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  1. #1

    Proper size a/c ducts for bedrooms in new construction home in NC???

    I'm having a home built and we've encountered several errors made by the builder which now leads to us question all the work for each subcontractor. Photos from the same floor plan show 2 a/c vents in the master bedroom and they are only installing one vent in my house. So my question is which would be the appropriate size duct for a bedroom that is 14x15 (210 sq feet)???
    The other house I saw had 2 6" ducts in the room. The builder in mines is installing one 7" duct. The master bedroom faces the front of the house with 3 exterior walls and we want to be sure the a/c will properly cool that room and be efficient at the same time. Please help!
    Also, what is the appropriate size duct for the other bedrooms sizes 11x12 (132 square feet)???

  2. #2
    Join Date
    Oct 2010
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    Anderson, South Carolina, United States
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    two 6's is more airflow that one 7, one 8 is the same as two 6's. If the ac contractor did load calculations then it may have only called for a 7" line. Lots of things go into a load calculation direction outside walls face, insulation values, # of windows, size of windows etc.

  3. #3
    That master bedroom has 3 exterior walls and a double window 3x5 (2 of these). Since the home is supposed to be energy efficient and they have made so many errors we are now concerned and hvac is not our expertise. The same floor plan in another community had two 6" in the room which is why we questioned it now.

    How about the other bedrooms, they are 11x12 (132 sq feet) with 1 single 3x5 window facing the rear of the house, one bedroom has 1 exterior wall and the other 2 exterior walls. Would a 6" duct with a 10x4x24 vent work well for the secondary rooms??

  4. #4
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,347
    Jtrammel is pointing your thinking in the right direction. Load calculations are supposed to dictate duct size and quantity, along with register size and quantity. There is no accurate "for 'x' square feet of bedroom, run a 'y' sized duct to it". Rather, there is "for 'x' heat gain or loss calculated for this bedroom, run a 'y' sized duct to it". That's the correct way to do it.

    Ask your builder how his subcontractor arrived at the duct sizes for each bedroom in your house. Anything other than "heat load calcuation" or "Manual J" is "seat of the pants" guessing and may or may not hit the target. Which is better: crapshoot poke n' hope, or engineering?
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  5. #5
    Join Date
    Jul 2004
    Location
    Massachusetts
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    6,837
    All the comments on required load analysis, proper Manual 'D' duct design, use of Manual 'T' or manufacturer's engineering data for the vent terminals and grills are of utmost importance. You'd most likely like to have a new home that is both energy efficient, comfortable and quiet. There is only one way to get all of that in your home as far as the HVAC is concerned and that's by applying the proper science to the project, not by guessing. If the installing company cannot produce the supporting documentation (science) for the project, you'd better sit down with your builder and have a talk. If the builder is of no help, you may need to consult with an attorney. But I'd recommend that under no circumstances, should you accept anything less than pure science. Everything else is guessing and BS!
    If YOU want change, YOU have to first change.

    If you are waiting for the 'other guy' to change first, just remember, you're the 'other guy's' other guy. To continue to expect real change when you keep acting the same way as always, is folly. Won't happen. Real change will only happen when a majority of the people change the way they vote!

  6. #6
    Join Date
    Oct 2010
    Location
    Anderson, South Carolina, United States
    Posts
    7,074
    Quote Originally Posted by skippedover View Post
    All the comments on required load analysis, proper Manual 'D' duct design, use of Manual 'T' or manufacturer's engineering data for the vent terminals and grills are of utmost importance. You'd most likely like to have a new home that is both energy efficient, comfortable and quiet. There is only one way to get all of that in your home as far as the HVAC is concerned and that's by applying the proper science to the project, not by guessing. If the installing company cannot produce the supporting documentation (science) for the project, you'd better sit down with your builder and have a talk. If the builder is of no help, you may need to consult with an attorney. But I'd recommend that under no circumstances, should you accept anything less than pure science. Everything else is guessing and BS!
    Yuuuuuup

  7. #7
    That's exactly what we requested (specifications!) and we got a letter saying they stand behind their workmanship and will repair any problems that arise at no cost but it gave no explanation. I don't want to wait for a problem to arise I want to avoid it by ensuring they get it right the first time. Thanks so much for your replies. Much appreciated.

  8. #8
    Join Date
    Dec 2002
    Location
    Arizona
    Posts
    4,412
    Ask if a manual J,D,S and T were done on your home. If yes could you have a copy? If no WHY NOT? A high efficiency home without this is not a high efficiency home, sorry!
    Make your expertise uniquely valuable.

    Make your influence uniquely far-reaching.

  9. #9
    Join Date
    Dec 2002
    Location
    Arizona
    Posts
    4,412
    Here is some info for you. Pay special attention to the HVAC portion!
    http://www.energystar.gov/ia/partner...Checklists.pdf
    Make your expertise uniquely valuable.

    Make your influence uniquely far-reaching.

  10. #10
    Join Date
    Dec 2010
    Location
    The Quad-Cities area (midwest).
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    2,701
    I agree with all that has been said here.....However, you'll be dead by the time you get a Manual J, D, S or anything like it. I hope I'm wrong and please let us know if they come through, but I would be totally surprised.

    That said, and this is sorta seat of the pants, the sq. footage is about the CFM you want to a room. This will change (of course) with the direction of the room/wall to the sun, insulation, quality of windows and the ceiling height.

    So, for your home: 2-6" or 2-7" in the master (you can always damper them if you have to much air) and 1-6" for a 11x12 interior bedroom with one exposed wall or 1-7" if it has a exposed wall.

  11. #11
    Join Date
    Mar 2009
    Location
    Mount Holly, NC
    Posts
    3,205
    what part of NC are you in? does your house have the same attic, shade, overhang, and insulation as the other house you found to have 2 vents in?
    "we'll stand behind our work" isn't very helpful when you are hot in your bedroom at night, and waiting to get on their service schedule because they are busy with installs...
    The TRUE highest cost system is the system not installed properly...
    The three big summer hearththrobs...
    Mel Gibson
    Dwane Johnson
    The A/C repairman

  12. #12
    Join Date
    Mar 2009
    Location
    Mount Holly, NC
    Posts
    3,205
    any chance their slogan has our techs do it best?...
    The TRUE highest cost system is the system not installed properly...
    The three big summer hearththrobs...
    Mel Gibson
    Dwane Johnson
    The A/C repairman

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