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  1. #1

    Do two small systems use more energy than one large one?

    We're trying to decide between replacing our two upstairs A/C units as they stand with two newer units (2-ton and 3-ton) or one large system (5-ton). Both units are more than 15 years old and very inefficient. I'm wondering what the pros and cons are for going to one unit.

    Obviously there's more flexibility in having two systems that can be controlled separately, but both of these units are cooling bedrooms, so that's not a feature I'd often take advantage of.

    We live in South Florida, very hot and very humid. The house is 4000 sf; the existing 2-ton unit cools a master bedroom addition to the house and the 3-ton unit cools four other rooms upstairs. We have a separate unit cooling the downstairs. Having three units in one house is no fun, and if it makes sense to reduce that to two units, I'd love to.

    Thanks!

  2. #2
    Join Date
    May 2006
    Location
    Raleigh,NC.
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    357

    logistics are your enemy!

    it would be nice to have the entire up stairs on one system. in fact, since your addition was an after thought, the upstairs load may not need the five tons. you will need a pro to answer that one. here's the but. the ducting will require a major re-do and now the electrical falls in the same delima. too many unanswered questions to ask us. find a good couple of good local contractors ,see what they say and then decide.
    remember, with electronics; when its brown,its cooking and when its black, its done!!!

  3. #3
    Quote Originally Posted by micdundee View Post
    it would be nice to have the entire up stairs on one system. in fact, since your addition was an after thought, the upstairs load may not need the five tons. you will need a pro to answer that one. here's the but. the ducting will require a major re-do and now the electrical falls in the same delima. too many unanswered questions to ask us. find a good couple of good local contractors ,see what they say and then decide.
    Yes, the ducts would need to be re-worked rather extensively (and I'm worried about what problems might ensue from that), AND the 5-ton system would require a larger wiring system as well, which would have to be installed and run. But given all that, I'm wondering about the energy usage on a monthly basis and how that might be affected. Our contractor feels that either option would use about the same amount of energy; I'm not so sure. On the one hand, it seems possible since the 5-ton uses a larger voltage, but still running two systems instead of one seems logically to use more energy. It *is* a dilemma!

  4. #4
    Join Date
    May 2006
    Location
    Raleigh,NC.
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    your dilemma spells better than mine, i see. you built the master suite for more room and privacy and you probably go to bed latter than the kids, so you can program your stat to that schedule and when they grow up and move out you will really save energy, not to mention money in general! haa! on an energy savings aspect only, yes, two systems.
    remember, with electronics; when its brown,its cooking and when its black, its done!!!

  5. #5
    Join Date
    Aug 2008
    Posts
    56
    Wow, 5 Tons and that doesnt include the system you have for the rest of the downstairs. What size is that?

    I would think that in theory a total of 5 Tons would be enough for the entire 4000 sf house in any combination.

    You may want to have a load calc done with the intent of determening if you can get the 2 Ton system for the master suite reduced to 1.5 Tons and the 3 Ton unit for the upstairs reduced to 2.5 tons, then start counting the savings in energy.

  6. #6
    Join Date
    Mar 2005
    Location
    burlington county n.j.
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    9,754
    i wouls stay with 2 properly sized systems for the upstairs if thats how its already set up.


    5 ton units are always the most inneficient in any model line.

  7. #7
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    68,308
    Your better off with 2 units.

    There is a good chance. That a niminal 5 ton unit. Won't actually meet the BTU capacity of your 2 smaller units.

    Which would mean you wouldn't be able to cool your house.
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  8. #8
    Join Date
    Nov 2008
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    Chapel Hill, NC
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    WAY better off with two systems in a 2 storey house in Florida. The downstairs heating/cooling requirements are invariably different from upstairs. About the only instance I can think of where a single unit is optimal is in a ranch house (1 floor).

  9. #9
    Join Date
    Nov 2000
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    Upstate, SC
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    Quote Originally Posted by sktn77a View Post
    WAY better off with two systems in a 2 storey house in Florida. The downstairs heating/cooling requirements are invariably different from upstairs. About the only instance I can think of where a single unit is optimal is in a ranch house (1 floor).
    I think he has a unit downstairs and two units upstairs the way I read the original post.

    I'd have a couple contractors do a load calc and see what is truly required for the upstairs. If it is truly a 5 ton load, I'd prefer the two smaller units, but no way to say for sure until the load calcs are done.

  10. #10
    Join Date
    Nov 2008
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    Quote Originally Posted by BobbyBJr View Post
    I think he has a unit downstairs and two units upstairs the way I read the original post.
    I think you may be right

    Sounds like the third system was added on when they added the additional bedroon so as not to mess up the original 2nd floor HVAC system. May or may not be possible to combine the ductwork with a single new system. While it may be cheaper to get a single system for the second floor, the savings might be offset by the ductwork modification costs.

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