Hey All,
I was incorrect yesterday, and Nick Caton called me on it offline.
I have always assumed that when zonal inputs are entered for outside air, that the hourly total system outside airflow would not drop below the cfm indicated on the SV-A reports (SUPPLY CFM * OA ratio) unless specifically controlled to do so such as by specifying a MIN-AIR-SCH or using DCV for the MIN-OA-METHOD.
Nick tried to verify this yesterday following the posts below, and discovered THIS IS NOT THE CASE.
It turns out that eQUEST gives priority to MIN-FLOW-RATIO values entered for the systems and/or the zones. This can result in simulations with hourly outside air flowrates below their intended values. Nick verified that zonal EXHAUST-FLOW inputs do take priority over zone MIN-FLOW-RATIO. I have created an input file (attached) to meet the 90.1-2013 VAV minimum flow requirement of G3.1.3.13. Minimum hourly zone flowrates are the greater of 30% of peak flow or the outdoor airflow rate.
The expressions in the input file do the following:
- create EXHAUST-FLOWs equal to zone outside air inputs
- set AIRFLOW-TRACKING to TRACK-EXHAUST so that hourly zone flow is the greater of exhaust (OA), MIN-FLOW-RATIO, or that required for space conditioning
- set EXHAUST-FAN-SCH equal to the system FAN-SCHEDULE
- set EXHAUST-STATIC to zero to avoid adding fan energy
- set zone MIN-FLOW-RATIO to 0.30 per G3.1.3.13 for Systems 5 and 7
I have tried the input file successfully in eQUEST 3.64 models and in 3.65 running both DOE-2.2 and DOE-2.3. Use File -> Import File? to give it a try!
You still need to enter a SYSTEM:MIN-AIR-SCH to model zero outside airflow during unoccupied fan cycling. Best to verify the hourly OA is correct by creating custom hourly reports for system(s) and tracking supply cfm and OA%.
There are many simulation changes and options for system sizing in eQUEST 3.65 running DOE-2.3 that I am just starting to explore, so the file may need edits.
Feedback welcomed!
Thanks,
~Bill
William Bishop, PE, BEMP, BEAP, CEM, LEED AP | Pathfinder Engineers & Architects LLP
Senior Energy Engineer
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