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  1. #1
    Join Date
    Sep 2019
    Location
    Northern Ontario
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    58
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    Return fan needed on meeting room AHU?

    Existing rooms were converted to our main meeting room in the 1980's. Approx 30' x 40' seats 180 people. It is an indoor ahu (Sheldons 4LP-2000) that consists of a supply fan and a heating coil and a cooling coil. Looks to be around 4000 cfm. There are 3 sets of dampers. Outdoor air inlet, mixed air and return air exhaust to outside. Original plans don't show a return fan.

    Fast forward to today. There is a return fan in place now, installed in 80's or 90's. The ducting at this return fan, seemingly do to limited space issues, is way undersized. 3 x 10" round flex ducts for that 4000 cfm. This is causing lots of noise and slow temperature response.

    Generally speaking, when would you install one or not? I ask because it was not originally spec'd and our cafeteria has almost the same system but no return fan.

    Thank you for any responses,

    Currently looks like this setup
    Name:  AirHandlingUnit_FreeCooling-700x650.png
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    Last edited by Gdh33; 09-20-2019 at 08:02 PM. Reason: Added photo

  2. #2
    Join Date
    Sep 2019
    Location
    Northern Ontario
    Posts
    58
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    Thread Starter
    Bumping this for ideas and to give an update. HVAC guy will be here next week. Would like the collective wisdom of you guys, so that way I know if HVAC guy is on the right path. Thinking of ducting straight from return on AHU, no return fan, and making sure the duct is the right size. So it would look like the picture above with out return fan, but the ducting will be replaced and right sized.

    Probably repurpose an old VFD for the supply fan. Still need to do a load calc, and maybe change sheaves sizes. And or both.

  3. #3
    Join Date
    Aug 2002
    Location
    Southold, NY
    Posts
    26,019
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    You don't need a HVAC man you need a Engineer.

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