To
gain some insight into this, consider the climate snap-shots
illustrated in the graph below.
The graph was created by plotting ambient temperature versus hours
of occurrence for different locations using
bin weather data obtained from NOAA . It’s easy to loose
sight of the fact that the climate and its implications on HVAC
processes varies from the location where you are currently working.
I remember walking out of the construction trailer one July day
right after I had moved to Portland, thinking how beautiful it was;
crystal clear blue skies, temperatures in the low to mid 80’s
in the late afternoon, a light breeze, and low humidity. In hot and
humid St. Louis, my former home, I would have been opening up the
house and enjoying the cool, pleasant weather. Just as I was
thinking this, the two women who kept things flowing smoothly back
in the construction office trudged by and both commented about how
unbearably hot and humid it was!
Now that I’ve lived in Portland for a while, I would
probably agree with them compared to the more typical summer
day. The point is that it’s easy to become acclimated
to the environment where we live, which may or may not be where the
project we are working on is located. Plots like the one presented
in the picture above are one way to gain some perspective on how a
local climate might interact with the HVAC systems and processes
you are working on. Consider the following with regard to the
Note the juxtaposition of climate data with the line that
represents 32°F. Water will freeze at or below this
temperature, including the water in our HVAC system coils. One of
the tricks associated with operating an economizer equipped system
is to maintain control of the mixed air temperature in a manner
that prevents the introduction of subfreezing air into the system.
Economizers operating in Key West, Los Angeles, and Yuma seldom if
ever have to deal with this issue. Economizers operating in
Portland, Oregon have to deal with it, but only occasionally.
Economizers operating in St. Louis and International Falls may
spend a significant amount of their time dealing with
subfreezing air and the issues it can create. In
International Falls, even a system with modest minimum outdoor air
requirements may require preheat to deal with the extreme winter
conditions that may be encountered.
Consider the different climates relative to the 75°F
line. While not a complete picture with out some indication of
humidity, this line does provide some insight for contrasting the
indoor environment we are trying to control with the outdoor
environment. In Key West, given the humidity, providing an
economizer on an air handling system may not be worth the time and
effort. In contrast, for environments like Portland, Oregon and Los
Angeles, economizers represent an opportunity to save significant
energy and the system serving them will operate at or near 100%
outdoor air most of the time given the mild climate. The energy
savings potential also exists in climates like St. Louis, Yuma,
Flagstaff, and International Falls, but economizers in these
locations will operate over a wide range of outdoor air flow rates
as the ambient temperatures vary.
Generally speaking, systems that must deal with a wide range
of ambient temperatures like those seen in International Falls will
need to be more robust than those that only deal with mild
temperatures concentrated in a narrow range like Los Angeles. A
system that operates with no perceived problems in LA may be a
nightmare to operate in International Falls. This implies that a
system in international falls will require more attention to the
details of design, construction, and operation for successful
results to be delivered.
It is not unusual for the same problem in two radically
different locations to lead to different conclusions and focuses
for design decisions and operating and commissioning efforts. In
the next post, I’ll look at how an economizer damper that is
stuck open results in diametrically opposite issues when it occurs
in two different climate zones.