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  1. #14
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
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    11,325
    Here's a few things for you to consider:

    • The "duct junction box" is properly called a "plenum". And if you measured on the cold side of the air handler, it's called a "supply plenum".
    • Air does not mix equally within a plenum like this. Every cooling coil has what is known as "coil bypass factor". This is air passing through the cooling coil that does not drop in temperature or humidity, or at least to the levels we want for air conditioning comfort.
    • What you may be measuring in your short and long run ducting is a combination of thermal stratification, radiant heat gain, and uneven mixing of air in the supply plenum. Without taking a lot more measurements all over the attic, this is my educated estimation of what's going on.


    And yes, before anyone asks, air in motion can thermally stratify. Meaning you can have moving air within a duct at one temperature at a lower level, and another temperature at a higher level, or vice versa. The first time I ever measured this and figured out what it was, I was blown away, pardon the pun.
    • 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.

  2. #15
    Join Date
    May 2013
    Posts
    41
    This is the Ask Our Pro's forum, and only Pro members that have been vetted by the AOPC may post advise, commentary, or ask questions of the OP here.

    You can find the rules for posting and qualifications here.

    Further infractions may result in loss of posting privileges.
    Last edited by beenthere; 07-12-2013 at 08:37 PM. Reason: Non Pro * member

  3. #16
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
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    11,325
    Quote Originally Posted by mankid View Post
    Regarding the color of the flex duct covering, I am no expert, but I thought black/dark colors were only warmer than lighter colors when directly exposed to a light source. ie sunlight. If in a dark surrounding, dark and light colors would absorb ambient heat the same. So in a dark attic the black material will be no warmer than a light material. If sunlight is shining on them then the dark material will certainly be warmer. Someone please correct me if I am wrong.
    Your post won't survive due to your non AOP Pro status, but I will answer it anyway.

    Mean radiant temperature is measured by a thermometer stuck inside a black globe. Not a white or blue one...it's black. Heat waves are in the infrared band...it's not visible light but it is still a form of wave energy, as is visible light. Therefore the black body is still a factor in heat transfer. It is also why radiant barriers work in the dark. Think about a thermos bottle. With the lid on, the volume inside the bottle, all shiny material, is completely in the dark.
    • 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.

  4. #17
    Join Date
    Jun 2010
    Location
    Philadelphia PA
    Posts
    2,190
    The mold concern I have is not in the attic it is concern for mold growing in the DUCTS. Since the heat inside the ducts seems to be swinging from 90+ (or more) in between cycles and then back down to about 55-60 degree range, that swing is probably going to cause mold, at least is what I'm thinking based on what I've seen in Florida. Such a quick temp differential causes moisture build-up, and being in a dark place like ducts it will then fester and grow.
    I think you'll find that you need LIQUID WATER aand Food ( dirt) to grow BUGS ( mold) and the temp range is not the controlling factor
    You have got to learn from other people's mistakes! Because God knows you don't live long enough to make them all yourself !!!!!!!!

  5. #18
    Join Date
    Nov 2010
    Location
    Central Florida
    Posts
    556
    Is there anything I could do to further insulate the ducts and the supply plenum? If I were to go buy a bunch of thin bat insulation and wrap it around (or lay it over) the main line and the supply plenum, would this likely help, or might I introduce other problems? Would it hurt to try? Because it is now sounding like my two options are to either better insulate the ducts or pursue a whole other attic ventilation solution, which may not have the desired outcome.

    Actually, the supply plenum feeds into a much larger flex duct (I'll call the main line) which feeds the air to probably half the house (attaches to another plenum on the far side), the remainder being smaller, shorter lines. If I simply added insulation to the supply plenum and also wrapped that one main line, I'm guessing I'd improve the situation by a good 50%. Or is the only alternative to have the ducts REPLACED with something rated more than R6?

  6. #19
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,913
    mankid, this is the Ask Our Pro's forum, and only Pro members that have been vetted by the AOPC may post advise, commentary or ask questions of the OP here.

    You can find the rules for posting and qualifications here.

    Your post has been deleted.
    Further infractions may result in loss of posting privileges.
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  7. #20
    Join Date
    Jun 2010
    Location
    Philadelphia PA
    Posts
    2,190
    pursue a whole other attic ventilation solution,
    This might make the problem worse with an even greater supply of humid air

    Consider replacing the flex with a R* duct board trunk with short flex tie ins, that will probably have half the surface area of your spider system. adding insulation may also complicate the problem with more than one vapor barrier
    You have got to learn from other people's mistakes! Because God knows you don't live long enough to make them all yourself !!!!!!!!

  8. #21
    Join Date
    Jun 2010
    Location
    Philadelphia PA
    Posts
    2,190
    That' R8 2 inch duct board
    You have got to learn from other people's mistakes! Because God knows you don't live long enough to make them all yourself !!!!!!!!

  9. #22
    Join Date
    Nov 2010
    Location
    Central Florida
    Posts
    556
    I have one other new theory/question to run by you guys...

    I had speculated at the beginning of the thread maybe the 5 degree increase being seen at most ducts was due to attic infiltration in the supply plenum over the AHU. However, what is the chance that instead there may be infiltration at each individual duct where it connects to the ceiling vents? If the flex ducts were not properly fastened/sealed at time of installation, might this explain how attic air could be infiltrating? I do know that one of my neighbors' HVAC company made this exact finding and she paid to have all the ducts properly fastened.

    Thoughts?

    If I were to go dig through the blow-in insulation and inspect a few vents, what would clue me in as to whether I need to call for inspection.... should I see tape, mastic, zip-ties, something else?

  10. #23
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
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    11,325
    Quote Originally Posted by SonicExplorer View Post
    I have one other new theory/question to run by you guys...

    I had speculated at the beginning of the thread maybe the 5 degree increase being seen at most ducts was due to attic infiltration in the supply plenum over the AHU. However, what is the chance that instead there may be infiltration at each individual duct where it connects to the ceiling vents? If the flex ducts were not properly fastened/sealed at time of installation, might this explain how attic air could be infiltrating? I do know that one of my neighbors' HVAC company made this exact finding and she paid to have all the ducts properly fastened.
    I said earlier that supply ducts do NOT induce infiltration into them. Supply ducts are under a positive pressure, so it is difficult for air from outside the duct to enter into it.

    That said, where the ceiling supply registers connect to the "boot" (the sheet metal box where the duct enters in), there is often a sizable gap between the ceiling drywall and the metal boot. Take the grill off and you'll see this. And when you see it, SEAL IT! Mastic tape works well for this.

    If I were to go dig through the blow-in insulation and inspect a few vents, what would clue me in as to whether I need to call for inspection.... should I see tape, mastic, zip-ties, something else?
    Hopefully you'll see duct wrap before you see duct. Beneath the wrap...it's anyone's guess from here whether it's sealed or not. Not likely is a safe bet. What you want to see is mastic on all joints, including where the boot hits the ceiling, even on the attic side. But hitting it with mastic from the interior of the house works just fine if you don't want to dig through insulation.
    • 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. #24
    Join Date
    Oct 2010
    Location
    Anderson, South Carolina, United States
    Posts
    6,806
    The boot to ceiling connection, if not sealed, can cause a Venturi effect that pulls in hot attic air and mixes with the cooled air as its thrown from the diffuser. http://www.alpinehomeair.com/related/Duct%20Leakage.pdf

  12. #25
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,325
    Quote Originally Posted by jtrammel View Post
    The boot to ceiling connection, if not sealed, can cause a Venturi effect that pulls in hot attic air and mixes with the cooled air as its thrown from the diffuser. http://www.alpinehomeair.com/related/Duct%20Leakage.pdf
    This is true...and about the only place I would expect to see infiltration occurring along the supply duct chain. The rest of the air escaping into the attic from the supply duct actually depressurizes the house, causing infiltration to occur through the building itself elsewhere.
    • 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.

  13. #26
    Join Date
    Nov 2010
    Location
    Central Florida
    Posts
    556
    Update:

    Today I pulled one register and the boot/box is flush to the drywall. I also put the thermometer up into the duct, well past the boot. Same result, still gaining about 5 degrees. So the boot is not the main source of infiltration. I also went over to the other neighbors' home who's house is exactly like mine, and who had her ducts re-fastened as well, and she has the same anomaly going on, gaining about 5 degrees between the AHU supply and the register of the shortest duct run.

    So....mystery remains.

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