[Equest-users] ASHRAE 90.1 - G3.1.2.8 Design Airflow Rates

[John T. Forester]

Hi Omar,

I believe what you want to do is set your airside system supply air temperature to be 11°C less than your space temperature setpoint for cooling.  For example, if your space cooling setpoint temperature is 22°C, your design supply air temperature setpoint is 11°F.  This is set on the cooling tab of the airside system.  This should be changed if you decide to change your room temperature setpoint.

You say that you’re not modeling for LEED, yet you are using App G.  If your baseline model requires you to follow App. G, you should not be manually entering your system airflow rates and should let eQuest size the system appropriately.  The calculated design airflow is going to depend on the envelope construction (walls and windows) in addition to internal loads.  Unless your building construction is identical in base and proposed buildings, your design airflows will be different.  Also your design airflows will vary when you rotate the building every 90 degrees.

If you are noticing a significant difference in system sizes between your two models something else may be impacting the calculations.  Check to be sure your design heating and cooling days are being specified properly.  Look at the LS-C report.  The firs section will show the Day, Hour, WB and DB temperatures of the design day.  These are the conditions, in addition to the internal loads and ventilation requirements, that eQuest uses to calculate your cooling/heating capacities and airflows.

Also, if you have central cooling in your proposed model, check your PS-C report to see if the equipment is oversized for the loads that eQuest is calculating.  If this is the case, your chillers, boilers, pumps will almost never operate above 60-70%.  Ideally, you want a few hours in the 90-100% bin.  This gives some confidence that your systems and equipment are sized appropriately for your location and loads.

John

John T. Forester, P.E., LEED AP, Mechanical Design Engineer I BVH Integrated Services I 617.658.9008 tel I 617.244.3753 fax I One Gateway Center Suite 506, Newton MA 02458 I www.bvhis.com<http://www.bvhis.com> I Hartford ● New Haven ● Boston
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From: equest-users-bounces at lists.onebuilding.org [mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of Omar Katanani
Sent: Monday, October 25, 2010 3:35 PM
To: equest-users at lists.onebuilding.org
Subject: [Equest-users] ASHRAE 90.1 - G3.1.2.8 Design Airflow Rates

Dear all,

I am modeling this same school for LEED 2009 purposes.

ASHRAE 90.1-2007 section “G3.1.2.8 Design Airflow Rates” states that System design supply airflow rates for the baseline building design shall be based on a supply-air-to-room-air temperature difference of 11°C or the required ventilation air or makeup air, whichever is greater.

Can someone please check the attached print screen and confirm if this is what they mean by supply air to room air temp difference?

Furthermore, if I leave the “Supply air flow rate” input for each zones as default in the baseline (instead of entering the supply CFM as in the proposed), I am getting a total consumption for the baseline to be less than the proposed! I am certainly not going to let eQUEST estimate the supply flow rates! Remember that I’m modeling for LEED and I aim for a 15% percent reduction in energy costs, which was a number I got when I assumed the baseline and proposed cases to have the same defined supply flow rates as per the ventilation rate procedure (provided to me by the design team).

Thanks for your help!
Omar

___________________________
Omar Katanani
Sustainable Design Engineer
Email: omar at ecoconsulting.net<mailto:omar at ecoconsulting.net>

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