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  1. #27
    Join Date
    Jul 2016
    Location
    Far North Dallas, Texas
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    29
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    Thread Starter
    Dan, lkapigian,

    I finally figured out how to look up your email addresses. I just emailed the manual-J reports to you. I did not want to post the full reports on the forum since they show my home address. That's not a good idea these days.

    Dan, thanks for taking the time to run a quick comparison manual-J calculation.

    Assuming that I have a conditioned unvented spray foam attic, high tech low infiltration Low E-366 windows with average SHGC of 0.17, tighter than average building envelope (ACH <2 vs <4 for the average), can I use a 3 ton variable speed, 3 zone Carrier Greenspeed HP to cool system 2 ? For the exceptional conditions of sustained 105F outdoor temps during the day, I can overcool the home overnight and in the morning when the outdoor temp is below the design temp and use the thermal mass of the home to carry me through the peak temp hours when the outdoor temp is above the design temp.

    Is this a reasonable strategy? And will the proposed 3 ton equipment handle the load at 98F-100F ?

  2. #28
    Join Date
    Jul 2016
    Location
    Far North Dallas, Texas
    Posts
    29
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    Thread Starter
    Quote Originally Posted by eehome View Post
    Hi lkapigian, I had a similar concern and asked the contractor about this. Here is a cut-n-paste fr our email exchange on this topic:


    My Question: I noticed the choice of a 5 ton coil on a 3 ton A/C.

    My Comments: A larger coil improves sensible heat removal capacity at the expense of latent heat removal capacity.

    HVAC Contractor response: Correct.

    My comments: Based on my limited knowledge, this would be a good choice for a dry climate like New Mexico or Arizona, but here in the warm, humid North Texas environment, we need to sacrifice a bit of SEER rating and run with lower coil temperatures. This requires running the coils at a lower temperature. Smaller coils running at lower temperatures and/or with lower airflow will be much more efficient at latent heat removal. But, these are old rules of thumb. Not sure what the electronic expansion control valves, variable air speed/compressor speed capability does to the old rules of thumb.

    HVAC Contractor Response: In a non-communicating system, a dry climate is the only place where oversizing the evaporator to this extent would make sense. In a communicating system, the advantage is the ability to control airflow and vary compression capacity based on suction pressure and temperature as measured by the suction pressure transducer in the heat pump. Minimum airflow on the 3 ton model matchup is 398 CFM in comfort mode and 830 CFM in efficiency mode. The indoor airflow delivered by this system varies significantly based on outdoor temperature, indoor unit combination, and system demand. Combined with the algorithms in the Infinity control, this gives us the best of both worlds in dehumidification (latent), and comfort (sensible) cooling.
    Hi lkapigian, did the response from the HVAC contractor in quotes above answer your concerns? Or generate addition questions/concerns?

    Note: I sent the manual-J via email Friday morning.

  3. #29
    Join Date
    Aug 2013
    Location
    Visalia California 93291
    Posts
    7,133
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    Quote Originally Posted by eehome View Post
    Hi lkapigian, did the response from the HVAC contractor in quotes above answer your concerns? Or generate addition questions/concerns?

    Note: I sent the manual-J via email Friday morning.
    Check you email, there are errors in the J and I attached the expanded ratings for your system- From what I see 4 is the better fit- 3 Ton @ best case is -8% unless you take the 50% excess latent which puts you @ +1 ( sensible)

  4. #30
    Join Date
    Jul 2016
    Location
    Far North Dallas, Texas
    Posts
    29
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    Thread Starter
    Quote Originally Posted by lkapigian View Post
    Check you email, there are errors in the J and I attached the expanded ratings for your system- From what I see 4 is the better fit- 3 Ton @ best case is -8% unless you take the 50% excess latent which puts you @ +1 ( sensible)
    Hi lkapigian, appreciate the effort to review and respond. I checked my email and all I see is the automatic notification from hvac-talk that you responded to my hvac-talk forum posting. I do not see a separate email. Sorry for the delayed response, I had to go out of town for a few days unexpectedly.

    I expect to select an installer today, so I'm hoping I can see your response before finalize the equipment selection.

  5. #31
    Join Date
    Aug 2013
    Location
    Visalia California 93291
    Posts
    7,133
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    Quote Originally Posted by eehome View Post
    Hi lkapigian, appreciate the effort to review and respond. I checked my email and all I see is the automatic notification from hvac-talk that you responded to my hvac-talk forum posting. I do not see a separate email. Sorry for the delayed response, I had to go out of town for a few days unexpectedly.

    I expect to select an installer today, so I'm hoping I can see your response before finalize the equipment selection.
    Check your email, just re sent

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