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  1. #1
    Join Date
    Mar 2007
    Location
    Twin Cities, Minnesota
    Posts
    10

    Two stage vs. single stage AC

    I live in Minnesota where our cooling season is very short, about 3 months. I have received bids on both two stage and single stage central AC units (about $2500 difference between the two). Is there a significant difference between a 15.5 SEER single stage and a 21 SEER two stage? I understand the "functional" differences, but will they really run that differently? and will I ever see any return on investment in energy savings if I go with the more expensive unit? It also seems that the higher end models run louder according to the manufacturer's specs. (More money and louder? Hmm.) The contractors naturally want to push the higher end unit, but I wish they would also be realistic about our actual AC usage. Comments appreciated.

  2. #2
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,095
    With Minnesota's cool summers and decent electric rates, nothing but a base 13 SEER will have any economic advantage. Only possible reason for going a notch higher is if the utility pays for it in a rebate. You probably will save $40-50 a year TOPS in electric savings if that much. That 2 stage will pay for itself in $50 years!

    Insulate the house, get new windows, something besides putting that fortune into a super duper A/C. Other than the summer of 1988 when I was there and we had 44 days over 90, I know what a real MN summer is like.

  3. #3
    Join Date
    Mar 2007
    Location
    Toronto
    Posts
    43

    Not worth it

    I sell the lennox xc21, and unless you have really bad humidiy where you live (which I think you dont) take a look at the XC15, about half the price, quietest unit on the market and it is extremly well built, comes with a 10 year parts and compressor warranty.

  4. #4
    Join Date
    Mar 2007
    Location
    Twin Cities, Minnesota
    Posts
    10
    Thanks for the responses. Minnesota is the land of 10,000 lakes, so yes, the summers can get terribly humid. I am leaning toward the less expensive model. But, is it true that the 2 stage cools to maintain a certain humidity level, where a single stage just cools to a given temperature? Don't know if this makes a difference since the job of any AC is to remove humidity.

  5. #5
    Join Date
    Nov 2004
    Location
    Naples, Fla.
    Posts
    1,403
    As stated - go w/ the 13 SEER, you have very little ROI any higher in your application.

    Spend the money on a Honeywell 9000 + DH90 dehumidifier + min MERV 10 air filtration (Aprilaire 2200) --- then you have ROI.

  6. #6
    Join Date
    Mar 2007
    Location
    Toronto
    Posts
    43
    System design is key to humidiy removal. Does your furnace have a varaible speed motor? Do you have a t-stat with humidity control. What is the ductwork lay out like in your home? Is you attic well ventilated? Is the system you are looking at properly sized? Have you had a heat gain calc completed? These are all important quesitons you need to answer before you select the type of equipment you would like to purchase.

  7. #7
    Join Date
    Jun 2004
    Location
    4H: Hot, Humid Houston H.O.
    Posts
    3,304
    >>is it true that the 2 stage cools to maintain a certain humidity level, where a single stage just cools to a given temperature?

    Sorry to say this is not true. Either type system will dehumidify due to passing air over a coil which is below indoor dewpoint (which you want to be 55-60F or lower). The two-stage system gains its advantage because of longer run times in 1st stage. Either type system can be connected with a dehumidistat or a thermostat which causes additional cooling for the purpose of dehumidifying.

    A variable speed air handler can be a very useful tool in a single stage system (and is mandatory in a two stage system).

    For comfort sometimes a two stage system will do the job better, and be worth it that way. I have never heard of a two stage system promising lower lifetime costs, even in an AC intensive climate like my S.Texas one.

    A dedicated dehumidifier is *the* tool for the job when outdoor temperature is mild and humidity high. It will give satisfactory results in spring and fall, while an AC generally will only do so in summer.

    Best of luck -- Pstu

  8. #8
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,372
    13 SEER simple a/c + 90 pint dehumidifier will keep you at <50%RH regardless the cooling load. MN like WI homes need strong dehumidification starting now. No cooling load right now with high humidity load. My 90 pint dehu removing 60 pints per day maintaining <50%RH. Green grass climate have months of high latent (dehumidification) without much sensible cooling. The honeywell, Ultra-Aire, Santa fe are the high eff. dehus that are ductable. Get written assurance about maintaining <50%RH with any system. Imagine that it is raining, 70^F outside temp, and the weather stays for a week or two. No a/c cooling with 70-80,000 btu latent load every day. This is 90 pints of dehumdification during the worst days. 2 stage will not help. The dehu TB

  9. #9
    Join Date
    Oct 2003
    Location
    Washington
    Posts
    7,405
    I would go with a 13 SEER AC and ducted dehumidifier(honeywell), mated with the IAQ stat. Best bang for your buck in my opinion.

  10. #10
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,372
    Keep us posted on results. Thanks, TB

  11. #11
    Join Date
    Mar 2007
    Location
    Twin Cities, Minnesota
    Posts
    10
    Thanks everyone, for the input. Decided to go with the single speed AC as our cooling season is too short to make a 2 speed worth the cost. I finally got a contractor to honestly say that I would never get a return on investment over the lifetime of the AC unit. Thank goodness that there are still some people out there that aren't trying to take advantage of a newbie homeowner, by selling a product that is really beyond our needs.

  12. #12
    Join Date
    Sep 2001
    Location
    kalamazoo,mich
    Posts
    2,174
    Well, what did you decide to buy?
    Have you hugged the Earth today?
    Donny Baker rules

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