Miles Ryan, P.E., writes a monthly column in Engineered Systems Magazine on Building Commissioning. Read June’s column below:

Introduction

There is a very nuanced and misunderstood situation which plagues a lot of AHUs. It occurs in AHUs which utilize airflow tracking for control of its return fan and either intentionally, or unintentionally, operate in a fully recirculating configuration. This is the second of several articles in a series written with the intent to explain the issue. The series provides several case studies for how I have seen the situation manifested and how a unique solution in each case was needed to rectify the issue.

Death Spiral Recap

The AHU recirculating mode death spiral occurs when the outdoor and relief air dampers are fully closed, the return damper fully open, and the return fan is attempting to control to a return airflow setpoint. That setpoint is reset to maintain an offset from measured supply airflow.

There are often scenarios where an AHU is intentionally put in such a configuration, and in such situations, it may seem appropriate to use a 0 CFM offset between real-time measured supply airflow and the return airflow setpoint. However, that approach is unsustainable and will result in one fan operating at full speed with the other operating at minimum speed. Refer to last month’s article for a demonstration as to why that is, as well as Case Study 1 which describes how this situation has played out.

There are also instances when AHUs unintentionally find themselves in a damper configuration that allows the AHU recirculating mode death spiral to manifest itself. Such a case study is the focus of this article.

Case Study 2

The AHU in question serves an existing hospital. Its sequence of operation followed a conventional sequence for its dampers in which the outdoor air and relief air dampers track each other, and the return air damper is modulated inversely of them. A minimum position for the outdoor air damper is set by a balancer to ensure minimum requirements for outdoor air is always met, even if the economizer is disabled or bottomed out. For the record, I am not a fan of this strategy for numerous reasons, but as a commissioning provider we often don’t have the influence we wish we had to get a more appropriate sequence specified. The AHU served multiple VAV boxes, its supply fan controlled to maintain supply air static pressure (SA-P) at setpoint (SA-P SP) and its return fan controlled to maintain return airflow (RA-F) at setpoint (RA-F EFF SP), which was reset to be 500 CFM below measured supply airflow (SA-F)

Several summers ago, the facility staff reached out to our firm to ask why the supply fan could not meet static pressure setpoint. We found the AHU in the configuration shown in Figure 1. The return fan was operating at minimum speed, as its airflow measuring station measured 1,556 CFM above the measured supply airflow (2,056 CFM above its active return airflow setpoint). It was summer and the economizer was disabled. The outdoor air damper goes to its minimum position in such instances. A minimum outdoor air damper position of 10% was being utilized. That value in and of itself seemed suspect, but the fact the setpoint was shown to be overridden (evident by the purple highlight in Figure 1) further heightened our suspicion. We released the override and found a default of 50% had been entered by the TAB contractor during some recent work on the unit. Engaging the TAB contractor, we confirmed 50% was indeed what they determined to be needed to ensure minimum ventilation requirements were met.

Figure 1. Initial operating conditions.

We released the override on the minimum outdoor air damper position and the AHU settled out as shown in Figure 2. The supply air static pressure was achieved, downstream VAV boxes’ airflow requests were satisfied, and the return fan started to control to its desired setpoint. We recommended a cleaning/recalibration of both supply and return airflow measuring stations to get them reading more accurately going forward.

Figure 2. Operating conditions after appropriate OAD MIN SP was restored.

What we had witnessed was the AHU being unintentionally set into a fully recirculating mode of operation. The AHU dampers were opposed blade, and even with a 10% open command to the outdoor and relief air dampers, they were basically still shut. This forced the supply fan to pull all its airflow from the return path. The return airflow measuring station read higher than the supply air’s, therefore the return fan slowed down until it eventually settled at minimum speed. With the return fan pulling little of the workload, the supply fan was forced to ramp up. However, the supply fan was not sized to overcome the pressure drops in both the return and supply ducts. Therefore, it could not meet its static pressure setpoint.

Case Study 2 – Version 2.0: (It happened again!)

Fast forward a few summers later, renovations to spaces served by this AHU were performed. Some of those spaces needed to be positively pressurized to their adjacent spaces. At the end of the project, those positive space pressure requirements were not being achieved, as those spaces in question were not getting the supply airflow they were requesting. We again found the AHU in the configuration shown in Figure 1, with a 10% minimum outdoor air damper setpoint being utilized. Facility staff informed us they were forced to implement such a setpoint override because the unit was unable to properly cool/dehumidify the air when the TAB-determined 50% minimum outdoor air damper position was utilized.

The TAB contractor was onsite and went to the unit. I remoted into the BAS and we worked together over the phone. I released the minimum outdoor air damper setpoint back to 50%, and the unit settled out in the configuration shown in Figure 3. Supply air static setpoint was again achieved, since the return fan ramped up and started to pull its weight in the process. This resulted in more airflow being sent to the VAV boxes, and space pressure requirements in the newly renovated spaced were then achieved.

Figure 3. Operating conditions after appropriate OAD MIN SP was restored for the second time. The AHU climbed out of the recirculating mode death spiral, but the cooling coil could not get the supply air temperature down to setpoint.

The TAB provider explained that the condensate drain below the cooling coil started “gushing” out water to the floor drain the moment I made the manipulation to the AHU. For a condensate drain, located in a negatively pressurized cabinet of the AHU to be “gushing” out water, there must have been a substantially high column of standing water within the cooling coil cabinet. A summer’s worth of condensate I would presume, since trends found the damper setpoint override had been in place for the past few months. Such a condensate drain would never work when the AHU was in the configuration shown in Figure 1, because an extreme negative pressure would be present at that section of the AHU in such an instance.

We then needed to address why the facility staff had overridden that minimum outdoor air damper setpoint in the first place. They had stated the cooling coil was underperforming. Review of the BAS (Figure 3) confirmed that to be the case. The supply air temperature setpoint could not be achieved. As it turned out, the cooling coil was getting 50°F entering water, when it should have been getting 44°F. This cascaded into us troubleshooting issues with the chilled water distribution system (maybe a story for another day). In the end when the AHU got 44°F entering water and the AHU was able to provide both the temperature and airflow the served spaces needed.

Conclusion

This second case study demonstrated the AHU recirculating mode death spiral can manifest itself even when the AHU was not intended to be in a fully recirculating configuration. An inappropriate override to the minimum outdoor air damper setpoint created the situation. That override was the result of an improper diagnosis of poor cooling coil performance. We will continue next month with another case study for a different AHU, with a completely different sequence of operation, that too fell victim to the AHU recirculating mode death spiral!