Troubleshooting Outside Air Airflow Measurement Stations

Hospital air handling units were periodically tripping freeze stats. Here's how the problem was solved.

By Scott Bilan, PE June 18, 2019

PBA VP S. Bilan shares lessons learned in health care engineering. A few years ago, a local hospital had post-occupancy challenges with a large project that PBA had designed.

Several indoor AHUs were periodically tripping freeze stats and there appeared to be no rhyme or reason as to when or why the stats would trip.

Discovering the Root Cause

The hospital spent several months “troubleshooting” the issue and modifying the sequence from PBA’s original design intent. However, none of the modifications would positively affect the outcome. Frustrated, and with no end in sight, the owner called PBA and suggested that an undersized heating system may be its root cause.

PBA spent several weeks on-site getting the sequence back to the original design intent. After the sequence was reverted, PBA began recommissioning the AHUs and heating system. Since tripping would occasionally occur at -10°F as well as +30°F, PBA believed the issue was not due to heating capacity. Upon further digging, several small issues were discovered and adjusted, which led the team to the airflow measurement stations – the root cause of the mysterious tripping freeze stats.

Déjà Vu

Recently, PBA received a similar call from a different client. Luckily, the sequence had not been significantly altered from PBA’s original design intent. The owner again suggested that the source of the issue was an undersized heating system. Because of the similar situation PBA had experienced with the previous health care client, PBA first suggested they check the outdoor airflow stations.

In order to resolve the issue as a team, PBA, the controls contractor, the balancer and the airflow measurement station manufacturer decided to meet on site. Due to the warmer weather, the team was able to put the units in full economizer (100% Outside Air). The balancer took a traverse inside of the unit and read about 51,000 CFM. The supply air (SA) fan airflow measurement stations were reading around 52,000 CFM. However, the outdoor air (OA) airflow measurement stations were reading around 24,000 CFM. Due to these low outdoor air readings, PBA made the determination that significantly more outdoor air was being brought in than intended. This resulted in the freeze stats tripping.

There was discussion of adjusting the unit’s K factor. However, PBA suggested that the OA airflow measurement stations may be dirty. The units in question were drawing “fresh” air from wall louvers that were six stories high. Once cleaned and reinstalled, the OA began tracking within 5% of the SA and measured values.

Maintenance | A Long-Term Solution

The owner was very grateful that PBA was able to lead them down the right path instead of unnecessarily modifying the existing, well-functioning heating system. They also now have a maintenance program in place for the sensors in order to properly maintain them and to avoid further freeze stats from tripping.

As with most of the systems PBA specifies, maintenance is Key. However, PBA strongly suggests to take heed if the AHU is critical and in a harsh environment. Consider other approaches or filtering the OA into the unit.

Lastly, it’s important to learn from your challenges; you never know how it may help you in the future and potentially strengthen your client relationship.

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This article originally appeared on Peter Basso Associates’ blog. Peter Basso Associates is a CFE Media content partner. 

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