[Bldg-sim] System 7 Min VAV setpoint

Dan Russell danr at engineeringinc.com
Wed Nov 12 15:06:47 PST 2008


Based on the fresh air volume in your example (108 l/s), the design air flow would have to be at least equal to that amount and probably higher unless the primary air handler is 100% fresh air.  The design air flow based on 20F delta T should always be trumped by the ventilation requirement, if ventilation requirement is larger than it.

In any case, I would maintain a minimum of 0.4 cfm/sf as a minimum level of ventilation simply as "good practice" to avoid stagnation.  This may mean using additional reheat energy or utilizing a fan-powered terminal unit to provide additional recirculation air.

However, if air stagnation is not a concern, and design air flow is less than 0.4cfm/sf then I would say that the minimum setpoint is at the discretion of the designer and any value less than or equal to 0.4 cfm/sf would not violate the intent of the requirement.  In the same way, if the cooling design load required 1.0 cfm/sf but the heating load for the space could be handled with less than 0.25 cfm/sf then I don't believe App G would require you to increase your minimum VAV setpoint to 0.4 since that would be a silly waste of energy.  The most extreme example of this logic is an interior zone, which has zero heating load and zero fresh air load...Since there is no reheat required, the 0.4 cfm/sf requirement does not apply and therefore the VAV damper could be allowed to fully close when the space is satisfied.  Due to Standard 62 this occurrence is highly improbable, but it helps to illustrate the point.

That's my two cents.

Thanks,

 Dan Russell, EIT

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From: bldg-sim-bounces at lists.onebuilding.org [mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Anis Ben Ayed
Sent: Wednesday, November 12, 2008 1:37 PM
To: bldg-sim at lists.onebuilding.org
Subject: [Bldg-sim] System 7 Min VAV setpoint

Dear all,

The VAV minimum set point for system 7 in Appendix G is calculated based on (0.4 cfm/ft2). For some spaces this number is bigger than the calculated design airflow based on 20F delta T. What should the VAV set points be in this case?

Example:
Room:
VAVmin = 131 l/s
Design Air Flow = 84 l/s
Fresh Air = 108 l/s

Thank you for your input,

Anis

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