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  1. #1
    Join Date
    Jun 2008
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    7

    4 Ton 21 SEER IQ drive or 5 Ton 16 SEER?

    I have a 14 yr old 10 SEER 5 ton Trane AC unit with a single speed blower. I have been looking into replacing the unit with a more efficient model. I have been looking at the 21-23 SEER IQ drive units but they only come up to 4 tons. An AC company did a heat analysis survey on my house to determine AC size and he said it is saying that I need a 4 ton system. I have had a few AC companies give me a quote and many of them say they would not recommend dropping a Ton. I live in a 2600 SqFt 2 story house in South Florida and I don't want to have an undersized system.

    I have also looked at the EER rating on this system and it shows 12.6 @ nominal capacity - 95 deg outdoor dry bulb / 80 deg indoor at 51% humidity.

    I also looked at a 5 ton 16 SEER Amana that shows up to 12.5 EER when matched with a certain blower / furnace.

    Now for my questions; If the 4 ton 21 SEER unit is running much more than the 5 ton 16 SEER unit is there going to be a big difference in energy efficiency given the fact that the EER values are so close? Would it be safer to go with a 5 ton, which is also about $900 cheaper than the 4 ton IQ drive? Should I listen to the AC people giving me quotes or the heat load analysis sheet that says I will be fine with a 4 ton?

  2. #2
    Join Date
    Jan 2004
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    Lancaster PA
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    The IQ will run more at lower capacity, so it will use less electric, and will most likely keep your humidity lower.

    Is the load calc from a contractor.
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  3. #3
    Join Date
    Jun 2007
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    The Twilight Zone
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    Quote Originally Posted by diyer23 View Post
    Would it be safer to go with a 5 ton, which is also about $900 cheaper than the 4 ton IQ drive?
    Pricing is not allowed on this Forum, but if the $$ difference between the systems is what you say it is, either the Amana is over-priced, the IQ Drive is under-priced, or the bids do not include the same scope of work.

    Good luck.

  4. #4
    Join Date
    Jun 2007
    Location
    The Twilight Zone
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    2,964
    The 4 ton IQ Drive (variable speed compressor) has a nominal capacity range of 16,000 btu to 54,000 btu. 54,000 btu = 4.5 tons.

  5. #5
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    Jan 2004
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    The 5 ton might only have a 53,000.
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  6. #6
    Join Date
    Jun 2008
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    7
    The load calc was done by a licensed AC company that is trying to sell me the 4 ton system. I asked for a copy of the report and the numbers show that I require 41,245 BTUh of cooling. Even though the analysis shows I need less than a 4 Ton unit, I currently have a 5 ton unit and I am still scared to drop. I would really like to have the significanlty higher SEER unit, but should I trust the analysis?

  7. #7
    Join Date
    Jan 2004
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    Lancaster PA
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    During your normal hot summer months, did your current 5 ton cycle off.

    If so, you can most likely drop to the 4 ton IQ with no problems. The IQ can increase its capacity to more then its rating for quick cool downs. And those few days that the OD temp exceeds design.
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  8. #8
    Join Date
    Dec 2007
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    Cedar Grove, Wi-Sheboygan
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    If the contractor who apparently did the heat load calc says you need a 4 ton I would go with the 4 ton, the other contractors are obvisously just going by what you already have installed to save themselves sometime and figure your going to believe them because you already have a 5 ton system in your home. Ask the other contractors who told you to go with a 5 ton unit to give you the numbers that told them you needed a 5 ton unit and ask the contractor who said 4 ton to give you his numbers to compare what each have but do not trade numbers from other contractors calc's with the other one's. I bet the ones that said 5 tons are just basing everything on the fact you already have a 5 ton. You know rule of thumb says customer already has on so that is what he must need for a replacment

  9. #9
    Join Date
    Mar 2001
    Posts
    285
    When it's 102* outside and 84* on your second floor, and you end up going to Lowe's or Walmart to buy a window unit because your 23 SEER undersized system can't keep up, you will probably say an ugly word.

    of course if you have R-40 in the attic, low-E Argon windows, and a super tight house 4 tons may be all that's necessary....

    but..

    I'm not a fan of one system serving two floors.

  10. #10
    Join Date
    Jan 2004
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    Lancaster PA
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    He's already got one system serving 2 floors. He should know the pit falls by now.

    How do you figure a 4 ton, with the same BTU rating of many 5 ton units is undersized.
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  11. #11
    Join Date
    Jun 2001
    Location
    DFW
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    683
    If your duct work is not in unconditioned space, make sure you have them tested for leaks. A leaky duct system is costly either way, but as the run time on the A/C goes up, the losses go up even faster.

    In other words, with leaky duct work, an oversized system may be more efficient than one that is right sized. Long run times with a system that is right sized leads to lots of energy lost to leaky duct work.

  12. #12
    Join Date
    Jun 2008
    Posts
    7
    Thanks for the information on the possibility of leaky ducts. I contacted my utility company and requested a duct leak test.

    In regards to having a single AC unit on a 2 story house, I have not really had much of an issue. The air handler is upstairs and the upstairs is usually a little cooler than the downstairs.

    All of the contractors that suggested a 5 ton unit have all suggested it because that is what I currently have. When I have asked about a load study, many of them say that the rule of thumb is 500sqft/ton and since my house is 2500 sqft that there really is no use in doing a load calc. They say they would just not recommend anything lower than a 5 ton. If this is true, I don't understand why the one contractor came up with a study showing I need a 3.7 ton machine (41,245 btuh). It seems like there is a big discrepancy between the "rule of thumb" (also my current system) and what the load calc shows.

    To answer beenthere's question, the 5 ton system I have does run a lot on hot summer days, but it doesn't run continuously. It does turn on and off throughout the day. Does this really mean that a 4 Ton will work for me?

  13. #13
    Join Date
    Jan 2004
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    Lancaster PA
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    Since your 5 ton is cycling. I think you'll find the 4 ton IQ will handle your load.

    Rules of thumb aren't reliable.

    Paul42 has a 2 ton for a 4000 sq ft home. His condstruction is of course much different then yours.
    But its the construction, windows, shade or lack of, along with insulation and infiltration that determine what size. And the one that did the load calc took these things into consideration to do the load calc. And thats why he says 4 tons will do your house.
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