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  1. #1
    Join Date
    Nov 2010
    Location
    Central Florida
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    556

    Theories on Temp Drop Please

    Hi,

    I recently noticed my split system Carrier heat pump seems to be running more than normal this Summer, and after checking the obvious culprits it appears something abnormal may be occurring. I should probably also mention that I had a new roof put on a few months ago which has a different type of ridge ventilation that is not as efficient in letting heat escape from the attic as rapidly as before. The AHU is inside the home and the ducts are in the attic (the round flex type, coil core with insulation wrapped by black plastic).

    The Delta-T looks within reason, about 22 degrees. However, when I measured the vents closest and furthest from the AHU, it yielded strange results. The closest vent (about a 5 foot run) measured 5 degrees warmer than at the AHU, while the furthest vent (about 50 feet from the AHU) measured 7 degrees warmer than at the AHU. It seems reasonable that the delta between the two vents is about 2 degrees. HOWEVER, clearly there is a 5 degree increase occurring in both vents that isn't making sense to me, and my best guess is the "duct juncture box" in the attic which the AHU connects directly to must have a leak that is sucking in enough hot attic air to cause a 5 degree increase in the air being blown throughout the home. The "duct juncture box" is made of duct board and mastic.

    Does this theory sound likely, or would attic temps of 125-135 degrees simply be to blame?

    Thanks,

    Sonic

  2. #2
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,376
    That black flex is a heat magnet. Think about it...hot attic heated primarily by radiant heat from the roof deck and a black body wrapping the flex duct.

    Did you go with a darker roof? It may not be just the ridge vents. I sure wish more people in cooling climates would get turned onto reflective roofing...the difference in comfort and energy use is amazing.

    Air leakage on the supply side in the attic would not affect a rise in supply air temp since the air would be leaking into the attic. In fact, supply side air leakage is making your house warmer and more humid by depressurizing the house. Return air leakage will add load to the evaporator, causing a loss in performance.
    • 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.

  3. #3
    Join Date
    Nov 2010
    Location
    Central Florida
    Posts
    556
    Actually, I did make the mistake of a slightly darker roof. But realize, when I say darker I'm talking just a shade darker. The new roof is gray as well but if the old roof was maybe a 5 on the gray scale, the new roof is maybe a 7. The old roof was a 3-tab shingle, the new roof has architectural shingles. And the old roof had 15# felt, the new roof has 30#. Also, due to Florida building code changes, the new roof has more nails per shingle and more deck re-nailing for hurricane reasons.

    Sounds like I need to pursue getting the roof ridge vent dealt with. The fool roofer used a roof vent that has HALF the NSF rating of old the vent. I'm sure this is part of the problem, but not yet convinced this is the primary problem since I measured a few other identical homes in the community that still have the original roofs/vents and their temps seem similar. Although just by sheer feel, it seems my attic is noticeably more hot and stuffy than before. However, as you implied, it is possible the new roof is more the cause of this than the ventilation situation.

    Either way, I am getting way too much heat induced into the ducts somehow. At peak day time, when the AC cycle is off for 10 minutes or so and then starts back up, the air coming out of the vent furthest from the AHU is about 90 degrees initially! For about 5 seconds, and then gradually cools, but it takes a good 5 minutes before it is down to where it should be. I'm now starting to get worried I will have mold build-up soon in the ducts due to the constant temperature variances in between cycles. And none of this explains why the vent nearest the AHU (only about a 5 foot duct run) is also 5 degrees more than at the AHU. This is what had me suspicious of a juncture box leak right above the AHU.

    I've also pondered maybe insulating the ducts more?? Any guidance on a direction would be appreciated.

    Sonic

  4. #4
    Join Date
    Feb 2013
    Location
    Sonora, California, United States
    Posts
    1,179
    how are you pulling temp measurement at the AHU? and at the ducts?

  5. #5
    Join Date
    Nov 2010
    Location
    Central Florida
    Posts
    556
    Quote Originally Posted by jacob-k View Post
    how are you pulling temp measurement at the AHU? and at the ducts?
    I used a digital probe-type thermometer, nothing high-quality but adequate enough for simple troubleshooting purposes. It's a fast-reaction type so I am able to watch temperature fluctuations. Checked measurements at the intake and exhaust of the AHU. (On the exhaust side it was inserted into the duct immediately above the AHU in the same hole the installers originally used to measure Delta-T) At the registers I simply inserted the probe into the grills (which by the way are in the ceiling in case that wasn't obvious)

  6. #6
    Join Date
    Feb 2009
    Location
    Tallahassee, FL
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    6,051
    Mold only grows in cool dark damp places. Sounds like your attic is hot and mold doesn't like hot.

  7. #7
    Join Date
    Jan 2009
    Location
    Keokuk, IA
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    5,520
    I wouldn't ever worry about mold in a vented attic. Too hot. Maybe a vented attic with a cool roof. That's why when you have a cool roof design, you're best to just seal the attic, use a vapor barrier underlayment (required with a metal roof anyway, and IMO should be mandatory in a hurricane climate) and insulate under the roof deck.

    If you have adequate soffit vents, then yes, reducing the ridge vents free area will reduce air movement in the attic. A few shades darker shouldn't be that big of a diffrence. However... were there multiple layers on the old roof? 2 or 3 roof layers might have acted as a insulator.


    In a humid climate, a 22F delta T is pretty good if you have clsoe to 400CFM/ton. Depending on how well sealed you attic pentrations are (ceiling fans, exhaust fans, light fixtures... and the worst... recessed lighting), and insulation, much of your heat gain is going ot be through radiation through the ceiling.


    I'm thinking that the ridge vent is the first place to look. DId they repalce hte plywood decking too? If so, I wonder if they cut or left a large enough gap at the peak. It's very possible to incorrectly install a ridge vent.

  8. #8
    Join Date
    Mar 2009
    Location
    Mount Holly, NC
    Posts
    3,536
    on the ridge vent, is the roof designed to have a ridge vent, or is your house equipped with BOTH ridge, and gable vents? it's gotta be one or the other, not both...
    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
    Join Date
    Mar 2009
    Location
    Mount Holly, NC
    Posts
    3,536
    also, on the flex duct insulation up there, is it marked what R value the ducts are insulated with?

    I am ALWAYS amazed at the number of houses with BLACK plastic insulated ductwork installed in attics... I mean it makes ZERO sense. maybe in cold climates where Heating is the PRIMARY design of the system, but for A/C... RADIANT heat is the biggie in an attic. and black absorbs radiant. ugh.
    The TRUE highest cost system is the system not installed properly...
    The three big summer hearththrobs...
    Mel Gibson
    Dwane Johnson
    The A/C repairman

  10. #10
    Join Date
    Feb 2013
    Location
    Sonora, California, United States
    Posts
    1,179
    Quote Originally Posted by vstech View Post
    also, on the flex duct insulation up there, is it marked what R value the ducts are insulated with?

    I am ALWAYS amazed at the number of houses with BLACK plastic insulated ductwork installed in attics... I mean it makes ZERO sense. maybe in cold climates where Heating is the PRIMARY design of the system, but for A/C... RADIANT heat is the biggie in an attic. and black absorbs radiant. ugh.
    this is an interesting concept as I would have never considered the thin black layer of plastic barrier to actually absorb heat so much it could make it through the layer of insulation and into duct and cause air temp problems. I cant say ive every ran into a problem with this and it was the dominant ducts ran in my area from around 98 to 2010.

  11. #11
    Join Date
    Mar 2009
    Location
    Mount Holly, NC
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    well, insulation is a TIME barrier to conduction heat transfer, radiant heat passes through it with ease.
    Quote Originally Posted by jacob-k View Post
    this is an interesting concept as I would have never considered the thin black layer of plastic barrier to actually absorb heat so much it could make it through the layer of insulation and into duct and cause air temp problems. I cant say ive every ran into a problem with this and it was the dominant ducts ran in my area from around 98 to 2010.
    The TRUE highest cost system is the system not installed properly...
    The three big summer hearththrobs...
    Mel Gibson
    Dwane Johnson
    The A/C repairman

  12. #12
    Join Date
    Feb 2013
    Location
    Sonora, California, United States
    Posts
    1,179
    Quote Originally Posted by vstech View Post
    well, insulation is a TIME barrier to conduction heat transfer, radiant heat passes through it with ease.
    well when you put it that way it makes alot of sense

  13. #13
    Join Date
    Nov 2010
    Location
    Central Florida
    Posts
    556
    Thanks for the replies thus far. Lots of questions, will try to answer them all here....

    No, roof was not layered before, always and still is a single-layer shingle, single felt underlayment.

    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.

    The ducts say "Thermaflex" and R6.0 (R6)

    There is only a ridge vent, no gable vents or other exhaust ventilation. And of course soffit vents for intake, which I have checked and are open, not blocked. The ridge vent is properly installed, not blocked. The decking is cut properly, the problem is the ridge vent used. The lazy contractors down here always opt for the cheapest, quickest thing unless you have enough knowledge to force them otherwise. Even if you are wiling to pay, they act like it's a hassle and STILL like to resist. What they did was replace an aluminium ridge vent having the typical 18 NFA with a plastic low-profile vent that has only 11 NFA. It is cheaper and can be installed with a nail gun. Cheap+Lazy=Homeowner get shafted. In spite of trying to be educated about any given job, it is just not possible for homeowners to know enough about every detail to be able to protect themselves here in Florida. I COULD go fight the roofer and force them to do something about the vent, but that introduces a whole new level of issues, they will probably screw something else up or damage the roof walking around in this kind of Summer heat. Before I do anything like that I wanted to eliminate all other potential causes and/or look into other solutions.

    I measured temps today again, and once again, there remains a 5 degree increase in the temps between the AHU and even the shortest duct run. I cannot believe this is simply radiant heat and am thoroughly perplexed as to what is going on. If it were primarily radiant heat penetrating the ducts, it would then be logical to assume the longer duct runs (50 ft) should show a much larger increase - but they do not. The temp difference between the longest and shortest runs are only about 2 degrees. There remains an initial 5 degree increase that is unexplained, which ALL ducts are incurring, no matter the distance.

    Sonic

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