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  1. #1
    Join Date
    Jul 2012
    Location
    Iowa
    Posts
    6

    sizing -- J needed?

    I am about to replace my 2.5 ton carrier unit. Spec is a new
    infinity carrier unit, price is reasonable based on other estimates for
    a new condenser, inside coil / lines and install. Probably be at 14.5
    as the new furnace is not being replaced. Old unit was a '99 410 unit,
    he estimated 10 seer. Living in Minnesota, probably on 1/4 of the days
    in the summer.

    After I gave him the upper level square footage, the HVAC guy started talking 'we know your house size type -- its a standard split level -- no need to do a standard J calc on my laptop.

    This was based he said that I don't use continually during the summer, so
    quicker cooling would be an advantage over lower humidity with a 2 ton
    unit.

    How much do customer use patterns / preference go into selecting the
    tonnage for a unit. That is, is there some fudge as to a 2 or 2.5 unit, or
    should I demand to go strictly by the J report?

    Thanks -- you expertise is greatly appreciate!

  2. #2
    Join Date
    May 2012
    Location
    San Diego, CA
    Posts
    352
    Have him do a calculation before undersizing your unit. He might end up with a 2 ton... so then his assumptions are backed up by science .

    Also, oversizing your unit is not good, but undersizing your unit is also just as bad. It shortens your unit's life and if you can only hold 80 F, then you might not be comfortable.

    Not all standard split levels are the same. Even houses of the same model and in the same tract have different orientations and sun exposures. Usually many of the older units are oversized but a little due diligence is never a bad thing.
    You can call me Sam

    It should be a crime to be a mechanical engineer in San Diego
    Summer Design Temperature: 83 F Dry Bulb ~ 69 F Wet Bulb (California Climate Zone 7)

  3. #3
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    33,898
    What's the point of spending the big bucks on an Infinity only to cripple it with the old furnace?

  4. #4
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    4,872
    In MN with long cold winters, I'd get a Home Energy Efficiency Audit & do what I could to reduce winter heat-loss. Then U can have the Manual j performed.

    That will save utility costs the year-round.

    Depending on the amount of airflow (1200-CFM or more) required for heating, you may need to up-size the tonnage of the indoor coil above the condenser's if the J then calls for 2-Ton.

    I'll leave your other questions for others to respond to.

  5. #5
    Join Date
    Jan 2012
    Location
    indy
    Posts
    471
    Quote Originally Posted by udarrell View Post
    In MN with long cold winters, I'd get a Home Energy Efficiency Audit & do what I could to reduce winter heat-loss. Then U can have the Manual j performed.

    That will save utility costs the year-round.

    Depending on the amount of airflow (1200-CFM or more) required for heating, you may need to up-size the tonnage of the indoor coil above the condenser's if the J then calls for 2-Ton.

    I'll leave your other questions for others to respond to.
    Im not sure what upsizing the coil deos for heating plz inform me???

  6. #6
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    4,872
    Quote Originally Posted by hvacmike85 View Post
    Im not sure what upsizing the coil does for heating plz inform me???
    Well, if your A/C would only need a 2-Ton indoor coil it's top airflow rating is 900-CFM, if heating required 1200 to 1300-CFM you would need a 3-Ton evaporator coil, for that 2-Ton condenser, to flow that CFM Volume; comprehend...

  7. #7
    Join Date
    Jan 2012
    Location
    indy
    Posts
    471
    Quote Originally Posted by udarrell View Post
    Well, if your A/C would only need a 2-Ton indoor coil it's top airflow rating is 900-CFM, if heating required 1200 to 1300-CFM you would need a 3-Ton evaporator coil, for that 2-Ton condenser, to flow that CFM Volume; comprehend...
    Oh ya I got ya here in indiana winters and summers are about equal so I've never had to do that, I've heard upsizing the coil can help dehumidify faster/better but I dont have facts to prove it, seems to work on a few I did. maybe that was for an orifice though?

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