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Central A/c sizing question

My three ton Goodman is due for replacement after 15 yrs of service and because we have increased our cooling space from 1500sqft to 1900 sqft.

The three ton can cool the house on the hottest days but only with extremely long and frequent run times.

I think a 3.5 ton would be adequate but two large contractors in my area are recommending 4 tons just by looking at the house.  The contractor who installed the 3 ton did the same thing and nailed it.  They say, I'd be happier with the four ton on a real hot day but I don't want to pay for a four ton or the power it will consume over it's life time if a 3.5 ton will do.  There are only 7-14 "really hot" days here in NC in the summer.  We hardly ever run a/c during the night and only during the waking and evening hours.

Please help me make the right choice.  I don't believe in bigger is better for the long haul.  maybe I am wrong.

Comments

  • Techman
    Techman Member Posts: 2,144
    Sizing

    Hay Joe,bigger than required is worse. Bigger = less run time and that=higher indoor humidity. Get a load estimate done . Your new area is 400 sq.ft. and generally speaking a half ton is not enough.but. real good insulation,vapor barrier, northern or eastern exposure, shaded by trees,etc, MIGHT do the trick . And if the 3t is slightly oversized then the 3.5 would do the job .Also consider a two speed or two stage system w/ a variable speed indoor blower .
  • Joe V_2
    Joe V_2 Member Posts: 234
    That's my line of thinking...

     On a scale of 1-10 the insulation is an 8.  It just seems to me that with a house that is close to 2000sqft that cools well with a three ton shouldn't require a 30% larger unit.
  • JStar
    JStar Member Posts: 2,752
    ...

    I agree with Techman's suggestions, and also...



    If you have the right CFM throughout the house, you'll be safer with a conservative cooling estimate. That way it will run longer and remove more humidity, and maybe even feel more comfortable at a higher temperature setting. I would even run the numbers on a 3.5 ton condenser with a 4 ton coil, TXV, and variable speed blower.
  • Eugene Silberstein
    Eugene Silberstein Member Posts: 1,380
    MANUAL J, MANUAL J, MANUAL J!

    Since you are changing out your system, you owe it to yourself to do it correctly. Rules of thumb no longer cut it! Have a professional come in and do a complete Manual J on your home. This will give you both the heat gain and heat loss of your home.

    As Terry (Techman) mentioned, bigger is definitely NOT better. Put in what you need... nothing more, nothing less.
  • Joe V_2
    Joe V_2 Member Posts: 234
    That's my problem

    Neither contractor performed a heat load/loss calculation.  They just paced off the perimeter, looked at the ceiling and said 4ton.

    I know it takes time.  And, my house is unusual with a granite exterior (who knows what the r-factor of granite is?).  But the red flag was when I explained that the existing a/c cools well but is old, the guys both skipped a size. 

    I'll try a couple more contractors I guess.  Thanks for your advice.
  • ronsick
    ronsick Member Posts: 2
    ronsick

    I am a hvac contractor that is now handcapped and all I do now is manual j 8 verzion heat load and cooling load and ductwork sizing if you want me to do it call me 304-754-0099
  • ronsick
    ronsick Member Posts: 2
    ronsick

    I am a hvac contractor that is now handcapped and all I do now is manual j 8 verzion heat load and cooling load and ductwork sizing if you want me to do it call me 304-754-0099
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