The last post finished our discussion of the field observations
made by the Pacific Energy Center
Retrocommissioning Class (PEC RCx class) attendees that caused
them to develop a test to assess the performance of the economizer
dampers on the air handling unit serving the center. The test they
developed actually had its roots in the work done by one of the lab
groups from the previous year’s class. Here is a picture of the
first year’s RCx class.
As a part of a group
exercise, the class as a whole had targeted investigating the
economizer change over signal used to disable the economizer in hot
or humid weather when it no longer provided a benefit and
The change over was based on outdoor air
The sensor that everyone thought was providing outdoor air
temperature and humidity data to the control system was actually
not wired into the system at all.
The sensor that was actually providing outdoor air
temperature and humidity information to the system was
significantly out of calibration, having essentially
The information documenting the change over sequence in the
system manual and project documents conflicted with itself,
indicating in one place that the change over occurred based purely
on outdoor temperature while indicating in another place that the
change over occurred based on comparing outdoor air temperature to
return air temperature.
The outdoor air dampers were frozen in the near 100% outdoor
air position and the system could not revert to minimum outdoor air
even if it wanted to.
Here is a picture of the class checking out the
outdoor air conditions sensor.
As startling as these findings may seem, they are actually not that
uncommon out their in the field. In my experience, many existing
buildings, if subjected to a retrocommissioning process, would
yield similar results. All of these issues are fairly easy to
address and yield a big bang for the buck and are great examples of
how the low cost/no cost benefits that can be uncovered and
resolved by a retrocommissioning process.
By the time the class had progressed to the point where it was
divided up into smaller groups (each of which was to develop and
execute a functional test) all of the problems identified in the
initial group exercise had been resolved. As a result, one of the
groups decided they would investigate the actual minimum outdoor
air flow delivered by the system and compare that to what might be
required by the facility. Abdi Farhang and Brent Lords spearheaded
this effort. Here is a picture of them at work in the unit.
Their test placed the unit on minimum outdoor air and then
documented the supply, return and outdoor air flows produced. Their
measurements were compared to the supply flow as measured by the
control system and to the outdoor air requirements associated with
Title 24 (the
California Energy Code) and the occupancy of the building on
the day of the test. Their conclusions were as follows:
The supply flow indicated by the control system was
The current minimum outdoor air setting provided about the
right ventilation flow rate for a full building under the operating
mode that the system was in when they tested it (about 50% of
Because the building occupancy varied considerably, the
ventilation requirement varied considerably.
Abdi and Brent focused some attention on the last observation
because they felt it might represent a significant opportunity.
What they recognized was that on some days, the building was only
occupied by the PEC staff, representing a minimum ventilation
requirement. But on other days, all of the class rooms might be
completely occupied, which would set the maximum ventilation
requirement. Furthermore, there would typically be a significant
number of days with occupancy between these two extremes.
Come back in a couple of days and I’ll post the results of the
estimate that Abdi and Brent did to identify the minimum outdoor
air flows required by the two occupancy extremes as well as their
recommendations that led to the test the 2nd class performed.