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Thread: A/C unit size

  1. #1
    Join Date
    Oct 2005
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    I'm in the process of getting estimates on replacing heating and cooling system. My current A/C unit is a 18 year old 3.5 ton unit and it had a hard time keeping the inside temp at 78 during the hottest parts of the past summer. A few contractors spent a great deal of time measuring and looking around, but only one has done a load calc while at my house and he said that I would need a 3.5 - 4.0 unit. He said that it was borderline (43,000 btu?) for using a 3.5 ton and would recommend a 4.0 ton unit. Would oversizing by .5 be that big of a deal?

  2. #2
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    Apr 2004
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    As long as the contractors numbers are correct, 43,000 is borderline. A 3.5 ton unit is rated at 42,000 btuh while a 4 ton is 48,000 btuh. Depending on your summer temps, on the extreme days a 3.5 ton may not keep up. I don't see where upsizing to the 4 ton would hurt anything. The other option would be to up-size the evaporator coil .5-1 ton bigger and change the metering device accordingly and keep the 3.5 ton air conditioner.
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  3. #3
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    Mar 2004
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    Few "four ton" units are actually rated for a true 48,000 BTU. Many will run around 45,000. If the load calc is correct, most four ton setups wouldn't be big enough to really qualify to be called oversized.

  4. #4
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    Nov 2005
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    Ditto...I agree. Make certain the calcs are done with your comfort level and hottest summer day in mind.
    The posts and comments made by me are in no way affiliated with any company or organization. They are simply my personal opinions.

  5. #5
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    Oct 2005
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    26
    I did an online calc and it came out pretty dang close to what the one guy told me, so I guess I shouldn't even condsider the guys who are quoting 3.5 tone units (almost all of them). Is there a huge price difference between 3 & 4 ton units?

  6. #6
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    Nov 2005
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    USA
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    No, not in the same category. I would go with 13 seer min and variable speed if you can.
    The posts and comments made by me are in no way affiliated with any company or organization. They are simply my personal opinions.

  7. #7
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    Nov 2005
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    BAKERSFIELD CA.
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    im gonna assume your house is 18yrs old some of the things you need to think about are up grades to the house ha any one put money into windows or more insulation in the attic (doesnt sound like it )like the 2 other guys said up a half ton dosnt hurt but you do need to consider the duct sizing if your gonna do something like that allot of times the duct has been sized at the min and that can bee a problem for return noise and proper air flow so you get a proper exchange across the evap and heat exchangers 4 ton a 18 return should do what you need and put a good barface grill on instead of the stamp face

  8. #8
    Join Date
    Nov 2004
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    SW FL
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    Question Load Estimate of What

    Originally posted by gtownguy

    A few contractors spent a great deal of time measuring and looking around, but only one has done a load calc while at my house and he said that I would need a 3.5 - 4.0 unit. He said that it was borderline (43,000 btu?) for using a 3.5 ton and would recommend a 4.0 ton unit. Would oversizing by .5 be that big of a deal?
    Is the 43,000 BTUH for the SENSIBLE Cooling Load only?

    4-TON UNIT will only provide ~36,000 BTUh SENSIBLE
    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

  9. #9
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
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    4,311

    Lightbulb

    Do you think that the 18yr old unit is still producing 3.5 tons of cooling?
    I'd go with the the energy savings of the 3.5 ton, worst case it's a wee bit warm inside on a 100 degree afternoon. If you did the calculation with blinds open or applainces running you could always do your cooking/drying at a cooler time of the day and close the blinds when it's hot outside

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