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How are outdoor air summary report numbers getting calculated?

asked 2015-10-23 13:26:52 -0500

Rohini's avatar

Under HVAC sizing summary, there is minimum outdoor air flow rate (cfm) for each zone. I can see how this is getting calculated. Under outdoor air summary report for entire facility, there is a table for average outdoor air during occupied hours and there is a table for minimum outdoor air during occupied hours. If I calculate the cfm from the mechanical ventilation (ACH) and volume listed in the table, the cfm values are not same as that of the minimum outdoor air flow under the HVAC sizing summary. How are the outdoor air summary report numbers getting calculated? As always, thanks for any help.

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I didn't find the minimum outdoor air flow rate in the HVAC sizing summary. Do you mean the calculated design air flow?

Yixing Chen's avatar Yixing Chen  ( 2015-10-23 16:22:03 -0500 )edit

It is under zone sensible heating and zone sensible cooling

Rohini's avatar Rohini  ( 2015-10-26 08:58:00 -0500 )edit

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answered 2016-01-08 13:01:38 -0500

If it is a small difference, then it's due to the use of different densities. All inputs of air volume flow rate are converted to mass flow using the standard density at the local elevation. The ACH values in the summary report use the current density of the zone air. You can check this with output variable "Zone Mechanical Ventilation Standard Density Volume Flow Rate" which should match the input OA flow rates. Of course, if this is a VAV system serving multiple zones, the distribution of outdoor air between zones may not match the input specification. If the sum of the zone OA flow rates at standard density does not match the inputs, then there may be a problem with occupancy schedule alignment (even one person in a zone is considered "occupied" for the summary report) or with the OA controls.

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Note there is a related suggestion to add some improvements related to OA reporting.

MJWitte's avatar MJWitte  ( 2016-01-08 13:04:38 -0500 )edit

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Asked: 2015-10-23 13:26:52 -0500

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Last updated: Jan 08 '16