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Thread: Hot in Bedrooms

  1. #1

    Hot in Bedrooms

    2100 sq ft. 3.5 ton unit. 18 months old. all of the bedrooms are much hotter than the rest of the house. Also, I'm not positive the ductwork looks quite right in the attic. I'm going to try to figure out how I can post pictures of the duct work. One of the flex lines is coming apart from the main duct and it appears to be very wet and there is water that has settled in the flex. All the flex runs are just sitting on the floor of the attic on top of the insulation. I am by far not an expert but something just doesn't seem right. Also, is there a max distance that a flex line should be used to go to a vent. There is one that looks like the line is 25 to 28 feet from main trunk.

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  2. #2
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    Depending on location and house design, that might be a little big for 2100sqft, but not far off. sounds liek the ductwork isn't sized right for the longest runs. Probably just cheap insualted flex, laid loose and maybe some runs have been stepped on and crushed. Airflow can almost double just by strething it tight nd getting rid of tight curves.

    I'd start with getting a good contractor in there ot clean up the poor quality ductwork, if needed increase teh size of one or 2 branches. Finally, you cna sometimes use higher flowing grills or increase the size of the boot/register to get more airflow. Then add dampers on each branch at the supply plenum, and balance the systme as needed. Start by reducing flow to the coldest rooms that are overconditioned.

    It's all about balancing. That's no excuse for every room in a hosue wiht a properly sized system not to be within 1F almsot all day long in a home over 1500sqft.

  3. #3
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    ductwork is supposed to be connected with UL listed connectors... duct tape can qualify, if installed properly, but a zip tie is preferred. mastic should be used to seal the boot, and the vapor barrier to the plenum.
    most flex duct is supposed to be limited to 10' in length, but there are flex systems that allow 25' before reductions are required. it all depends on what type of duct system is used, and how it's sized. attic installs are fine to lay on top of the insulation. the tight bends are not good though.
    The TRUE highest cost system is the system not installed properly...
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  4. #4
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    In a nutshell, if a room or rooms are not getting cool enough, they are not getting enough airflow.

    There are two ways to address this issue.

    Trial & Error:
    Many pros have seen this scenario before and know what will likely make it better. Often adding a supply duct to the room(s) in question is a good start.

    Engineered calculations:
    Starting with a room by room load calculation, and your blowers capabilities, the correct duct sizes for your particular layout can be determined and compared. The exact modifications required are known when this is done. The benefit of this engineered approach is that all rooms will be at the same temperature and your system will perform as rated and have a long life.

    The first option may not cost very much and even though it's based on "rules of thumb", it has a fair chance of working, at least to your satisfaction. The second option may cost more but will get your system operating as rated.

    "If perfection is your goal, you may end up with good enough, what might you end up with when good enough is your goal?"
    Wayne Pendergast, efficientcomfort.net

  5. #5
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    Quote Originally Posted by vstech View Post
    ductwork is supposed to be connected with UL listed connectors... duct tape can qualify, if installed properly, but a zip tie is preferred. mastic should be used to seal the boot, and the vapor barrier to the plenum.
    most flex duct is supposed to be limited to 10' in length, but there are flex systems that allow 25' before reductions are required. it all depends on what type of duct system is used, and how it's sized. attic installs are fine to lay on top of the insulation. the tight bends are not good though.
    Flexible duct connectors are limited to 10ft. Flexable duct is unlimited in length. There is a differance
    It's hard to stop a Trane. but I have made one helluva living keeping them going.

  6. #6
    Thanks all. Did the pictures I posted show up?

  7. #7
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    In terms of the photos, I've seen a lot worse, but it could be improved. I suspect it's mostly an issue of supply length to those underconditioned rooms. What's easier to blow air through, a 6" long straw or a 20" long straw?

    Whiel I would fix some of the sealign and isnulation issues for sure and fix where there are tight radiuses. The easist solution is to add dampers to the runs with too much airflow. The caveat, being that you can maintain minimum airflow on the system overall. If you can;t then you'l have ot go the other route and increase duct size on the long runs.

  8. #8
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    Quote Originally Posted by Southern Mech View Post
    Flexible duct connectors are limited to 10ft. Flexable duct is unlimited in length. There is a differance
    true, there is a difference, but my statement is valid. code states any ductwork exceeding 25' needs to be upsized from the plenum to maintain air velocity. meaning, if 6" is needed to flow the air to a register, and it's 40' away, you need to run 8" for at least 15' to feed the 6" vent.
    the reason for it is the friction on the ductwork reduces the velocity of the air, and limits the CFM getting to the boot.
    The TRUE highest cost system is the system not installed properly...
    The three big summer hearththrobs...
    Mel Gibson
    Dwane Johnson
    The A/C repairman

  9. #9
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    Quote Originally Posted by vstech View Post
    true, there is a difference, but my statement is valid. code states any ductwork exceeding 25' needs to be upsized from the plenum to maintain air velocity. meaning, if 6" is needed to flow the air to a register, and it's 40' away, you need to run 8" for at least 15' to feed the 6" vent.
    the reason for it is the friction on the ductwork reduces the velocity of the air, and limits the CFM getting to the boot.
    Good point, I tend to forget the upsize reason being just like the OP's problem...I would have run 1 larger duct back to those bedrooms then branched off individually. But that is difference in design from 1 man to another.
    It's hard to stop a Trane. but I have made one helluva living keeping them going.

  10. #10
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    I don't like to see flex duct sitting on the floor of an attic, simply because, as your photos show, it hardly ever gets stretched tightly that way. Of course, just hanging it in the air doesn't guarantee that will happen, either, but it is easier to stretch it tightly between points of suspension vs. just snaking it over the insulation on the attic floor.

    Bottom line is for a flex duct system to flow air at the proper amounts, all duct runs need to be sized correctly, accounting for length of run, with minimal turns, absolutely no kinks, and all runs stretched tightly. All fittings should be mastic sealed, and all supply boot should be mastic sealed to the drywall wherever they penetrate the ceiling underneath the attic. If you have anything less than this, you will have uneven cooling of rooms in your house.
    • 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.

  11. #11
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    The big deal here IMHO is the restrictions caused by using flex duct not fully stretched to the maximum. Fully stretched duct work makes your actual duct size approximately 1 1/2 to 2 diameters less that of metal duct.

    Say a 8" flex is the equivalent of a 6" metal round duct. Now if it is not fully stretched then you are looking at major restrictions here. A 8" flex could be the equivalent of say a 5" or 4" metal round duct. Check out this study done on flex duct losses on fully stretched duct vs duct compressed more than just 4%.

    Basically it says that at compression values over 4% flexible duct will exhibit 2 to 10 times the presser losses of a metallic duct.

    If you can afford it run metal duct to the rooms otherwise then stretch the ducts out to the max and use metallic elbows which will allow you to stretch the duct and eliminate the excessive losses through a flex bends. Thank you very much

    http://www.mmmfg.com/pdfs/060601_CC-...tTechPaper.pdf
    Last edited by glennac; 06-28-2013 at 02:09 PM.
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