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

[Nathan Miller]

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

Mechanical Engineer/Senior Energy Analyst 

RUSHING | D 206-788-4577 | O 206-285-7100

Our new web site:  <http://www.rushingco.com/> www.rushingco.com

 

From: bldg-sim-bounces at lists.onebuilding.org
[mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Brian Tysoe
Sent: Thursday, April 11, 2013 3:09 PM
To: 'bldg-sim at lists.onebuilding.org'; bldg-rate at lists.onebuilding.org
Subject: [Bldg-sim] VAV Minimum Flow Setpoint and Supply Air Temperature
Reset in ASHRAE 90.1-2007

 

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:



 

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:



 

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 

 

 <http://www.mcw.com> 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) 598-5394 

 

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