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  1. #1
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    Apr 2012
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    Princeton NJ
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    upstairs AC 2 or 3 tons?

    Hi,
    I'm in the process of replacing my 80k single stage heater upstairs with a 60k trane modulating unit. The heat load is 41k.

    I'm considering taking advantage of soon to expire rebates, and replacing the AC. It is 3 tons, and 17 years old. It is on the second floor, 1500 sq ft. The load calc program I used (as recommended here) came back at 24k btu. However, when it is mid 90's outside, the temp upstairs rises to 81 or so. The old "oversized" unit can't keep up. Is it because the calc is bad, or the unit?

    If i were to replace it, what size should I get?


    Steve

  2. #2
    Join Date
    Jan 2004
    Location
    PA
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    68,758
    Probably your duct work is causing the A/C problem. Check for leaks in your duct work.
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  3. #3
    Join Date
    Oct 2010
    Location
    Anderson, South Carolina, United States
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    7,745
    Quote Originally Posted by beenthere View Post
    Probably your duct work is causing the A/C problem. Check for leaks in your duct work.
    X2, as well as the coil etc being dirty so the capacity is diminished the old unit probably has an actual output of 2.5 tons or less now add in duct leakage to unconditioned space and too small duct and you've got a 1.5 ton unit or worse

  4. #4
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    Oct 2010
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    Anderson, South Carolina, United States
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  5. #5
    Join Date
    May 2000
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    Indianapolis, IN, USA
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    The calc also doesn't look at how much heat rises from downstairs and adds to the load of upstairs.

  6. #6
    Join Date
    Aug 2010
    Location
    West Monroe, LA
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    1,537
    Oh the dredded upstairs discussion! While I have ran into this problem many times in my career. What sticks out most as others have advised is the duct work. Often I find the same thing you are talking about but the square footage is less. The upstairs stills has a (3) ton and doesn't keep up.

    What I find is the duct system is undersized and leaking as others have posted. So you truly are not getting the rated btu out of your existing system nor if you installed a new system the same size.

    A good example would be: if your duct work will only handle (2) tons. An the load cal. Calls for (2.5) ton and someone installs that you still only have (2) tons worth of supply and return! No matter how big or small your upstairs system is. It can be limted by duct undersized or leakage you will never get the rated btu's out of the system. In other words you could install a bigger system but if it want supply and return the amount of air needed. So capcity is reduceded.

    So the real question is has anyone looked at your ductwork for problems? Also as a system ages and the inside coil becomes dirty or the outside the unit will not deliver the rated capacity. I would suggest having a pro out to do a detailed load cal., review the duct work and size your system properly. Upstairs is always harder to cool due to heat rising. The only way to combat this is ensure the load is correct, the duct system is sized properly and all ductwork is sealed propely. Also this might be a good time to get a energy rater out to review over the entire home and make recommdations to correct heat gain/loss. What is your location? As this plays a big role in the sizing of the system.

  7. #7
    Join Date
    Nov 2004
    Location
    SW FL
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    6,428
    Quote Originally Posted by central nj View Post
    Hi,
    I'm in the process of replacing my 80k single stage heater upstairs with a 60k trane modulating unit. The heat load is 41k.

    I'm considering taking advantage of soon to expire rebates, and replacing the AC. It is 3 tons, and 17 years old. It is on the second floor, 1500 sq ft.

    The load calc program I used (as recommended here) came back at 24k btu. However, when it is mid 90's outside, the temp upstairs rises to 81 or so.
    The old "oversized" unit can't keep up. Is it because the calc is bad, or the unit?

    If i were to replace it, what size should I get? Steve
    WHY would you question the calculated cooling load (24,000), IF you believe in the heating load (41,000).?

    TESTING
    http://www.buildingperformancebydesi...ctblaster.html
    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

  8. #8
    Join Date
    Nov 2004
    Location
    SW FL
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    6,428
    Quote Originally Posted by duckman06 View Post
    Oh the dredded upstairs discussion! While I have ran into this problem many times in my career. What sticks out most as others have advised is the duct work. Often I find the same thing you are talking about but the square footage is less. The upstairs stills has a (3) ton and doesn't keep up.

    What I find is the duct system is undersized and leaking as others have posted. So you truly are not getting the rated btu out of your existing system nor if you installed a new system the same size.

    It can be limited by duct undersized or leakage you will never get the rated btu's out of the system.

    So the real question is has anyone looked at your ductwork for problems?

    I would suggest having a pro out to do a detailed load cal., review the duct work and size your system properly.

    What is your location? As this plays a big role in the sizing of the system.
    What is the OPs name?

    NJ = mid 90's
    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
    Apr 2012
    Location
    Princeton NJ
    Posts
    125
    Ok, thanks for all the comments. I will ask them to clean the coil, and seal whatever they can. I would love a duct blaster test, and will ask about it.

    Does my air conditioner "require" more air than my furnace? The oversized 80k btu furnace doesn't seem to have any problems short cycling, and jacking the heat from 70 to 73 in 5 minutes. Does that imply that there is enough return/air in the system?

    I think the basic structure of the ducts is ok, in that the furnace is in the center, and both return ducts and supply ducts split off like a T. I am having the plenum/connection to return increased (it is 11x18, and will be 12x25).

    I only have 5 between stud returns (14x3?). Would it be worth it to have more return air?

    Steve

  10. #10
    Join Date
    Jan 2004
    Location
    PA
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    More return would help a lot.

    While your furnace may not be cycling on its high limit. that does mean its not running lose to tripping the high limit.
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  11. #11
    Join Date
    Nov 2004
    Location
    SW FL
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    Quote Originally Posted by central nj View Post
    The oversized 80k btu furnace doesn't seem to have any problems short cycling, and jacking the heat from 70 to 73 in 5 minutes.
    1. Does that imply that there is enough return/air in the system?

    2. I am having the plenum/connection to return increased (it is 11x18, and will be 12x25).

    3. I only have 5 between stud returns (14x3?). Would it be worth it to have more return air?

    Steve
    1. NO, with lower air flow the delivered air will be hotter - nearly same Heat Capacity.

    2. Good improvement - 2.08 sq. ft should be adequate for 2-ton

    3. 325 Sq Inch more generally needed for a 3-ton, so 3-ton VERY Likely Underperforming.
    ------ 1.46 square feet total should be adequate for 2-ton or ~ 50,000 BTU Output furnace/ 800 CFM with ~55'F Rise, BUT # of sharp turns significantly increase resistance and decreases air flow.
    _ ACCA Manual D _ is needed to determine analytically where air distribution may be inadequate.
    _ or One may Measure total air flow and suspect low air flow areas.

    14 inch
    15 inch
    210 sq inch
    144 SqIn/ Sq Ft
    1.458 Sq feet
    600 Ft / Min
    875 CFM
    1200 CFM needed for 3 ton
    325 Sq. Inch More

    12 inch
    25 inch
    300 SqInch
    144
    2.083 Sq Ft
    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
    Nov 2004
    Location
    SW FL
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    IF Cooling Load Calc is accurate and requires 24,000 BTU/hr (Total or Sensible?), what size unit is one going to select?
    2.5 Ton _
    2.0 Ton _

    2.5 Ton will provide about 24,000 BTU/Hr Sensible Capacity.
    2.0 Ton ......" ....... " .... 19,000 BTU/Hr Sensible if the Relative Humidity is being controlled.
    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

  13. #13
    Join Date
    Apr 2012
    Location
    Princeton NJ
    Posts
    125
    Thanks very much all, and Dan for calculations. Suckky. But i will at least be able to make several improvements.

    Thanks again!!!
    Steve

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