VAV Design

[cuse]

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.)?

 

[GMcD]

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).

 

[lilliput1]

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.

 

[walkes]

Here's a link to the Advanced VAV System Design Guide, put out by the California Energy Commission in 2003.

http://www.energy.ca.gov/reports/2003-11-17_500-03-082_A-11.PDF

 

[Tvrm]

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