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
My two cents.
1) Is the change an admission that the previous requirement could
lead to an inefficient baseline system? Yes.
2) Are you justified in "exploiting" it to achieve high energy
savings? Yes.
These are the rules of the game. Sometimes they benefit the baseline
system, sometimes they benefit the proposed system.
If that doesn't make you feel comfortable, you could always see if there
was a specific addendum to 90.1-2007 where that change was made and
"elect" to apply that addendum in its entirety to your model.
Nathan Miller - PE, LEEDRAP BD+C, CEM