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  1. #1
    Join Date
    Jun 2011
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    Washington DC Area
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    1

    Question Need help choosing: 2 1/2 ton single-stage or 3 ton 2-stage A/C?

    I'm having my 17-year-old gas furnace and A/C unit replaced. My current A/C is a single-stage 2 1/2 ton unit, and that is the correct size based on the load calculation.

    I live in a 3-story townhouse and on very hot days (all of late spring & summer), the top level is more than 10 degrees hotter and more humid than the main level of the house. It's VERY hot upstairs.

    I'm getting a new Trane 2-stage gas furnace with variable speed blower, and I'm trying to figure out if I'd be better off with a 2 1/2 ton single-stage A/C or a 3 ton 2-stage A/C (the 2-stage units do not come in half-ton sizes). I'm hoping to achieve maximum cooling/dehumidification on my upper level. HERE'S MY MAIN CONCERN: It's my understanding that if an A/C unit is too big, it will short-cycle. So if the right size unit for my house is 2 1/2 tons, will getting a 2-stage 3 ton unit be a problem and cause short-cycling on the hottest days? My top priority is properly cooling the upstairs and I'm not really concerned with monthly $ savings.

    I'm worried if I make the wrong decision I might be stuck for many more years with an unbearably hot upper level. My contractor thinks either unit would be fine. Anyone who has experience with this type of situation, please help me make an educated decision. Thank you!

  2. #2
    Join Date
    Jun 2004
    Location
    4H: Hot, Humid Houston H.O.
    Posts
    3,304
    Quote Originally Posted by knickerbocker View Post
    I'm having my 17-year-old gas furnace and A/C unit replaced. My current A/C is a single-stage 2 1/2 ton unit, and that is the correct size based on the load calculation.

    I live in a 3-story townhouse and on very hot days (all of late spring & summer), the top level is more than 10 degrees hotter and more humid than the main level of the house. It's VERY hot upstairs.

    I'm getting a new Trane 2-stage gas furnace with variable speed blower, and I'm trying to figure out if I'd be better off with a 2 1/2 ton single-stage A/C or a 3 ton 2-stage A/C (the 2-stage units do not come in half-ton sizes). I'm hoping to achieve maximum cooling/dehumidification on my upper level. HERE'S MY MAIN CONCERN: It's my understanding that if an A/C unit is too big, it will short-cycle. So if the right size unit for my house is 2 1/2 tons, will getting a 2-stage 3 ton unit be a problem and cause short-cycling on the hottest days? My top priority is properly cooling the upstairs and I'm not really concerned with monthly $ savings.

    I'm worried if I make the wrong decision I might be stuck for many more years with an unbearably hot upper level. My contractor thinks either unit would be fine. Anyone who has experience with this type of situation, please help me make an educated decision. Thank you!
    Don't know which model Trane you are looking at, but the old XL19 line did come in 2.5 ton size. Here's the funny part: 1st stage was identical between it and the 3.0 ton size, the only difference was size of 2nd stage! Even the airflow rating was the same between the two sizes, both in 1st and 2nd stage. I am a homeowner and only know this because I have one -- needless to say I chose the 3.0 ton size.

    But... it may be an expensive error to think this will solve your problem of too-hot rooms. One of the likely possibilities is attention to ductwork could solve this problem using a single stage unit. Some professionals might suggest zoning to solve it, which is expensive. That implies a real world solution might be to choose the single stage AC and spend more money on other parts of the system.

    A professional who can measure airflow to your various rooms might have the kind of skills and tools you need to solve the problem.

    Hope this helps -- Pstu

    P.S. The expected behavior of a 2-stage AC is very long run times on 1st stage most days, with occasional bumps up to 2nd stage when that fails to maintain temperature. That is what I see with mine. There is no reason to suspect short cycling because of two-stage, rather the opposite.

  3. #3
    Join Date
    Jul 2004
    Location
    Massachusetts
    Posts
    6,829
    It's been a few years since we were a Trane dealer but at the time we were, all their 2-stage AC units were 50% 1st stage and 100% 2nd stage. So a 3.0-ton unit would cycle between 1.5-tons and 3.0-tons. I don't see that as a short cycling problem. You could however end up with noise if the duct system is barely big enough to 2.5-tons and the variable speed blower is asked to produce airflow for 3.0-tons. It could be a little noisy when on 2nd stage.

    On the other hand, I don't see it as any solution to your temperature difference between the 1st and 2nd floors. Normally I see that kind of imbalance when there's no or not enough return air from the 2nd floor. Many homes had systems installed by Neanderthals who didn't think return air was necessary, given they were installing just heat and not AC. Truly, it's not such a big issue when heating as heat rises anyway and the absence of a return air on the 2nd floor would not be as noticeable. But when the cooling season arrives, hot air stacking on the 2nd floor with no escape route is a text book case of poor duct design. You should entertain the idea of having the installing company provide some return air for the 2nd floor.

  4. #4
    Join Date
    Dec 2005
    Posts
    1,909
    Quote Originally Posted by knickerbocker View Post
    I'm having my 17-year-old gas furnace and A/C unit replaced. My current A/C is a single-stage 2 1/2 ton unit, and that is the correct size based on the load calculation.

    I live in a 3-story townhouse and on very hot days (all of late spring & summer), the top level is more than 10 degrees hotter and more humid than the main level of the house. It's VERY hot upstairs.

    I'm getting a new Trane 2-stage gas furnace with variable speed blower, and I'm trying to figure out if I'd be better off with a 2 1/2 ton single-stage A/C or a 3 ton 2-stage A/C (the 2-stage units do not come in half-ton sizes). I'm hoping to achieve maximum cooling/dehumidification on my upper level. HERE'S MY MAIN CONCERN: It's my understanding that if an A/C unit is too big, it will short-cycle. So if the right size unit for my house is 2 1/2 tons, will getting a 2-stage 3 ton unit be a problem and cause short-cycling on the hottest days? My top priority is properly cooling the upstairs and I'm not really concerned with monthly $ savings.

    I'm worried if I make the wrong decision I might be stuck for many more years with an unbearably hot upper level. My contractor thinks either unit would be fine. Anyone who has experience with this type of situation, please help me make an educated decision. Thank you!
    The key thing is your top floor not cooling well.

    Putting 2 stage system will not help you any. Instead it make it worse. BECAUSE, when it run 1st stage, which has only 50 to 70% of the total amount of air. Thus, the blower WILL NOT be able to push the air up to 3rd floor. Remember, it is bad enough with full force of blower on high stage cannot get enough air on 3rd floor. What you think will happen with lower force of air.

    Your best solution is to size the duct to 3rd floor correctly. Other wise, you wil run in circle again and again and again.

    Good luck.

    P.S. I don't know ehere you live. But think before you buy 2 stages system. I may take (or may not) too long to get your money back in cooler climate.

  5. #5
    Join Date
    Nov 2008
    Location
    Chapel Hill, NC
    Posts
    1,070
    Sounds like you have a ductwork/air distribution/insulation problem. The new A/C and air handler equipment won't fix this.

  6. #6
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    4,908

    Arrow

    Quote Originally Posted by knickerbocker View Post
    I'm having my 17-year-old gas furnace and A/C unit replaced. My current A/C is a single-stage 2 1/2 ton unit, and that is the correct size based on the load calculation.

    I live in a 3-story townhouse and on very hot days (all of late spring & summer), the top level is more than 10 degrees hotter and more humid than the main level of the house. It's VERY hot upstairs.

    I'm getting a new Trane 2-stage gas furnace with variable speed blower, and I'm trying to figure out if I'd be better off with a 2 1/2 ton single-stage A/C or a 3 ton 2-stage A/C (the 2-stage units do not come in half-ton sizes). I'm hoping to achieve maximum cooling/dehumidification on my upper level. HERE'S MY MAIN CONCERN: It's my understanding that if an A/C unit is too big, it will short-cycle. So if the right size unit for my house is 2 1/2 tons, will getting a 2-stage 3 ton unit be a problem and cause short-cycling on the hottest days? My top priority is properly cooling the upstairs and I'm not really concerned with monthly $ savings.

    I'm worried if I make the wrong decision I might be stuck for many more years with an unbearably hot upper level. My contractor thinks either unit would be fine. Anyone who has experience with this type of situation, please help me make an educated decision. Thank you!
    Okay; four things to get more Supply & Return air to & from the upper-floor areas.

    Put nearly all of the return-air on that upper-floor with an oversized Return air filter grille. If you use a single stage 2.5-Ton unit, using 450-cfm per/ton of cooling is 1125-cfm / 300-fpm is 3.75-sq.feet of free-open-air area.

    3.75 * 144 is 540-sq.ins. If U use 2-cfm per sq.in., 1125-cfm / 2 is 562.5-sq.ins., but that is only the (Ak) open-air-area of the filter.

    At .65% open area: 562.5 * 1.35 is 759-sq.ins., or around a 30X26 = 780-sq.in. filter area. Figure how much air you're going to let through the main floor filter, so U could go to a narrower upper-floor filter.

    Then the main floor would also need a filter grille & covered to greatly reduce its return CFM; NO filter in the furnace.

    The room T-stat has to be on the upper-floor.

    Also, may have to add more supply air runs to upper-floor area.

    You need a room by room heat-gain calc on that upper-floor so you know how much air has to be cycled through each room.

    Install dampers on all the take-off points of the SA branch runs.

    Sounds like a lot but it usually isn't that difficult to do.
    Last edited by udarrell; 06-09-2011 at 08:09 PM. Reason: to do...

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