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Thread: makeup air

  1. #1
    Join Date
    Nov 2009
    Posts
    28

    makeup air

    Hi,
    We need makeup air units of 2500 cfm, 12 tons with 100% outside air, no heating. Can you refer a product.

    Can I use a standard rooftop unit (+/- 15% OA) as a makeup air (100% OA)? The supply air temperature must be under 75F.

    Thanks
    Alex

  2. #2
    Join Date
    Dec 2002
    Location
    SouthEast NC ICW & Piedmont Foothills
    Posts
    7,619
    check Reznor
    It`s better to be silent and thought the fool; than speak and remove all doubt.

  3. #3
    Join Date
    Jan 2011
    Location
    WI
    Posts
    79
    Greenheck, Captiveair are two others.

  4. #4
    Join Date
    Feb 2006
    Location
    Atlanta,GA.
    Posts
    811
    you will have to have one custom designed. i believe heatcraft will do that but wont be cheap

  5. #5
    Join Date
    Feb 2006
    Location
    Atlanta,GA.
    Posts
    811
    also look at applied air smu models

  6. #6
    Join Date
    Apr 2010
    Location
    southern california
    Posts
    535
    There are some flaws already in your design. If you are wanting to bring in 100% osa and there is 12 tons of cooling, 2500 cfm is not enough air moving across your evaporator. No you can not use a std. rooftop unit , damage will eventually occur to the compressors. I do a lot of make up air jobs. Mfgs that come to mind are Reznor, Greenheck, Semco, AAon just to name a few. Most makeup unit have enthalpy controls , reheat coils , ervs, etc. The quantity of osa is determined by the ventilation requirements and the quality of air needed in the space.Exhaust will also be needed. Heating will more than likely be needed with reheat coils to help control humidity. Osa systems can become complicated in a hurry do to the constant changing conditions of the outdoor enviroment. Unless you are very good with psychrometrics, I would recommend consulting with a mechanical engineer.

  7. #7
    Join Date
    Mar 2008
    Location
    NC
    Posts
    1,445

  8. #8
    Join Date
    Oct 2011
    Location
    NJ
    Posts
    149
    You can use an insulated bypass curb on a roof top unit to allow a standard roof top unit to provide higher than normal outside air requirements while recirculating enough air (before it can get to the space) to maintain +-80 deg F mixed air temp (This is key to making this work). The unit fan supplies say 4000 CFM at 55 deg F and 1500 CFM at 55 deg F runs through the bypass while 2500 goes on to the space and is made up by 2500 CFM of outside air at say 95 deg F that mixes with the 1500 CFM of 55 deg F bypass air which gives a 80 deg F mixed air. Lennox, amoung others, can do this arrangement. It is inefficient using more fan power than required for the room, but a cheaper unit can be used. The problem is, if you raise your discharge temp from 55 deg to 75 deg, you will need much more total air, 9000 CFM total air, 6500 CFM bypass air at 75 deg F to bring 2500 at 95 deg down to 80 degrees mixed. Outside of the range of the 12 ton unit fans I believe.

    If you need that 75 deg F discharge temp, or want a more efficient/better method consider Valent, Innovent, Greenheck, or Semco these all make decent 100% OA units. Trane has a new line of 100% MUA units that look pretty nice as well.

  9. #9
    Join Date
    Jun 2010
    Location
    Philadelphia PA
    Posts
    2,159

    You can't use Heat Recovery?

    You must have 100% OA? Why? What is your application
    You have got to learn from other people's mistakes! Because God knows you don't live long enough to make them all yourself !!!!!!!!

  10. #10
    Join Date
    Nov 2009
    Posts
    28

    makeup air

    We use Daikin cassettes for cooling and makeup air units at 100% OA for ventilation. No recirculation (exhaust from washrooms). A mechanical eng. told me to use the rule of thumb of 200cfm/ton.

    Can somebody tell me why I can't use a standard rooftop. Why this would damage the compressors as mentionned in the previous thread?

    This is in the South and no heating is required (no heat recovery)

  11. #11
    Join Date
    Nov 2009
    Posts
    28

    rtu 100% OA

    Hi,

    Can someone explain me why I can't use a standard roof top unit as a makeup air with 100% outside air. Why this can damage the compressor?

    Thanks for your help
    Alex

  12. #12
    Join Date
    Aug 2005
    Location
    iowa
    Posts
    1,649
    i guess the first thing i think of is why? i hate to answer a question with a question but what are you trying to accomplish? what are you trying to do?

  13. #13
    Join Date
    Nov 2009
    Posts
    28
    We use Daikin cassettes for cooling and makeup air units at 100% OA for ventilation. No recirculation (exhaust from washrooms). A mechanical eng. told me to use the rule of thumb of 200cfm/ton.

    Makeup air units must be 2500 cfm / 12 tons. Supply air temperature must be under 75F.

    This is in the South and no heating is required (no heat recovery)

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