ASHRAE 62.1 Calculator

For constant volume systems, Ds is 1.0 at both the system and zones. For VAV systems, Cooling condition: Ds is typically 1.0 for the system

I'm understanding it in this way. Ds is the supply efficiency. However, this is typically assumed to be 95% considering 5% as supply loss. If your design cfm (Vdzd) is 2800, if Ds is 95%, then Vpz becomes 2800*0.95=2660. The supply efficency for the best & worst conditions are same as the duct design remains same.

Ds is a population diversity factor, not a supply efficiency factor. If you don't have a good sense of the diversity to apply, 1.0 is acceptable as a default.

Dear Chang,

Percent of total design airflow rate at conditioned analyzed is Ds interms of %. Occupant diversity is D in terms of % again. The latest version of LEED online templates accounts for these two factors. These two values have to be fed in the template. These are not automatically calculated.

Please refer to page 14 in the following link - http://www.accessengineeringlibrary.com/mghpdf/0071741828_ar022.pdf

I have referred to the above document & came to this assumption.

It will be great if you can guide my on my understanding.

Thanks :-)

Thanks for the clarification about which variable you were inquiring about. The D variable should be adjusted to reflect the realistic zone airflow rate under the condition you're looking at. For heating, this would typically be 30% of the design cooling airflow rate for a VAV system (or the VAV box min setting). D is not a measure of supply efficiency, but rather to account for different operating conditions. You'll find that higher VAV box minimum settings are sometimes necessary to keep the system level ventilation fraction from driving up to 100%. Hope this helps!

For cooling, assume 100% for Ds, if you have no other information. You've touched upon one of the challenges with ASHRAE 62 compliance for VAV systems. You'll find that the design heating condition is going to drive your system level ventilation requirement.

Hi Roger,

I saw your comments from above regarding Ds both for cooling and heating. We used 100% Ds (Cooling) and 30% (Heating). However, the result from the template is "Yes" to IEQp1 and "No" to IEQc2 requirement. Does it mean we need to adjust the CFM for office space? I tried to use 45% (Heating) and adjusted cfm for office space. Both requirements are "Yes". Is it acceptable or we just need to provide narrative?

Appreciate respond.

thanks,
Susan