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  1. #14
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    You said it was in the attic? Air leakage in the ductwork could be a source. The air leakage may correlate more to runtime than to static pressure and total air flow.

    R22 system just seemd to dehumidify better. I think they just had less surface area and less total mass.

  2. #15
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
    Posts
    4,503
    The furnace/coil is in the garage, all the ductwork is in the attic. There may be some duct leakage, but it didn't seem to affect the larger A/C as much. The new A/C cools more at night, the old A/C was about the same no matter the temperature. Once temperatures get below 90 the new unit cools about as good as the older unit that was 50% larger. It's almost as the TXV is feeding more at lower outside temperatures, the piston on the old A/C seemed to feed LESS at lower outdoor temperatures. Humidity is holding steady in the mid 40's, even with the rain we had last night.

    No 410a was added, just used the factory charge, 25' lineset. It might be a little low but I don't hear any "flash gas" at the TXV. The guy helping me do the install seemed surprised at the size of he coil for begin a 2 ton. Lots of surface area, but it's a 2 row coil with 1/4" tubing instead of the standard 3 rows of 3/8" tubes for the freon. Coil "slab" was about 1/2 the thickness of the old 3 ton coil.

  3. #16
    Join Date
    Feb 2009
    Location
    Tallahassee, FL
    Posts
    6,051
    What you stated about txv vs piston is precisely what you should notice between txv and piston.

    The cooler outside temps lowered head psi on old system which meant less refrigerant through hole.

    The txv opens up more to allow capacity at wider range of id/od temps. By far a superior device.

    Go ahead and take some readings next 100 afternoon.

    Accurate ll temp/pressure and ambient temp

    Record vapor line temp leaving indoor coil as well as condensor.

    Lets make sure valve has solid column under conditions.

  4. #17
    Join Date
    Feb 2009
    Location
    Tallahassee, FL
    Posts
    6,051
    Not to mention the added efficiency and protection of compressor hard shut off txv offers

  5. #18
    Join Date
    Jun 2005
    Location
    Near Chicago, IL
    Posts
    3,317
    Unsealed ductwork in an attic is a killer- it can wipe out 30% of your capacity.

    You should check your return temperature at a far room (in the room, next to the grille) and again at the unit. Do the same for the supply.

    Odds are, you will see high superheat with your gauges.

    The resi stuff I have worked with is all pre-charged for a 15' lineset, so if the OEM got it right, you are low on charge.

    Even builders level equipment needs to be charged properly, and higher SEER stuff is even more picky.

  6. #19
    Join Date
    Oct 2010
    Location
    Anderson, South Carolina, United States
    Posts
    7,750
    If the duct was sized for 1200 cfm and now you have 800cfm the velocity is lower so the cooled air stays in the ducts longer and gains more heat from the attic, this could be the reason for the 14f drop vs 18f drop.

  7. #20
    Join Date
    Feb 2009
    Location
    Tallahassee, FL
    Posts
    6,051
    Quote Originally Posted by jtrammel View Post
    If the duct was sized for 1200 cfm and now you have 800cfm the velocity is lower so the cooled air stays in the ducts longer and gains more heat from the attic, this could be the reason for the 14f drop vs 18f drop.
    Good point.

  8. #21
    Join Date
    Nov 2011
    Location
    Phoenix,AZ
    Posts
    345
    I would certainly check the refrigerant charge I always do on a split system no matter what the length of the line set is

  9. #22
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
    Posts
    4,503
    I would think if much attic air was leaking into/out of the ductwork a 2 ton A/C wouldn't be able to keep a 75-77 degree inside temperature on a 100 degree day with a 1600sqft house. Either that or the "500sqft per ton" rule so commonly used in our area needs to die immediately for homes under 20yrs old. Part of me thinks builders oversize to cover up crappy installation practices.

    The duct was "Sized for 1200CFM", but I don't think it ever got that much. 2 return of 12" diameter flex, one is about 20' long, the other 10' long. I don't see those hitting 600CFM each. I do think I'm really getting about 900CFM according to the blower curve in the furnace specs.

    On 100 degree afternoons the suction line is warm enough that it doesn't sweat, but when temps get below 95 it sweats. Below 90 and it feels cold. Discharge air temperatures drop dramatically as outdoor temperatures drop. At 100+ ambient I'm surprised the volume of hot air coming out of the 2 ton condenser, seems to be twice what a typical 2 ton condenser does.

  10. #23
    Join Date
    Oct 2010
    Location
    Anderson, South Carolina, United States
    Posts
    7,750
    What r value is the duct wrap? If its 4.2 it will help a good deal to change to r8 with duct in attic, while you're at it you can shmear some pooky on all the joints to seal it up

  11. #24
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
    Posts
    4,503
    Quote Originally Posted by jtrammel View Post
    What r value is the duct wrap? If its 4.2 it will help a good deal to change to r8 with duct in attic, while you're at it you can shmear some pooky on all the joints to seal it up
    It's standard R-6, some pooky wouldn't hurt it though. I figure if the ductwork was that bad it wouldn't be able to keep up @ 800sqft per ton. Most units around here are sized 500sqft per ton...

  12. #25
    Join Date
    Oct 2010
    Location
    Anderson, South Carolina, United States
    Posts
    7,750
    Quote Originally Posted by 54regcab View Post
    It's standard R-6, some pooky wouldn't hurt it though. I figure if the ductwork was that bad it wouldn't be able to keep up @ 800sqft per ton. Most units around here are sized 500sqft per ton...
    I dont think its leaking badly but 99% of duct systems leak, lots leaks 25-50% of capacity of system. It may keep up even better and cycle on a 100f day at 76f set point even more disproving the rule of thumb and all you need is a bucket o mastic. Some of the worst leaks and easiest to fix are the boot to floor/ceiling/wall connections.

  13. #26
    Join Date
    Jan 2004
    Location
    PA
    Posts
    68,759
    Recheck your SC on the next 95 degree you get a chance to.
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    How many times must one fix something before it is fixed?

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