Required Central Air Tonnage?

Discussion in 'HVAC Heating & Cooling' started by bjferri, Jul 14, 2009.

  1. bjferri

    bjferri DoD Army

    Jun 5, 2007
    Department of the Army Civilian (DAC)
    Okay - I'm going with Trane XL15i and the HVAC originally quoted me a 2.5ton unit for my 1400 square foot home. I want 16 SEER and this tax break. Weeks later after all the negotiating muck he came over with the contract and before we signed, I pointed out the contract stated 3 tons, not 2.5. Even though it is a typo, he's offering me now the 3-ton, over the 2.5, at no extra charge. Do I take it? Am I sacrificing anything taking it?

    Thanks in advance!
  2. jimbo

    jimbo Plumber

    Aug 31, 2004
    San Diego, CA
    Sounds like they are playing fast and loose with the tonnage number. Did anyone do a manual J heat load/heat loss calculation to see what you really need?

    Shooting from the hip, 2½ sounds a little small for 1400 sqft, but a lot depends on the house and where you are.
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  4. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Sep 2, 2004
    Retired Systems engineer for defense industry.
    New England
    The worst thing you can do is have one too big. You won't know what the 'right' size is unless they actually run the calculations, and x btu per sq ft rough estimate is NOT the way to do it. There's a fairly well defined science about this. It takes into account the insulation, window location, quality, orientation, size, square footage, ceiling height, and the local temperature and humidity level extremes plus, the temp you want to keep the house. Only then will you know if you are getting a bargain. 3 may be right, 2.5 could be too big, if you have really good windows and insulation. Or you might need a bigger one!
  5. FloridaOrange

    FloridaOrange Plumbing Designer

    Dec 2, 2005
    Plumbing Designer
    SW Florida
    As said above calculations need to be done to see what is right for your house. Too big isn't good. A system that is too big will cycle on/off more, it can cool very fast but it won't pull any (or enough) humidity out of the air, this could lead to mold problems. 1/2 ton difference may not be too bad but you should have a correctly sized system.

    From an air control system (obviously not from $$ standpoint) the best system is one that would run all or almost all the time.
  6. Dana

    Dana In the trades

    Jan 14, 2009
    What they said- run a real calculation!

    Trom both a comfort & efficiency point of view you're better off being a ton low than a ton oversized. If undersized it'll dry the air out (the "latent load") on the few hours/days where it won't keep up with the load. If oversized it'll chill the temp faster (the "sensible load") without drying it out as much.

    In the mid-Atlantic region latent loads are typically several times the sensible loads, so if your Manual-J calculations (which factors in both sensible & latent load), ends up between sizes, erring to the small side is almost always the right thing to do, since a 100% duty cycle improves the latent load cooling factor. On those hazy days where it's just a wicked-muggy 80-85F outside an undersized unit will definitely "feel" better since by running longer cycles it'll be dealing with the humidity better.

    Also, Manual-J isn't perfect- there's some fudge-factor oversizing built-in. If you're planning to do any significant upgrading to the insulation or tightness of the house, an ideal-size for the existing condition of the house may prove oversized after envelope upgrades. If that's in your plan, be sure to run the Manual-J on the anticipated, not current condtions. In the size range of compressors you're talking, undersizing it a half-ton from Manual-J is usually "safe" all the way around. A full ton undersized might not always cut it, but it should most of the time.

    Undersizing generally reduces wear & tear on the unit too, since the bulk of the wear is at startup, and the fewer cycles it experiences, the longer it will last.
  7. PEW

    PEW DIY Senior Member

    Sep 2, 2004
    Check out the rebate to be sure, apparently the qualifications were strangely written (big surprise).

    I am updating my home system with a Trane XL15i 4 ton, does not qualify for a tax rebate.
    Last edited: Jul 16, 2009
  8. Thatguy

    Thatguy Homeowner

    Aug 27, 2008
    A bounty hunter like in "Raising Arizona"
    Kinda' late here. . .

    Last edited: Dec 18, 2009
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