Hi everyone,
I wanted to do a gut check of whether I'm interpreting ASHRAE 90.1-2007
Section G3.1.3.13 correctly.
That section says:
G3.1.3.13 VAV Minimum Flow Setpoints (Systems 5 and 7). Minimum
volume setpoints for VAV reheat boxes shall be 0.4 cfm/ft2 of floor area
served or the minimum ventilation rate, whichever is larger.
I have a number of System 5s for a warehouse facility with very low
heating/cooling loads.
If I set the minimum flow for all zones to 0.4 in this screen, it ends
up substantially over-supplying the baseline zones.
Since there are maximum cooling supply temperatures (60 degrees), I end
up overcooling the zones even at minimum load, so there is reheat.
There is also cooling energy to get the supply temps down to 60.
Am I doing anything "wrong"?
I seem to be following the requirements of 90.1, just want to make sure
there isn't an exception somewhere that I'm missing.
Thanks!
James Hansen, P.E., LEED AP
Hi James,
I'm no doctor, but I think your gut checks out =).
The minimum 0.4 cfm/ft2 is going to be more than necessary for some spaces, and will in turn cause reheat that wouldn't otherwise occur for such spaces. If this is causing issues of unmet hours or other related headaches, I think you could turn the minimum down in select spaces and stay within the intent of the requirement.
It's my sense the intent of this requirement is to set a "floor" of sorts, (however arbitrary) to pick up the realities of VAV system minimum airflows to maintain ventilation requirements, minimize/prevent stratification, and to avoid cool air "dumping" from registers when velocities get too low. This sets a bar so that actual designs addressing these realities/issues aren't put at an immediate disadvantage.
Regards,
NICK CATON, P.E.
90.1-2010 has removed the 0.4 cfm/ft2 criteria, leaving the 30% or ventilation air minimum flow criteria.
Paul Riemer, PE, LEED AP BD+C