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  1. #1
    Join Date
    Apr 2012
    Location
    Princeton NJ
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    125

    When is an air conditioner oversized?

    Hi, this forum has been so good to me already. Thank you.

    I'm debating between a 4 or 5 ton trane 19i air conditioner, the stages would match my zones nicely. I currently have a 10 seer 5 ton trane that is 17 years old. When working correctly, there were no humidty or other problems (other than it didn't match my 3 zone duct work!)

    The load calc is between 4 and 4.3 depending on addition of several hundred feet of space with no registers (hallways, pantry---conditioned, but no register so the sales guy said they didn't have a load?) , and insulation estimates.

    The sales guy, and lead installer were adament that a 5 ton was the way to go.

    I'm worried that if I go with the 5 ton, I will experience the short cycling humidity issue you folks keep speaking of. Do you think this would be possible with the bigger unit? How much bigger/oversized does an airconditioner have to be to cause problems?

    Steve

  2. #2
    Join Date
    Jan 2012
    Location
    indy
    Posts
    471
    Quote Originally Posted by central nj View Post
    Hi, this forum has been so good to me already. Thank you.

    I'm debating between a 4 or 5 ton trane 19i air conditioner, the stages would match my zones nicely. I currently have a 10 seer 5 ton trane that is 17 years old. When working correctly, there were no humidty or other problems (other than it didn't match my 3 zone duct work!)

    The load calc is between 4 and 4.3 depending on addition of several hundred feet of space with no registers (hallways, pantry---conditioned, but no register so the sales guy said they didn't have a load?) , and insulation estimates.

    The sales guy, and lead installer were adament that a 5 ton was the way to go.

    I'm worried that if I go with the 5 ton, I will experience the short cycling humidity issue you folks keep speaking of. Do you think this would be possible with the bigger unit? How much bigger/oversized does an airconditioner have to be to cause problems?

    Steve
    I'm not the load calc genious on here but I can tell you you go under what the load calc says, so if it said 4.3 you go 4 ton, you could do a two stage also. Shoert cycling is bad and wont remove humidity, I'd rather have my unit not keep up on hot days by a couple deg at least the humid will be low and the cool feeling when you walk in everybody loves ahhhhh. The guys on here will prob tell you to get another calc done, small mistakes can lead to 17more years of a unit not working right. (If its too big) good luck

  3. #3
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
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    6,436
    It would be nice to be able to handle the above design conditions that we routinely are getting during the hot summers. Also nice tobe able to keep the home cool when the company is over. What of this hub bub is about is the fact that all a/cs are unable to maitain <50%RH when there is low/no cooling loads, outdoor dew points are +55^F, and the home is occupied.
    I suggest that you will supplemental dehumidifications if you want <50%RH during low/no cooling load condition. This more important if you getting the correct amount of fresh air ventilation/infiltration with damp cool weather. A whole dehumidification cost about the same as a multispeed a/c upchatge.
    Consider that even the most expensive varispeed a/c is unable to keep a home <50%RH without a significant cooling load.
    Keep us posted.
    Regards TB
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

  4. #4
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    4,989
    Quote Originally Posted by central nj View Post
    Hi, this forum has been so good to me already. Thank you.

    I'm debating between a 4 or 5 ton trane 19i air conditioner, the stages would match my zones nicely. I currently have a 10 seer 5 ton trane that is 17 years old. When working correctly, there were no humidty or other problems (other than it didn't match my 3 zone duct work!)

    The load calc is between 4 and 4.3 Ton, depending on addition of several hundred feet of space with no registers (hallways, pantry---conditioned, but no register so the sales guy said they didn't have a load?), and insulation estimates.

    The sales guy, and lead installer were adamant that a 5 ton was the way to go.

    I'm worried that if I go with the 5 ton, I will experience the short cycling humidity issue you folks keep speaking of. Do you think this would be possible with the bigger unit? How much bigger/oversized does an air conditioner have to be to cause problems? Steve
    Trenton NJ summer design is 88F DB 74F WB or, a fairly high 52% RH.

    Because of your low 88F outdoor design humidity & location conditions, I would never go above a 4-Ton A/C. You need adequately long runtimes on the milder days

    That will create a more efficient match with the duct system & the longer runtimes increase the operating SEER level plus, provide more uniform room comfort levels.

    On my closer to 700-sf first floor 1937 farm home with a lot of windows, I have a Half-Ton window A/C (that is; - an amazing 1400-sf per ton; yes it amazes me too) that keeps me totally comfortable even in 110F Heat Indexes.

    With my setup I have kept the humidity at close to 50% no matter if it is 65 or 70F outdoors...

    The unit is only supposed to cool a 250-sf room; I use a fan to circulate the air throughout the 3-rooms & hallway; so don't tell me a properly installed & setup 4-Ton won't work within your Central NJ conditions.

    Why would you ever go to 5-Ton?
    Last edited by udarrell; 08-08-2012 at 09:32 PM. Reason: close to 50% no matter if it is 65 or 70F outdoors...

  5. #5
    Join Date
    Aug 2007
    Location
    Northern VA 38 degrees N by 76 degrees W
    Posts
    5,060
    Quote Originally Posted by central nj View Post
    Hi, this forum has been so good to me already. Thank you.

    I'm debating between a 4 or 5 ton trane 19i air conditioner, the stages would match my zones nicely. I currently have a 10 seer 5 ton trane that is 17 years old. When working correctly, there were no humidty or other problems (other than it didn't match my 3 zone duct work!)

    The load calc is between 4 and 4.3 depending on addition of several hundred feet of space with no registers (hallways, pantry---conditioned, but no register so the sales guy said they didn't have a load?) , and insulation estimates.

    The sales guy, and lead installer were adament that a 5 ton was the way to go.

    I'm worried that if I go with the 5 ton, I will experience the short cycling humidity issue you folks keep speaking of. Do you think this would be possible with the bigger unit? How much bigger/oversized does an airconditioner have to be to cause problems?

    Steve
    Find you a different dealer, or remove all your registers and there will be no need for AC.

  6. #6
    Join Date
    Dec 2002
    Location
    Arizona
    Posts
    4,411
    Quote Originally Posted by second opinion View Post
    Find you a different dealer, or remove all your registers and there will be no need for AC.
    LOL.

    By the way if your sales person is trying to sell you a 19i you need to tell him Trane replaced them about 2 years ago with the 20i.
    Make your expertise uniquely valuable.

    Make your influence uniquely far-reaching.

  7. #7
    Join Date
    Apr 2012
    Location
    Princeton NJ
    Posts
    125
    Wow. Thanks for the help. It is a 20i btw, my mistake.

    Regarding heat load, would a register-free pantry, surrounded on all 4 walls by conditioned space, a conditioned second floor and a basement really have a heat load??

  8. #8
    Join Date
    Dec 2002
    Location
    Arizona
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    4,411
    Quote Originally Posted by central nj View Post
    Wow. Thanks for the help. It is a 20i btw, my mistake.

    Regarding heat load, would a register-free pantry, surrounded on all 4 walls by conditioned space, a conditioned second floor and a basement really have a heat load??
    Nope. It would not, as long as yor basement is conditioned. Just don't turn on that light ( just kidding).
    Make your expertise uniquely valuable.

    Make your influence uniquely far-reaching.

  9. #9
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    With zoning, in general, all zones are rarely needing maximum capaicty at the same time. In most cases you want to lean towards beign a little undersized than oversized. So if a load calc called for 4.5 tons, you would install a 4 tons. The benefit being long runs tiems and on low stage, only needing the ductwork in each zone to be able to handle about 850-1000 CFM depending on how it's set-up. With a 5 ton, you'd need to handle 1100-1300CFM.

    In some situations, you could actually consider a basement as a negative load. Ground temps will usually remain below indoor conditions. I have all registers closed in my basement all summer, so other than air leaks, it's not conditiioned. It remains around 70-74F even though the upstairs is 76-77F. In the winter I open the registers and it stays over 66F. But I have enough registers on the 1st floor that i can close the basement registers.

  10. #10
    Join Date
    Apr 2012
    Location
    Princeton NJ
    Posts
    125
    Udarrell--thanks for the comment. I probably will get the 4 ton. Why the 5 ton? It has been over 95 about 10 days so far this summer---i'm fine with the temp inside being 20deg cooler---75/76, even 77. I'm really not happy if a load of 4.3, means a 4 ton will only cool to 80 when its 95. How likely would that senario be?

    Also, a 5 ton, is "only" 25% more than a 4 ton. I understand going from a 2 to a 3 is a 50% increase in size, and would cause humidity issues---but I wonder if a 25% increase would do it.

  11. #11
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
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    Quote Originally Posted by central nj View Post
    Udarrell--thanks for the comment. I probably will get the 4 ton. Why the 5 ton? It has been over 95 about 10 days so far this summer---i'm fine with the temp inside being 20deg cooler---75/76, even 77. I'm really not happy if a load of 4.3, means a 4 ton will only cool to 80 when its 95. How likely would that senario be?

    Also, a 5 ton, is "only" 25% more than a 4 ton. I understand going from a 2 to a 3 is a 50% increase in size, and would cause humidity issues---but I wonder if a 25% increase would do it.
    This is funny. You buy an undersized a/c to get humidity control. Yet when the cooling loads decline and outside dew points are +65^F, your undersized a/c will not run enough to provide <50%RH. You need a good dehumidifier to provide <50%RH during low/no cooling loads with an oversized a/c or an undersized a/c. Get an straight simple a/c that handles the occasional high cooling loads, reasonable t-stat setup when occupied, and the a/c gets old. Get a good 70-90pint whole house dehu which will keep the home at 50% regardless of cooling load. Keep investigating the problem, the solution will come to you. All a/cs are unable to remove latent loads without significant sensible cooling loads.
    Regards TB
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

  12. #12
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    How warm it gets indoors when it's above design conditions has a LOT of vairables. The most important is the air volume and the mass of walls, floors and furnishings. And those amounts as a ratio of the system capacity. Older homes with plaster, thick wood floors and subfloors, full dimensional lumber and often real brick, stone, stucco or wood siding, will heat up and cool off mcuh slower. They also burn a lot slower too BTW. You'll have a lot more time to escape a fire in a old home than a newer home for much of the same reasons.

    The most improtant thing you cna do on really hot days is shut your blinds! Especially on the West and South. Even if you have Low-E windows, they reduce infrared heat transfer, but do not provide a significant amount of shading so visible light energy will still get transferred. That's good in the winter but bad in the summer. You can cut your heat gain by something like 20-25% by closing blinds vs. leaving them open depending on you percentage of window area to wall surface.


    A 4 to 5 ton is a smaller percentage increase, but in terms of airflow requirements, will make a big difference on a zone system.

  13. #13
    Join Date
    Jul 2008
    Location
    Rochester NY
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    Quote Originally Posted by central nj View Post
    Udarrell--thanks for the comment. I probably will get the 4 ton. Why the 5 ton? It has been over 95 about 10 days so far this summer---i'm fine with the temp inside being 20deg cooler---75/76, even 77. I'm really not happy if a load of 4.3, means a 4 ton will only cool to 80 when its 95. How likely would that senario be?
    If that were to happen, spend the $ you saved on the smaller unit on caulk and foam. You can always improve the home to match the unit.

    How big is this house anyway?
    Which makes more sense to you?
    CONSERVATION - turning your thermostat back and being uncomfortable. Maybe saving 5-10%
    ENERGY EFFICIENCY - leaving your thermostat where everyone is comfortable. Saving 30-70%

    DO THE NUMBERS! Step on a HOMESCALE.
    What is comfort? Well, it AIN'T just TEMPERATURE!

    Energy Obese? An audit is the next step - go to BPI.org, or RESNET, and find an auditor near you.

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