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  1. #1
    Join Date
    Jun 2008
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    23

    Duct sizing questions

    Two questions for the experts:

    1. How can I measure the size of a flex duct? My house has two flex supply runs that have no indication of size. The branch lines clearly state its size but the supply runs do not. I'm trying to determine if my current ductwork is sized properly for my 3 tons of cooling.

    2. I spoke with an HVAC contractor yesterday about potentially replacing my flex supply ducts with metal ducts. He mentioned that a round metal trunk would move air better than a rectangular metal trunk. What do you guys think? Any certain one you prefer?

    Thanks!

  2. #2
    Join Date
    Dec 2007
    Location
    Cedar Grove, Wi-Sheboygan
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    1,582
    Use a tape measure and measure the width of the trunk of flex will tell you how big it is. Yes round ducting does move air better than retangular ducting does but if your gonna use retangular the sizing will most likely be smaller in size than round ducting, If you go with round ducting sizing will more than likely be larger than the suare or retangualr ducting to give you same size but different shape round, I believe this site warmair.com will give you some info on ducting and it's size comparisions and other question you may have regarding duct size or cfm, and oter pertitent question you might have.

  3. #3
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,603
    In order to find the size by measuring the diameter, you would need to know the thickness of the insulation.

    A 12" round metal duct does flow more air the a 12" flex duxt at same length.
    But, if you have a 12" flex duct, just going to a 12" round may not be enough, you may need a larger size. Don't just replace same size. Replace with proper size.

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  4. #4
    Join Date
    Jun 2004
    Location
    4H: Hot, Humid Houston H.O.
    Posts
    3,304
    I have had some success measuring the circumference of ducts, when the inside diameter is not printed on the outside. There remains some guesswork and likelihood of me being wrong by an inch, but I *think* I got it right for each of my ducts.

    You can pull a supply grille off somewhere and directly measure inside diameter at that point. Depending on the duct layout that may give you a clue, or maybe even the direct answer you are looking for.

    I am a homeowner but would recommend for many people, spending $XX on the ACCA Manual D book of duct design. I got that a couple years ago and it was tough reading at first, but makes a lot more sense to me now. Even better would be teaming up with a 1st-class HVAC pro who can critique your duct system for you (and design changes if needed).

    But all this may be putting the cart before the horse. If your HVAC pro can measure your static pressure on return and supply side, that will reveal a lot about whether your duct system is adequate.

    Best of luck -- Pstu

  5. #5
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    As above static pressure test can tell.


    Why do suspect that the ducts aren't the right size?

    RoundRound 12 " metal handles the same cfm as 12 X 10 metal,flex would be 14" ,based on 600 cfms and .08 Friction Rate.

  6. #6
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    5,025
    It is critical to know what CFM of airflow is being delivered to each room, & the total through the coil.

    Energy Conservation should be taught in school & discussed on the satellite & cable TV & radio channels!

    CFM (ft3/min)= Air Velocity (ft/min) x Area of duct (ft2)
    Round duct sq. ins. = 8”dia. X 8”dia. 64 -sq.ins. X .7854 = 50-sq.ins. rd duct
    (8" rd.) (550-fpm) (50-sq.ins / 144) = 0.34722222-sq.ft x* 550-fpm= 190-cfm

    For energy conservation measures, low cost anemometers with which to measure air velocity in fpm should be made available, one way or another to home owners. In my county there is nothing available to anyone, In my county I would go & do duct system analysis for mileage & a small fee.

    I was talking to a guy working at a local HVAC/R supply house, & he said
    the customer's are not willing to pay several hundred dollars to get the airflow checked.

    If we are going to conserve energy,
    evaluation of the tightness & delivered airflow of the duct system ought to be done! - Darrell

  7. #7
    Join Date
    Jun 2008
    Posts
    23
    Quote Originally Posted by dash View Post
    As above static pressure test can tell.


    Why do suspect that the ducts aren't the right size?

    RoundRound 12 " metal handles the same cfm as 12 X 10 metal,flex would be 14" ,based on 600 cfms and .08 Friction Rate.
    After measuring, I have the follow flex runs

    1. Return flex: 8" (goes to a 12x12 return grill)
    2. Return flex: 14" (goes to a 20x20 return grill)

    3. Supply flex: 10"
    4. Supply flex: 14"

    I thought the returns HAD to be at least the size of the supply runs. According the setup at our house, the returns are not large enough.

    Second, it appears that my supply runs are adequate for the ducts they are feeding; however, they are flex. In other words, even though a 14" metal round supply would work, I have a 14" flex supply run. It appears that I would need a 16" flex supply run instead.

    I am concerned about the return flex runs not being as big as the supply runs. What do you guys think?

  8. #8
    Join Date
    Jun 2008
    Posts
    23
    Quote Originally Posted by udarrell View Post
    It is critical to know what CFM of airflow is being delivered to each room, & the total through the coil.

    Energy Conservation should be taught in school & discussed on the satellite & cable TV & radio channels!

    CFM (ft3/min)= Air Velocity (ft/min) x Area of duct (ft2)
    Round duct sq. ins. = 8”dia. X 8”dia. 64 -sq.ins. X .7854 = 50-sq.ins. rd duct
    (8" rd.) (550-fpm) (50-sq.ins / 144) = 0.34722222-sq.ft x* 550-fpm= 190-cfm

    For energy conservation measures, low cost anemometers with which to measure air velocity in fpm should be made available, one way or another to home owners. In my county there is nothing available to anyone, In my county I would go & do duct system analysis for mileage & a small fee.

    I was talking to a guy working at a local HVAC/R supply house, & he said

    If we are going to conserve energy,
    evaluation of the tightness & delivered airflow of the duct system ought to be done! - Darrell
    Darrell:

    Your website is great by the way. Extremely informative! Do you think the smaller sized flex RA trunk lines are a concern? I'm trying to be as informed as possible, so I can have the work done right by a reputable HVAC company.

    I'm just not a big fan of all the unstretched/sagging flex duct runs in my house. Plus, I think, based on charts I have looked at, that the flex supply and return lines are too small for the required CFMs per room. If the ducts were metal, I think it would be fine, but I've always heard (please correct me if I am wrong), but typically your flex ducts should be 2" larger in diameter than the standard metal ducts.

    What do you think?

  9. #9
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
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    The sizes could be correct,a lot depends on the length,turns ,and fittings in the duct system.

    The 8" return could be the correct size for the area it serves.The 14" supply could have been selected because a 12.5 " flex was correct,so they went to the next size up.

    Need a Pro to test the static to find out.

    What comfort issues are you having??

  10. #10
    Join Date
    Jun 2008
    Posts
    23
    Quote Originally Posted by dash View Post
    The sizes could be correct,a lot depends on the length,turns ,and fittings in the duct system.

    The 8" return could be the correct size for the area it serves.The 14" supply could have been selected because a 12.5 " flex was correct,so they went to the next size up.

    Need a Pro to test the static to find out.

    What comfort issues are you having??
    Thanks for the reply, Dash. The air flow in our house is not very good (except for the master suite). We have two supply runs from the supply plenum. The first run (10" flex run) only goes to the master suite. The master suite has good air flow and cools and heats well. However, the rest of the house, fed by a 14" flex run, has very bad air flow. This causes the rest of the house to not heat or cool adequately.

    The Delta T is fine, the tech measured a good 20 degree drop from the return air to the supply air; however, it's just an air flow issue. The blower fan is set to high. We have a 3 ton unit for our roughly 1,500SF house.

  11. #11
    Join Date
    Aug 2002
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    Office and warehouse in both Crystal River & New Port Richey ,FL
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    Does the 14 go to one distribution bow 9which the branch runs come out of),and then to another with a smaller duct?

    What brand and model numbers??

  12. #12
    Join Date
    Jun 2008
    Posts
    23
    Quote Originally Posted by dash View Post
    Does the 14 go to one distribution bow 9which the branch runs come out of),and then to another with a smaller duct?

    What brand and model numbers??
    Brand is Comfortmaker. Air conditioner model number is as follows:

    1. Model No: ACS036A2C1
    2. Mfr. No: NACS036A2c1

    I don't know what the model number is for the furnace, I can't find it anywhere on the air handler. I just see Comfortmaker "RPJII".

    The 14" run goes to two distribution boxes. The first distribution box it goes to has three branch runs off of it to the side (7", 6", 6"). The end of this first distribution box is connected to the second distribution box by a 14" flex duct. This second distribution box has four branches off to the side (6", 6", 6", 4").

    The 10" flex run goes to only distribution box whic has five branches (4", 4", 6", 6",6")

    Also, this is a 1,500SF ranch style house so the flex runs are pretty long.

  13. #13
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
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    Which rooms do you have issues with,and which distribution box is the branch from??

    Have you checked the air flow to the rooms with issues,when the door is open ,versus closed?Lack of return or return path could be the problem.


    At some point you are going to need to call around and find a Pro that understands duct design,might be good to do so soon.

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