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  1. #1
    Join Date
    Oct 2019
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    Replace upstairs or downstairs AC?


    I am new to the forum. I am replacing one of my AC units in my home to try and lower my energy bills and remove potential for any future system failures. We are paying between $250 to $350 on electricity each month! I live in upstate South Carolina. I am torn on whether to replace the upstairs or downstairs unit.

    According to HVAC contractors both units have issues.
    Downstairs is Trane 2 Ton heat pump:
    - requires adding a plenum to air handler return
    - needs additional return to get the CFM required for system. Currently only have one return (14x25 grate) which has a 13" diameter flex duct.
    - system is 10 SEER and seems to have a good cooling differential. Never have had major problems with the unit
    - 12 supply vents
    - cooling 1200 sq ft with vaulted ceilings in living room and foyer

    Upstairs unit is Trane 2 Ton heat pump:
    - requires adding a plenum to air handler return
    - two returns, one 12" and one 8". The 8" is teed into 12" currently and improperly installed. HVAC contractor said he will enlarge both and route back to a plenum for a proper install to give me more CFM
    - compressor outside is rated at 2.5 Tons which he said was stupid because I only have a 2 Ton evaporator coil
    - 7 supply vents
    - cooling 1000 sq ft
    - system has leaked before and lines had to be purged, sealed, and re-charged with R22. Had to top it off again this past summer.

    My gut is telling me to replace my upstairs unit since I have had more problems with it and the system is mismatched. Also R22 is about to be crazy expensive. Also I am paying to run a 2.5 Ton compressor and can only handle up to 2 Tons. HVAC contractor did load calcs and said I am basically running at half capacity on both units because of low CFMs on return side due to issues mentioned above. I know the downstairs unit runs a hell of a lot more, around 800 hours over the year according to smart thermostats. Replacing the downstairs makes also sense based on my thermostat data and replacing with a more efficient 14 SEER system but then again that is basing my analysis off of systems not performing adequately. HVAC contractor is convinced I will get twice the air output from system by adding additional return. Hope he is right.

    Any insight is welcomed!

  2. #2
    Join Date
    Nov 2006
    Athens, Ohio
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    The upstairs unit is in the area that has the biggest cooling load. If that system is correctly sized and installed you won’t need much cooling downstairs.
    AOP Rules: Rules For Equipment Owners.

    Free online load calculator:

    There = not here. Their = possessive pronoun. They're = they are
    It's = contraction of it is. Its = the possessive form of it
    Too = also. To = expressing motion. Two = 2
    Then = after that, next. Than = indicates a comparison.
    Questions should end with a question mark "?" Statements end with a period "."

  3. #3
    Join Date
    Jan 2012
    Missouri, USA
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    Replace the upstairs, fix the return downstairs, and let the cool air from upstairs help cool the lower level.

  4. #4
    Join Date
    Aug 2002
    Southold, NY
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    Replacing "Just" the equipment will have a minimal effect on the monthly energy usage.

    Tightening the envelop
    Properly sizing the equipment
    Properly sizing the ducts
    Proper start up and commissioning

    That will help reduce overall energy usage!

    Start with the upstairs.

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