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  1. #1
    Join Date
    Aug 2011
    Location
    Boston, MA
    Posts
    13

    Discussing Manual J Sizing Calc with HVAC Installer

    Thank you all for your responses earlier related to my concerns over installing undersized equipment. I think I am going to go with the smaller 2 ton condensor and see if I can make any improvements to the existing ductwork without tearing the house apart. It also seems like 15 SEER vs 16 might make more sense as well.

    I am meeting with my installer tomorrow to go over the manual J-sizing but don't know much about the calculation except that it tells me what size cooling and heating system I should be running.

    Are they any questions I should be asking him when I go over the calc (I know this seems silly to some but I don't know much about HVAC)?

    Also I heard the systems need to match in output (i.e. if I have a 2 ton condensor but the furnance is 3 ton is that a problem)?

  2. #2
    Join Date
    Dec 2008
    Location
    North Carolina
    Posts
    2,858
    The best advice I have is to download this and have it printed at meeting: https://www.acca.org/Files/?id=177.

    And do not be afraid to go down the checklist and ask if it applies to you or not. Checklist is pretty good on explaining things, what you aren't clear on...then it is a great question to ask your contractor you are considering, they should definately not feel uncomfortable in discussing these things.

  3. #3
    Join Date
    Aug 2011
    Location
    Eastern South Dakota
    Posts
    28
    on the matching size, the indoor evap needs to be at least as big as the outdoor unit but having a inside coil that is one size larger will work the biggest advantage to that is you will get better dehumidification it usualy will help some with your seer rating.

  4. #4
    Join Date
    Jan 2011
    Location
    Metro Atlanta
    Posts
    777
    Quote Originally Posted by bryantman21 View Post
    on the matching size, the indoor evap needs to be at least as big as the outdoor unit but having a inside coil that is one size larger will work the biggest advantage to that is you will get better dehumidification it usualy will help some with your seer rating.
    I thought there could only be a half ton difference between outdoor coil and the indoor coil.

    I will have to check on that one when I get to school tomorrow.

    I also thought that if the seers do not match up that you will only get the value of the lowest rating. So if you have a 13 seer indoor coil and 10 seer outdoor unit, I thought you would only get the value of the 10 seer

    I will have to check on that one tomorrow as well.

    I have the program director for two classes this semester, so I will see what he has to say.
    The opinions expressed by me are not that of my employer.


    insulation modern marvels
    http://www.youtube.com/watch?v=g37riSkyZzM

  5. #5
    Join Date
    Aug 2011
    Location
    Eastern South Dakota
    Posts
    28
    Going a ton larger is just fine with all manufactures I have delt with. As for the seer rating a 16 seer unit for example, that's the best case scenario rating but if you look up the ahri ratings for a condensor evap and blower unit together you usually don't hit the max seer rating and a larger evap can help with that ( not all cases).

  6. #6
    Join Date
    Feb 2010
    Location
    Cal
    Posts
    1,596
    Equipment is rated at 80* indoors, so if you expect to maintain a lower temp, now would be the time to bring it up.

  7. #7
    Join Date
    Dec 2005
    Location
    Cincinnati, Oh
    Posts
    5,042
    Tonnage doesn't matter on the coil.
    Instead of using a rule of thumb, just look up the rating (as bryantman has said) with AHRI. Thats the best way to determine if a coil is rated.

    Bryantman, there are some that would argue your statement about having a large coil providing more dehumidification is incorrect.
    The idea with a "normal" sized coil would be a lower coil temp, thus being much more below the dew point, thus providing additional dehumidification.
    "Better tell the sandman to stay away, because we're gonna be workin on this one all night."

    "Dude, you need more than 2 wires to a condenser to run a 2 stage heatpump."

    "Just get it done son."

    Dad adjusted

  8. #8
    Join Date
    Dec 2005
    Location
    Cincinnati, Oh
    Posts
    5,042
    Quote Originally Posted by NY2GA01 View Post
    I thought there could only be a half ton difference between outdoor coil and the indoor coil.

    I will have to check on that one when I get to school tomorrow.

    I also thought that if the seers do not match up that you will only get the value of the lowest rating. So if you have a 13 seer indoor coil and 10 seer outdoor unit, I thought you would only get the value of the 10 seer

    I will have to check on that one tomorrow as well.

    I have the program director for two classes this semester, so I will see what he has to say.
    ARI: 4594726

    Thats a ASZ18036, (3ton)with a CAPF4961 coil. One could say that the coil is "rated" up to 4-5 tons. On top of that, the odd numbers indicated it's a beefy coil. Has an extra row. 18 seer rating.

    ARI:3068859

    Thats a SSZ14036, with a ASPF4260 air handler. One could say that the coil in the air handler is rated for 5 tons. 15 seer.

    The old way of thinking, was: Find AC tonnage. Find a coil of the same tonnage, that would fit in the plenum. Don't worry about seer.

    Things have changed. Just because the "tonnage" is the same, doesn't mean that your going to get the correct seer out of the unit. You can usually get away with that method if your just slamming in 13 seer equitment, but you still have to watch out.

    For instance, an ASZ18036 (18seer 3ton) won't match up with a CAPF3636, even though they are the same tonnage.
    "Better tell the sandman to stay away, because we're gonna be workin on this one all night."

    "Dude, you need more than 2 wires to a condenser to run a 2 stage heatpump."

    "Just get it done son."

    Dad adjusted

  9. #9
    Join Date
    Aug 2011
    Location
    Eastern South Dakota
    Posts
    28
    Maybe I am thinking wrong but larger coil larger txv and orfice, the more freon that can boil off giving you a more constant coil temp under high load?

  10. #10
    Join Date
    Jul 2008
    Location
    Rochester NY
    Posts
    4,729
    Carrier bryant matches coils up to 1 extra ton on communicating equipment. More surface area gains a little efficiency. May also grab a little moisture, irrelevant with evolution since it dehumidifies so well. If that unit doest keep the house comfy you need a separate dehumidifier.
    Which makes more sense to you?
    CONSERVATION - turning your thermostat back and being uncomfortable. Maybe saving 5-10%
    ENERGY EFFICIENCY - leaving your thermostat where everyone is comfortable. Saving 30-70%

    DO THE NUMBERS! Step on a HOMESCALE.
    What is comfort? Well, it AIN'T just TEMPERATURE!

    Energy Obese? An audit is the next step - go to BPI.org, or RESNET, and find an auditor near you.

  11. #11
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,295

    Thumbs up checklist at ...

    Quote Originally Posted by DGIO-Not View Post
    The best advice I have is to download this and have it printed at meeting: https://www.acca.org/Files/?id=177.

    And do not be afraid to go down the checklist and ask if it applies to you or not. Checklist is pretty good on explaining things, what you aren't clear on...then it is a great question to ask your contractor you are considering, they should definately not feel uncomfortable in discussing these things.
    https://www.acca.org/Files/?id=186
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  12. #12
    Join Date
    Aug 2011
    Location
    Boston, MA
    Posts
    13
    Quote Originally Posted by DGIO-Not View Post
    The best advice I have is to download this and have it printed at meeting: https://www.acca.org/Files/?id=177.

    And do not be afraid to go down the checklist and ask if it applies to you or not. Checklist is pretty good on explaining things, what you aren't clear on...then it is a great question to ask your contractor you are considering, they should definately not feel uncomfortable in discussing these things.
    Thanks Designer Dan. This is a great checklist. I will go over this with my contractor tonight.

  13. #13
    Join Date
    Aug 2011
    Location
    Boston, MA
    Posts
    13
    Thanks for all the advice. I will make sure the equipment is AHRI rated and discuss it with the contractor to make sure the coil and condensor match up properly for the best ratings.

    Any comments on the gas furnance tonnage matching up with the coil and condensor? Do they have to be the same. The current proposal has a 2 ton condensor and hopefully a matching coil with a 3 ton 60K BTU variable speed two stage A/S 96.7% furnace. The coil and condensor are also American Standard (Allegience 16).

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