×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

VAV Design

VAV Design

VAV Design

(OP)
In designing a VAV system for a typical office building, are there any rules for determining the minimum CFM for each VAV box (air changes/hour or CFM/sq. ft.)?

RE: VAV Design

The rules are:  maintain minimum fresh (outdoor air rate) for the occupancy;  maintain local temperature; and any local ordinances for air change rates.  The most difficult issue is to maintain the minimum ventilation air rates to satisfy ASHRAE with varying primary air quantities.  There are some fairly convoluted calculations and control algorithms to achieve this that Trane and some of the other major suppliers can provide with their central air handling equipment controls.  Normal rule of thumb is don't go below 30% full flow (and hopefully you have supply air temperature reset and a reheat coil so you don't over-cool the space at minimum airflows).

RE: VAV Design

Make sure your diffusers do not dump at minimum CFM. The diffuser must be rated at the minimum flow or else air will dump out the diffusers closes to the air terminal box and not get to the remote diffusers. Also check that you have enough CFM to heat the space. Normal control is for the VAV box to go to minimum CFM then the reheat sequences in as required. It is possible to program critical boxes to then increase CFM when space still calls for heat & the reheat coil control valve already is full open.

To not dump. minimum limit is usually about 50% of design max.

RE: VAV Design

In many cases some designers don't go below 4 ACH.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources