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  1. #1
    Join Date
    Aug 2011
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    Round or Rectangle ducting?

    Ok so I have a brand new Goodman gmvc95 furnace with Honeywell HZ311 using two zones. I am getting ready to have my third zone installed (basement 550 sqft). So had the HVAC guy out and with the current dips my 1st stage heat/cool is around 900cfm so I need the duct work to move about that much. Anyways he said I can do a 10" ROUND duct along one wall with registers off it and then a 8" ROUND in the opposite direction with registers and that will satisfy the cfm needs OR can do several RECTANGLE ducts that will get the same cfm and will be tucked in between the joints. Both options are sheet metal. The rectangle costs a little more too.

    Just FYI because it is a basement the thermostat with have a swing temp setting, there will be a de-humidifier, and there will be a automated bypass just for the basement.

    Which option is better Round or rectangle sheet metal?

  2. #2
    Join Date
    Oct 2010
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    Anderson, South Carolina, United States
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    7,775
    Doesn't matter, rectangle will have a lower profile and you can hide it easier. As long as its sized, sealed, fabricated, and installed properly it doesn't make a difference to the air if its round, rectangle, oval etc

  3. #3
    Join Date
    Aug 2011
    Location
    Kansas City
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    78
    Great. Thank you!

  4. #4
    Join Date
    Feb 2009
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    Tallahassee, FL
    Posts
    6,051
    Round flows the best. And 10" wont move 900cfm.

  5. #5
    Join Date
    May 2007
    Location
    DC Metro Area (MD)
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    3,371
    So the contractor is proposing one 10" round and one 8" round to move 900 CFM? I would not pick that option.

  6. #6
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    Oct 2010
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    Anderson, South Carolina, United States
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    7,775
    550sq ft doesn't need 900cfm, no way.

  7. #7
    Join Date
    Aug 2011
    Location
    Kansas City
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    78
    Ok now I'm confused.

    SBKold - How does Round flow the best? It is cheaper too, but what will it do vs rectangle?

    RyanHughes - a 10" and 8" is not enough? What should I ask for? What is going to happen if a 10" and 8" is installed? (remember a bypass is going to be installed)

    jtrammel - the HVAC guy said this too, but he said it needs to be able to flow 900cfm in case the zone panel goes out and all the 1st air is pushed to the basement. He said if I went to small and this happened it will destroy the "blower wheel"?

  8. #8
    Join Date
    May 2007
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    DC Metro Area (MD)
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    A 10" and an 8" may be "enough" for 900 cfm, but it certainly isn't ideal. A variable speed blower may be able to push 900 cfm through them provided there is enough available static. Remember variable speed blowers are only rated to move a constant CFM up to 1.0" of external static pressure in general. But if your return is already restrictive, moving that 900 CFM through a 10" and an 8" will certainly be pushing the blower to its limit. High static causes premature failure for variable speed motors. 550 CFM through a 10" and 350 CFM through an 8" is over 1000 fpm (velocity). Won't be quiet. Even with a bypass, how much airflow does the basement require (per load calculation, if one was done)? Probably considerably less than what will be provided. Basements generally don't carry much of a load.
    Last edited by RyanHughes; 10-27-2012 at 11:12 PM.

  9. #9
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    Aug 2011
    Location
    Kansas City
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    78
    No load calculation done. He did one for the upstairs when the furnace was installed which is why I went down from 90k to 70k but didn't count the basement. I really don't know what 1000 fpm means but I'll take your word it will be noisy. What do I want in terms of fpm?

    I typically doesn't get hot down there, does get chilly though. You are making me rethink a third zone in the basement.

  10. #10
    Join Date
    May 2007
    Location
    DC Metro Area (MD)
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    3,371
    I didn't take into account the bypass in my previous post. With the bypass in place, zoning the basement with those sizes ducts shouldn't be an issue since it will be getting less than 900 cfm with a good amount of the air being bypassed back to the return. Should the bypass or zone panel fail, however, then you would likely have issues with that much airflow. It probably won't take long to satisfy the basement zone with that much airflow (even with the bypass). Are there any returns in the basement?

  11. #11
    Join Date
    Aug 2011
    Location
    Kansas City
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    78
    Oh ok thanks. Right now there is a 8x8 return. The HVAC tech said it was added years ago, probably, when the AC was installed. What he talked about was adding a 10x8 damper that would open when the basement calls for AC or Heat.

    The whole install would be 1 10" round duct, a 10" round zone damper, 1 8" round duct, a 8" round zone damper, 1 10x8 return, a 10x8 normally closed damper, and 3 or 4 registers. So the system would make a call the zone dampers would close off zones 1 & 2, zone 3 (basement) dampers would open and the 10x8 return damper would open. The upstairs returns and the other 8x8 return will always be open.

  12. #12
    Join Date
    Feb 2011
    Location
    North East Ohio
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    786
    Consider using a barometric bypass damper. You won't have to worry about it failing

    Round flows better because it has less surface area and therefore lower static pressure

  13. #13
    Join Date
    Aug 2011
    Location
    Kansas City
    Posts
    78
    NCHeat - HVAC guy said I can't use a barometric bypass because it will confuse the VS blower speed? Something about the barometric bypass opens and closes with cfm and the VS tries to maintain an even cfm. It sounded really smart and made sense, but I don't know.

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