[Bldg-sim] DOAS and baseline OA

Michael A. Eustice MAEustice at htlyons.com
Tue Aug 16 06:18:50 PDT 2011

The critical zone fraction as explained by ASHRAE 62.1 will, in most cases, increase outside air intake in a VAV system.  This is due to the need for the entire system to increase its outside air fraction if one zone's airflow dips to its minimum VAV airflow.  To maintain the ventilation fraction in the one zone that reduced airflow, the entire system ingests large amounts of outside air to make sure it gets enough air to that particular zone.

It's a bit more complicated than that, but that's the simplest explanation.  There are ways around it, but it takes time to 'correct' for small, high ventilation-fraction areas that tend to drive the system towards high ventilation rates when they modulate to minimum position.

Hope this makes sense.


From: bldg-sim-bounces at lists.onebuilding.org [mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Jim Dirkes
Sent: Tuesday, August 16, 2011 9:12 AM
To: 'Anne Juran'; bldg-sim at lists.onebuilding.org
Subject: Re: [Bldg-sim] DOAS and baseline OA

Dear Anne,
I'm not sure why you think the OA requirement varies by system.  I'm not well versed in Standard 62, but my basic understanding is that one of the calculation methods is the result of building area and number of occupants.  Those are unchanged by system selection ... which is why it makes sense for Appendix G to require matching volumes.

The Building Performance Team
James V. Dirkes II, P.E., BEMP , LEED AP
1631 Acacia Drive NW
Grand Rapids, MI 49504
616 450 8653

From: bldg-sim-bounces at lists.onebuilding.org [mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Anne Juran
Sent: Tuesday, August 16, 2011 8:33 AM
To: bldg-sim at lists.onebuilding.org
Subject: [Bldg-sim] DOAS and baseline OA

I'm curious how everyone is handling the OA for Appendix G simulations when using a DOAS in the proposed system.  We have a design that is DOAS + FCU with a  total OA of 8,000 CFM.  When you run the OA calcs for this same building with a VAV system (the baseline), the total OA required to meet Standard 62 is 39,000 CFM.  This difference in OA represents a significant energy savings (in climate zone 4A), yet Appendix G requires the OA volumes to match.  It does not seem "fair" to me that the proposed case cannot take credit for design choice when it comes to OA.  I feel like Appendix G should make an exception for DOAS.  Am I missing something?  Is there a way around this?  Any thoughts are appreciated!


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