[Bldg-rate] VAV Minimum Flow Setpoint and Supply Air Temperature Reset in ASHRAE 90.1-2007

Brian Tysoe BTysoe at mcw.com
Fri Apr 12 06:43:40 PDT 2013


I am doing an ASHRAE 90.1-2007 PRM analysis for a LEED 2009 CS project, credit EAc1. The baseline system is System 7 - VAV with Reheat.

Appendix G has the following requirements for System 7:
[cid:image001.jpg at 01CE3762.36BA8840]

In keeping with the above requirements, when the temperature is satisfied in any zone, I am supplying 0.4 cfm/ft2 at 59F (54F + 5F reset). This is causing many zones to be overcooled and, therefore, reheated. The result is that I have a significant amount of simultaneous heating and cooling in my baseline model. Section states that reheating, recooling and simultaneous heating/cooling is not allowed unless the air volume is no greater than 0.4 cfm/ft2 (or the larger of 0.4 cfm/ft2, the ventilation requirement, 30% of max flow, etc.). Therefore, my baseline is allowed to overcool and reheat since it is running at 0.4 cfm/ft2.

My proposed model does not have this problem as it is allowed to go below 0.4 cfm/ft2 and it is allowed to reset by more than 5F. I am achieving unreasonably high savings as a result of this issue.

What I find interesting is that G3.1.3.13 has been revised for ASHRAE 90.1-2010 to the following:
[cid:image002.jpg at 01CE3762.36BA8840]

The minimum is no longer an absolute 0.4 cfm/ft2 but a relative 30% of zone peak air flow. In mild or heating dominated climates 30% of zone peak air flow can be much less than 0.4 cfm/ft2.

By changing the requirement for 90.1-2010 is this an acknowledgement by ASHRAE that the previous requirement could lead to an inefficient baseline model? Am I justified in exploiting it to achieve high savings?

Thank you!

Brian Tysoe M.A.Sc., P.Eng., LEED AP
Associate, National Manager of Energy Modelling Services

MCW Consultants Ltd. <http://www.mcw.com>
Queen's Quay Terminal
207 Queen's Quay West, Suite 615
Toronto, ON, Canada M5J 1A7
Phone: (416) 598-2920 ext:519
Fax: (416) 598-5394

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