Proposing the Use of MIN-FLOW-SCH for altering MIN-FLOW/AREA...especially for Labs and Healthcare

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Fellow modelers,

I'm wondering what the general consensus might be for proposing that the Min Flow Schedule (MIN-FLOW-SCH) on the airflow tab be utilized as a modifier to the Min Flow Ratio or Minimum CFM/sf for the zone (MIN-FLOW/AREA) rather than a direct modifier of peak design CFM.

We experience a bit of a glitch when dealing with this, most often with laboratory and healthcare type projects with minimum ACH requirements and 100% OA (where load drives up the design flow, but not minimum flow). These requirements sometime vary throughout the course of a day (occupied/unoccupied hours). When the CFM/sf is constant, it's as simple as putting in a CFM/sf Min Flow value (usually the same for most zones) that equates to the minimum ACH value. However, if there is an occupied/unoccupied ACH, things become more difficult. For the zones being modeled in this manner, it would be great if the Min Flow Schedule (MIN-FLOW-SCH) on the airflow tab was utilized as a modifier to the Min Flow Ratio or Minimum CFM/sf for the zone (MIN-FLOW/AREA) rather than a direct modifier of peak design CFM. This would allow one schedule to adjust airflow rather than creating multiples to accommodate different peak design flow values for each zone.

For example, we might need to simulate a minimum of 6 ACH when occupied and 4 when unoccupied for a particular zone. If a value of 1 cfm/sf for minimum flow/area was input to achieve 6 ACH (10' ceiling), and no schedule was used, the minimum flow would be 1 cfm/sf regardless of Peak Design Flow. However, if we use the same 1 cfm/sf in conjunction with a Min-Flow Schedule (with a fraction of 1 for occupied and 0.667 for unoccupied hours), the Min-Flow schedule modifies the Peak Design Flow value to determine the new hourly min flow ratio, rather than multiplying 1 cfm/sf by 1 or 0.667 to calculate the new hourly min flow ratio.

Some might suggest that the new features added to the OA tab should help us deal with this. However, though great in their own ways (thanks eQuest team), there are also limitations with these features when dealing with 100% OA when a space is ventilation driven rather than load driven (OA fraction doesn't necessarily stay at 100%).

Considering both of these items, we'd like to propose that the Min Flow Schedule-to-Peak Design Flow relationship be changed to a Min Flow Schedule-to-Min Flow Ratio relationship. Perhaps the coding makes such a rabbit hole too deep, but if not, the applicability of this to labs and healthcare projects, as well as some scenarios with DOAS systems (now surging in popularity) might make it worth considering.

All of this said, if others out there feel that we've missed something in our assessment of the functionalities, please help make this a conversation. Thanks.


Paul Erickson LEED(r) AP
SR. Sustainable Design COnsultant

5802 Research Park Blvd. | Madison, WI 53719

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