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  1. #1
    Join Date
    Jan 2008
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    196

    Using square duct vs. round

    I have a very tight crawl. The original contractor was going to use a 16" round trunk, then he switched to a 12" round duct because of space limitations. But it is a mess. New possible contractor is telling me a 8x20 duct will work fine and it does fit into the required space. It would start under the downflow furnace connected to the base/supply plenum in the crawl. One side would turn up through the subfloor going vertical alongside the air-handler to feed 6 supplies from the ceiling of the second floor. The other side would continue the length of the crawl (near 55') to feed nine supply registers. He would supposedly build a directional baffle to help flow.

    Before I go cutting holes and reframing some floor joist, does anyone see a reason why I should question this?

  2. #2
    Join Date
    Dec 2003
    Posts
    709
    What size furnace and a/c? model #s? Technically round duct is going to be most efficient, but not if you have to undersize it or smash it to get it into its space. Think of it as a water hose, I would prefer a round hose over a rectangular because the water will flow better in the round, same goes for air.

    The rectangular duct will be fine as long as he designs the duct system properly.

  3. #3
    Join Date
    Jan 2008
    Posts
    196
    Thanks hivacer...I find myself leaning on this forum, as truly it is the only place I hear proffesional level HVAC science being applied.

    I personally think munincipalities should require engineered HVAC plans for any building permit. How on earth are the mandated higher efficiency systems going to meet that efficiency if the duct work is the limiting factor?

    After my local interaction with the HVAC trade many of them would be making half as much money doing installs to someone elses design, much like how framing has become. The industry needs to embrace and promote this science or it will eventually be done for them with-out them.

    I'm still amazed, I have not found one person capeable or willing to perform a manual D. I have talked with probably 15 HVAC contractors in the last few weeks representing many different manufacturers. Most of them seem to think using a ductalator is all that is needed and seem proud they are able to do that, as many do not.

  4. #4
    Join Date
    Mar 2005
    Location
    Suppy NC
    Posts
    4,516
    20x8 duct will handle about 1000cfm of the top of my head witch is 2.5 ton
    and will work well in a tight crawl. Realize it does need to be insulated with duct wrap or bubble rap

    what size system are you having installed

    round duct is bulky and requires a lot more room

    as for as witch is less restrictive you are drawing straws

  5. #5
    Join Date
    Jan 2004
    Location
    PA
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    Big difference between 16" round, 12" round, and 20x8.

    Still didn't tell us what size unit.
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    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

  6. #6
    Join Date
    Jul 2004
    Location
    Massachusetts
    Posts
    6,841
    We run round whenever we can, usually just in attics. In basements, to maintain ceiling height, we usually end up with rectangular. The biggest problems with installs, IMO, is a the gross lack of understanding about total equivalent length, which translates to lack of knowledge about fittings. Tell someone a straight takeoff has 35-EF, while an offset transition has 10-EF and they don't know what you're talking about. They just know the straight T/O costs less!! We also plan our residential systems using rectangular trunk to have a maximum of 50-EF for the main. This allows plenty of tolerance for the round branch supplies.
    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!

  7. #7
    Join Date
    Jan 2008
    Posts
    196
    If I can believe the original installer the system was to be a 3.5 ton. But a 3 ton was actually chose because of availability.

    The second stories 6 registers are fed by a 12" duct running vertical from under the main level under stair.

  8. #8
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    Jan 2004
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    PA
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    Quote Originally Posted by mlo1 View Post
    If I can believe the original installer the system was to be a 3.5 ton. But a 3 ton was actually chose because of availability.

    Availability is not what determines the size unit you need.

    The second stories 6 registers are fed by a 12" duct running vertical from under the main level under stair.
    You need a contractor that will do a load calc to know what size you really need.
    Then they can use manual D to determine duct sizes required.
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  9. #9
    Join Date
    Feb 2004
    Location
    chicago suburbs
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    4,422
    Quote Originally Posted by tinknocker service tech View Post
    20x8 duct will handle about 1000cfm of the top of my head witch is 2.5 ton
    and will work well in a tight crawl. Realize it does need to be insulated with duct wrap or bubble rap

    what size system are you having installed

    round duct is bulky and requires a lot more room

    as for as witch is less restrictive you are drawing straws
    round pipe is best for airflow.
    FILL OUT YOUR PROFILE!!

  10. #10
    Join Date
    Mar 2005
    Location
    Suppy NC
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    4,516
    Quote Originally Posted by mlo1 View Post
    If I can believe the original installer the system was to be a 3.5 ton. But a 3 ton was actually chose because of availability.

    The second stories 6 registers are fed by a 12" duct running vertical from under the main level under stair.

    they say 3.5 needed then want to install 3 ton because it is what they can get. Then want to undersize the duct because the hieght of the crawl space

    some light bulbs should be going off here.

    what beethere has said get someone that can do what you need

    20x8 or 30x8 will only take up 8 inches of hieght in the crawl plus insulation and clearances. The duct needs to be sized for the system and the system needs to be sized for the house not for what is available and easer to handle

  11. #11
    Join Date
    Mar 2005
    Location
    Suppy NC
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    Quote Originally Posted by tinner73 View Post
    round pipe is best for airflow.
    i know i am being bated but why not for the sake of dicussion

    this not actuly true

    air travles in circular motion and round pipe fources it to travel in a straight motion thus causing more restriction.

    retangular or sqrare duct will alow air to travel in it intended circular motion and less restriction acording to Smacma. Elbows and such with round backs and square throats cause turbulants and restrictions making it more restrictive in the end. But with turning vanes with square backs and throats then your turbulants and restrictions or minimal and flo is increased making it better in the long run

    again like i already said you are drawing straws as to witch is better

    if i find my smacma book i would coppy and post but over the years seems i cant find it

    some day i will get a new one

  12. #12
    Join Date
    Jan 2008
    Posts
    196
    Yes Sir...there were problems. I was slow in acting but I see them now after getting an education from this forum.

    My house design has near 300 sq.ft. of glass. It will also have a hybrid insulation technique consisting of both closed cell foam and cellulose. Exterior walls have 1" wood sheathing and interior are 5/8" gypsum.

    I have and appt tomorrow from a contact throught the local HVAC wholesaler. I have had to wait for this guy but hopefully things will get back on track. I have been at a standstill for nearly three weeks.

  13. #13
    Join Date
    Jul 2004
    Location
    midwest
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    2,868
    Quote Originally Posted by tinknocker service tech View Post
    i know i am being bated but why not for the sake of dicussion

    this not actuly true

    air travles in circular motion and round pipe fources it to travel in a straight motion thus causing more restriction.

    retangular or sqrare duct will alow air to travel in it intended circular motion and less restriction acording to Smacma. Elbows and such with round backs and square throats cause turbulants and restrictions making it more restrictive in the end. But with turning vanes with square backs and throats then your turbulants and restrictions or minimal and flo is increased making it better in the long run

    again like i already said you are drawing straws as to witch is better

    if i find my smacma book i would coppy and post but over the years seems i cant find it

    some day i will get a new one
    Less friction to move the same amount of air through a round pipe. Look at the surface area on round versus square to move the same amount of air.

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