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Thread: Confused

  1. #1
    Join Date
    Sep 2010
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    South Georgia
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    6

    Confused

    Hello all,
    I've had a load calc completed by two competent HVAC companies and have two quotes but I'm receiving different stories from them concerning their systems and my duct work.

    My existing system is a 20 year old Trane XL900 heat pump with a 7 year old 3.5 ton American Standard Air Handler (AH). My trunk line is flex and about 40 foot long. The AH is in the attic. I live in south Ga and it is hot and humid here. The air flow seems ok at the fartherest line but I'm sure more would be better.

    Quote A: 3.5 ton Carrier Cond #25HCC542A003, VS AH # FV4CNF005T00, with a Thermidistat. He states his sytem will do a fantastic job of controlling humidity as well as temperature. However, he says I need to get rid of my existing flex trunk line and replace it with pipe for it to work correctly. His price for the system is comparable to the Trane below...but the extra cost for the duct work puts him much higher than the Trane.

    Quote B: 3.5 ton Trane Cond # 4TWR5042E, VS AH # 4TEE3F65. Originally he had a non-programable Tstat in the quote but when I told him I wanted to have humidity control he quoted the Trane TCONT800 and said it would give me the himidity control I'm looking for. He stated, "it's all in the Tstat". He states my existing duct work is fine and no changes are needed.

    My questions are:
    1. Would changing out the flex trunk line to pipe be a huge difference in air flow at the fartherest register?

    2. Which system above will give me the best humidity control? My son is allergic to dust mites and I want to be able to keep the humidity below 50% to kill the dust mites.

    3. Which system is going to be most efficient?

    Thank you very much for your assistance!

  2. #2
    Join Date
    Mar 2007
    Location
    Myrtle Beach, SC
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    2,919
    Did anyone actually measure air flow or duct static? If not then they are just guessing. I have seen flexduct systems that delivered adequate air flow and balance, but that us not the norm. I have seen sheet metal duct systems that delivered way less than the required air flow.

    The key is testing. Without testing, you are just guessing!
    Remember, Air Conditioning begins with AIR.

  3. #3
    Join Date
    Dec 2002
    Location
    Houston,Tx.
    Posts
    15,506
    Hold On!
    __________________________________________________ ____________________________
    “Reality is merely an illusion, albeit a very persistent one.” –Albert Einstein

    “Experience is a hard teacher because she gives the test first, the lesson afterwards.”-Vernon Law

  4. #4
    Join Date
    Dec 2002
    Location
    Houston,Tx.
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    In my opinion your putting the cart before the horse unless you start off by getting a complete home energy audit by a BPI field certified company. A load calculation will not find infiltration in your home, and it may be possible that the load calculation might even call for a smaller system, but if your loosing half that capacity through a half sealed envelope “home” what good have you done? Trust me, I have seen load calculations here in Houston performed on older homes“25 year old or more” and customers not satisfied, they burn up in the summer, winter is not much an issue here, you could stay warm with a Coleman stove. I am not saying a load cacl. Is not a good thing, but if your going to go that far, why not go all the way and do it right, get a complete home energy audit and have a peace of mind.

    Here is an example below written by one of our Pro's here Al "Classical" that lives in Houston on why a load calc. depending on location could be wrong.

    Houston is not just one climate we have about four climates here and they can all manifest themselves on the same day in different parts of the region.

    The Manual J builds the same system for Galveston/Baytown as it does for Katy and
    Cypress. Now that means nothing to you but Galveston and Baytown are on the coast and virtually surrounded by water and Katy and Cypress are prairie. Design temps are for 95* and 59 Grains but if you design for that in Katy or Cypress you will have one upset customer, along the coast you will have mold growing on the walls.

    You must design for a 100* degree day and still be able to remove some serious humidity on a 72* day. If you go with a tax credit system with essentially oversized blower it will never provide real comfort or efficiency.

    Attention to the building envelope is critical as is attention to radiant heat as opposed to conductive heat transfer. Yes a BTU is a BTU is a BTU but how you deal with them is more important than just removing them.
    __________________________________________________ ____________________________
    “Reality is merely an illusion, albeit a very persistent one.” –Albert Einstein

    “Experience is a hard teacher because she gives the test first, the lesson afterwards.”-Vernon Law

  5. #5
    Join Date
    Sep 2010
    Location
    South Georgia
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    No one measured actual airflow. The Carrier man told me his "calculations" indicated a need for replacing the flex duct with pipe or the system would never achieve it's full capacity.

  6. #6
    Join Date
    Dec 2002
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    Houston,Tx.
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    Quote Originally Posted by SouthGaHVAC View Post
    No one measured actual airflow. The Carrier man told me his "calculations" indicated a need for replacing the flex duct with pipe or the system would never achieve it's full capacity.
    With pipe? can you elaborate on this a little? Ask him about an energy audit and see what he says, ask him if the system will still satisfy your needs, without an energy audit, and if he says yes, have him guarantee that in writing.
    __________________________________________________ ____________________________
    “Reality is merely an illusion, albeit a very persistent one.” –Albert Einstein

    “Experience is a hard teacher because she gives the test first, the lesson afterwards.”-Vernon Law

  7. #7
    Join Date
    Sep 2010
    Location
    South Georgia
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    I understand your concern about a home energy audit. I'm not a certified professional but I'm very knowledgeable around the home and believe my home to be very energy efficient. It was built in 1990 by the developer of the small neighborhood...so he did it right from the beginning. I've been all over the home and attic and made minor tweaks to improve efficiency.

  8. #8
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    Sep 2010
    Location
    South Georgia
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    Mr Bill, By pipe I mean sheet metal rigid duct instead of flex duct. Thanks!

  9. #9
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    Dec 2002
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    Houston,Tx.
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    Quote Originally Posted by SouthGaHVAC View Post
    I understand your concern about a home energy audit. I'm not a certified professional but I'm very knowledgeable around the home and believe my home to be very energy efficient. It was built in 1990 by the developer of the small neighborhood...so he did it right from the beginning. I've been all over the home and attic and made minor tweaks to improve efficiency.
    Then you "seem" set, I think it's only fair, to make folks aware that a load calc. is no silver bullet for an old home, unless there has been an energy audit. Not everyone, manages their home and attic and do tweaks to improve the envelope, as you have.
    __________________________________________________ ____________________________
    “Reality is merely an illusion, albeit a very persistent one.” –Albert Einstein

    “Experience is a hard teacher because she gives the test first, the lesson afterwards.”-Vernon Law

  10. #10
    Join Date
    May 2008
    Location
    Portland, OR
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    236
    I agree with Mr. Bill. Without testing the air flow we don't know how well the flex duct is working. It is possible that the contractor used a slide calculator to determine what size the flex run should be and found it significantly under/over-sized. Ask him to explain exactly how he came to the conclusion that it's not sized correctly. If he just 'guessed', then we're back at square one.

    If the duct system is under or over-sized, then yes, it needs to be changed or the system will not reach it's full capacity.

    Both system matches are great and comparable to each other!

  11. #11
    Join Date
    Sep 2010
    Location
    South Georgia
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    Thanks for the duct responses. The Carrier man is coming back out today to look at the duct work again. I'm make sure to ask about an air flow check.

    Any comments on the humidity control for both of these systems? Do they both use an "overcooling" to dehumidify? Is one better than the other? "Overcooling" to dehumidify sounds like it may increase my electric bill...thoughts?

    Thanks!

  12. #12
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
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    6,063
    Quote Originally Posted by SouthGaHVAC View Post
    Thanks for the duct responses. The Carrier man is coming back out today to look at the duct work again. I'm make sure to ask about an air flow check.

    Any comments on the humidity control for both of these systems? Do they both use an "overcooling" to dehumidify? Is one better than the other? "Overcooling" to dehumidify sounds like it may increase my electric bill...thoughts?

    Thanks!
    If want as much humidity control as you get by extending the cooling cycle by 3^F, you are on the right track. Under moderate cooling loads, 3^F over cool helps.
    If you want to maintain <50%RH during wet cool weather when is low/no cooling load, no way will over cooling maintain ,50%RH. A home with a couple of occupants, needs 25-36 lbs, 3-4 gals. per day of dehumidification when the outside dew point is +68^F.
    I suggest a high eff. small whole house dehumidifier as the best way of being able to maintain <50%RH during the different weather patterns that you have. WHDehus like the Ultra-Aire 65H, Honeywell 65 etc will keep a 2,500 sqft. home at <50%RH. If your a/c is set-up properly, it will be able to main <50%RH during the high cooling load weather. As the outside cooling load declines, but the outside dew point is high, the dehumidifier will supplement the dehumidification required to maintain <50%RH throughout the home. This avoids overcooling and the complex multispeeding that is unable to remove enough moisture during wet cool weather.
    Regards TB
    Regards TB
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

  13. #13
    Join Date
    May 2007
    Location
    DC Metro Area (MD)
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    3,350
    If they set the thermidistat up to do it's job correctly, it can reduce blower speed to control humidity. The TCONT803 will just overcool by a few degrees, when necessary, to drop humidity. For the Trane, for better humidity control, ask the dealer about Honeywell's IAQ thermostat. It is not the same as Trane's 803, which many believe to be the case.

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