Results 1 to 13 of 13
  1. #1
    Join Date
    Oct 2010
    Posts
    4

    VAV Box (Cooling Min. CFM)

    Hello

    I'm getting mixed answers about what value the Minimun value CFM should be
    on a VAV box with Reheat.

    Is it 30-50% of the design cfm (Max. CFM)? 300cfm? or untill there's enough air for ventilation?

    I hope that's enough information.

  2. #2
    Join Date
    Mar 2009
    Location
    Triad - North Carolina
    Posts
    328
    The installation manual will detail those settings and parameters
    Every problem gives us the opportunity to choose from several solutions: stupidity, ignorance, indifference, or brilliance. Which one will you choose?

  3. #3
    Join Date
    Sep 2010
    Location
    NY
    Posts
    325
    I think it depends on the box size, static pressure and damper position. (vav) I think that is what is going to dictate the cfm flowing through the vav box. There might be a chart which intersects cfm with static for the box size. I will interested to see what others post.

  4. #4
    Join Date
    Oct 2010
    Location
    Baton Rouge, LA
    Posts
    1,110
    Its also going to depend on the reheat set up and how big it is.
    Is it electric reheat or hot water? Does it have a fan?
    If you're too "open" minded, your brains will fall out.
    Artificial intelligence is no match for natural stupidity.

  5. #5
    Join Date
    Jun 2010
    Location
    Philadelphia PA
    Posts
    2,190
    The min is a design concern not a system requirement meaning under the old rules for fresh air, you needed to make sure you had X CFMs per person delivered to the space. Now that we are monitoring CO2 some of that may have gone away. You may need to look at the drawings, original design intent before you take it upon yourself to "deny the occupants a healthy space" If that sounds like what a lawyers complaint might sound like, well it is!
    Also whoever asked about electric reheat also understands you need min airflow so the elec coils/elements don't burn up is also right on
    You have got to learn from other people's mistakes! Because God knows you don't live long enough to make them all yourself !!!!!!!!

  6. #6
    Join Date
    May 2000
    Location
    Stafford, Va. USA
    Posts
    213
    Hi Shrend,
    You are correct, this setting should be used for minimum outside air ventilation, when the cooling requirement is satisfied.
    In the days of inlet guide vanes (IGV), this setting would be higher because the IGV could only throttle the the supply air flow down to 30% of the total supply air flow from the supply fan.
    With the use of variable frequency drives (VFD), the cooling minimum can now be set much lower. This provides additional benefits, better comfort for the occupants and more energy saving because the supply fan can be run at lower speeds for lower air flows.
    Allen
    UA Local 602
    A man who says that he can't learn anything in a class is exactly right.

  7. #7
    Join Date
    Mar 2003
    Posts
    7,321
    occupancy type is also a consideration. in medical suites for example requirements are higher in some cases.

  8. #8
    Join Date
    Oct 2010
    Location
    Baton Rouge, LA
    Posts
    1,110
    Gracia Genduct.
    If you're too "open" minded, your brains will fall out.
    Artificial intelligence is no match for natural stupidity.

  9. #9
    Join Date
    Oct 2010
    Posts
    4
    Thanks guys.

    Thanks for all the information

  10. #10
    Join Date
    Oct 2010
    Posts
    12
    Most VAV's are set at @ 10% as a rule of thumb if you don't have access to design spec's.

  11. #11
    Join Date
    Feb 2008
    Posts
    31
    Quote Originally Posted by shrend View Post
    Hello

    I'm getting mixed answers about what value the Minimun value CFM should be
    on a VAV box with Reheat.

    Is it 30-50% of the design cfm (Max. CFM)? 300cfm? or untill there's enough air for ventilation?

    I hope that's enough information.
    If Reheat is electric and VAV is cooling only, then 70 CFM per KW is typical. Some people use 100 CFM per KW to be safe. Most velocity sensors are accurate down to .03" of velocity pressure, fall below that and you will have problems with control system measuring the flow. Hope this helps.

  12. #12
    Join Date
    Oct 2010
    Location
    Baton Rouge, LA
    Posts
    1,110
    Hey someone answer this one it has me kinda boggled. I have a VAV box with a 3 stage electric reheat. I have a normal sized classroom of 14x17x12. No equipment(computers, projectors, so on). I have 6 tables with 4 chairs a table. So I am looking at 30ish people in the room. It has a I/A series DDC with a minimum CFM of 1330?!? At first I thought well maybe there is a CO2 hidden somewhere but nope. The AHU blows 1.2 WC and 55 degrees 24/7.

    I showed up because the room was cold at 60 degrees with a setpoint of 76 degrees. Turns out after a few hours of testing it was a .5 WC pressure safety switch for the reheats mounted upside down(according the manufacture specs) and a clogged Hi pneumatic line.

    But I still can't figure out why you would have a 1330 CFM on a room that size with that many people. I could understand if it was a 25x25x12 with 40+ people.
    If you're too "open" minded, your brains will fall out.
    Artificial intelligence is no match for natural stupidity.

  13. #13
    Join Date
    Sep 2002
    Location
    Hampton Roads, Virginia
    Posts
    1,593
    Quote Originally Posted by XcelTech View Post
    Hey someone answer this one it has me kinda boggled. I have a VAV box with a 3 stage electric reheat. I have a normal sized classroom of 14x17x12. No equipment(computers, projectors, so on). I have 6 tables with 4 chairs a table. So I am looking at 30ish people in the room. It has a I/A series DDC with a minimum CFM of 1330?!? At first I thought well maybe there is a CO2 hidden somewhere but nope. The AHU blows 1.2 WC and 55 degrees 24/7.

    I showed up because the room was cold at 60 degrees with a setpoint of 76 degrees. Turns out after a few hours of testing it was a .5 WC pressure safety switch for the reheats mounted upside down(according the manufacture specs) and a clogged Hi pneumatic line.

    But I still can't figure out why you would have a 1330 CFM on a room that size with that many people. I could understand if it was a 25x25x12 with 40+ people.

    A classroom should have 15CFM of outside air per occupant, 30*15 = 450 CFM, but what % of the delivered air is outside air ? Lets say its 25% then you would need to deliver 1800 CFM of air.

    Ideally classrooms should have a DOAS delivering 68 50%RH air at 15 CFM/Occupant, and then use the main system to meet the cooling/heating needs only.


    Kevin
    "Profit is not the legitimate purpose of business. The legitimate purpose of business is to provide a product or service that people need and do it so well that it's profitable."

    James Rouse

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
Comfortech Show Promo Image

Related Forums

Plumbing Talks | Contractor Magazine
Forums | Electrical Construction & Maintenance (EC&M) Magazine
Comfortech365 Virtual Event