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  1. #1
    Join Date
    Jul 2012
    Location
    michigan
    Posts
    5

    Getting different opinions on A/C size

    Hello,
    I am planning to replace a furnace and AC in my house but so far I have been getting different opinions regarding the size of AC I should get. The first contractor actually did manual J calculation and recommended replacing my existing 4 ton unit with new seer 13 5 ton unit. According to him my house requires at least 49000 BTU to cool with all windows covered and 60000 BTU with all windows shades open. The other contractor also preformed manual j calculation but came out only with 49000 btu and recommended 4 ton unit since the return ducts are undersized and can handle up to 1700 cfm if I understood him correctly. I am not sure who is right and who is wrong. Any advice would be appreciated? My house is 3100 square feet and located in michigan

  2. #2
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,310
    Close the shades on a hot day!

    Most of the time we find that ductwork in a house is undersized for what you have. So then if you want to go 25% bigger you need at least 25% more ductwork. Reduce your heat gain and go 4 ton. Blowing a little more insulation in the attic can make a world of difference year round.

  3. #3
    Join Date
    May 2012
    Location
    San Diego, CA
    Posts
    352
    If your ducts are undersized, you might have to go with a 4 ton. Did your old AC keep up with the cooling you needed during the summer?
    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)

  4. #4
    Join Date
    Oct 2004
    Location
    So Cal
    Posts
    3,547
    I seriously doubt your ducts could take a 5 ton unit.
    Go 4 ton & make home more efficient. More attic insulation (at least R38) & change windows to dual pane. Then re-run the calcs. I would have ran the calcs based on what you have now & with the upgrades I recommended above. Then I would have sold you a smaller unit & window/insulation upgrade package at same time.

  5. #5
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,323

    Hmm Reasonable Expectations

    Quote Originally Posted by mi2006 View Post
    I am planning to replace a furnace and AC in my house but so far I have been getting different opinions regarding the size of AC I should get.

    The first contractor actually did manual J calculation and recommended replacing my existing 4 ton unit with new seer 13 5 ton unit. According to him my house requires at least 49000 BTU to cool with all windows covered and 60000 BTU with all windows shades open. The other contractor also preformed manual j calculation but came out only with 49000 btu and recommended 4 ton unit since the return ducts are undersized and can handle up to 1700 cfm if I understood him correctly. I am not sure who is right and who is wrong.

    Any advice would be appreciated?
    My house is 3100 square feet and located in michigan
    I seriously doubt you need a 5-ton.!.. !!
    ... unless your house is > 50 years old.

    Do you have > 450 total square feet of windows/ sliding glass doors?

    Have the windows/sliding glass doors facing West TINTED (SHGC < 0.45, but check light transmittance effects) professionally and you could reduce the load almost 9,000 BTU/hour (assuming > 200 square feet of WEST facing windows).


    I could VERY EASILY live with 76'F inside on days over 94'F outside.
    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

  6. #6
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,323

    Thumbs down Cost - Benefit

    Quote Originally Posted by precision hvac View Post
    Go 4 ton & make home more efficient. More attic insulation (at least R38) & change windows to dual pane.
    .
    What is the SPECIFIC advantage of changing to dual pane windows for cooling in THIS application?
    I wonder if this residence currently has dual pane windows.

    ~800 BTU/ hour at a cost of $28,000 for new windows?
    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

  7. #7
    Join Date
    Jul 2012
    Location
    michigan
    Posts
    5
    I do have dual pane windows and insulation in my attic is R-35 according to the inspection report i got when we bought the house. Most of the windows are new and all them are either covered with energy efficient drapes or regular shades. I will take your advice and look into getting more insulation and getting a 4 ton unit. Thank you for your help
    Last edited by mi2006; 07-20-2012 at 12:43 AM. Reason: grammar

  8. #8
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,323
    4,000 BTU/ hr benefit at cost of $28,000 for dual pane windows in MI might reduce total seasonal cooling energy by Maximum of 4,000,000 BTU or 330 Ton-Hours / 400 kW-hrs / ~$40 per year. Payback on new windows based soley on cooling would be 700 years or actually NEVER if one factors in the co$t of money.
    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
    Nov 2004
    Location
    SW FL
    Posts
    6,323

    Thumbs up R.H. should be < 47% with right sizing

    Quote Originally Posted by mi2006 View Post
    I do have dual pane windows and insulation in my attic is R-35 according to the inspection report i got when we bought the house.

    Most of the windows are new and all them are either covered with energy efficient drapes or regular shades. I will take your advice and look into getting more insulation and getting a 4 ton unit.

    I would look into a 4-ton, 2 stage unit with variable speed air handler for all the hours when Outside Air is < ~88'F when a 4-ton might actually be OVERsized.

    LOCATION ?
    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

  10. #10
    Join Date
    Jul 2012
    Location
    michigan
    Posts
    5
    I am located in metro Detroit area. My A/C cannot keep up if outside temperature goes above 90.

  11. #11
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,323
    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

  12. #12
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,310
    May be duct issues, blower issues, low capacity from current A/C but 2000 sq ft in Motown shouldn't need a 5 ton with good windows and attic insulation.

  13. #13
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    5,006
    Well, a 2.5% summer design in Detroit: 88-F dry bulb 72-F wet bulb & about 46% relative humidity.

    With that outdoor humidity level, if there is too much air-infiltration you'll need to run the blower for 4-Ton @ 350-CFM per-ton of cooling; or, 1400-CFM, which will help make the duct system, filter areas & everything work more efficiently.

    Additionally, the better matched 4-Ton unit will have longer run cycles necessary for humidity control.

    You're much better off to invest in more insulation, plugging air holes in your home &, sealing & better insulating all the ductwork, if necessary, than ever going to a 5-Ton condenser.

    That action will additionally pay big dividends in the very long heating seasons in Detroit, MI.

    The 5-Ton option will cost you more money in every way nearly every day!
    Last edited by udarrell; 07-20-2012 at 03:58 PM. Reason: 4-Ton @ 350-CFM per-ton of cooling...

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