VAV Minimum Flow Setpoint and Supply Air Temperature Reset in ASHRAE 90.1-2007

2 posts / 0 new
Last post

Hello,

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:image003.jpg at 01CE36DF.AC800100]

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 6.5.2.1 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:image004.jpg at 01CE36DF.AC800100]

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

Brian Tysoe M.A.Sc., P.Eng., LEED AP Associate, National Manager of Energy Modelling Services MCW Consultants Ltd. Queen’s Quay Terminal 207 Queen’s Quay West, Suite 615 Toronto, ON, Canada M5J 1A7 Phone: (416) 598-2920 ext:519 Fax: (416)
Brian Tysoe's picture
Offline
Joined: 2011-09-30
Reputation: 200