David Sellers

Damper tests provide valuable insights, Part 5; Test Background
June 23, 2007

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 discovered that:

The change over was based on outdoor air temperature.

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 failed. 

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 reasonably accurate. 

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 design flow).

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.

Posted by David Sellers on June 23, 2007 | Comments (0)