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  1. #1

    Are mismatched evap coil and compressor sizes a bad idea?

    Hi all, I posted earlier about some estimates we got. We got another one on a Trane today but this contractor is suggesting a 2 ton outdoor unit (max our electrical system can handle) paired with a 2.5 ton coil. He says this would cool better. Until the current coil failed on our 25 year old 2 ton Ruud, we didn't have any trouble keeping the house at 80 which is fine for us while we're home. (we live in the DC suburbs, so summers aren't particularly brutal, just humid) Does it make any sense to mismatch like this? None of the other contractors suggested this. What would the benefits to this be? Possible drawbacks?

  2. #2
    Join Date
    Dec 2002
    Location
    Houston,Tx.
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    15,504
    Quote Originally Posted by hitthewall55 View Post
    We got another one on a Trane today but this contractor is suggesting a 2 ton outdoor unit (max our electrical system can handle) paired with a 2.5 ton coil. He says this would cool better..
    I believe if and old Indian Chief had a say today in this business, he would say, "much advise causes much confusion". This is why the more folks tire kick, the more confused they will be. I think it's great to get a couple/three bids, but everyone will have a different opinion and price. Back in the old school days, the 1/2 ton larger evaporator coil was very common, but this is a new age with newer technology, the best advise I can give you and I would hope most pro's here would agree is to, go strictly with what the mfg. of your equipment says is the best/perfect match, that way if down the line you encounter any issues and a mfg. rep. has to come out, your covered on your end.
    “Reality is merely an illusion, albeit a very persistent one.” –Albert Einstein
    “Experience is a hard teacher because she gives the test first, the lesson afterwards.”-Vernon Law

  3. #3
    Join Date
    Dec 2010
    Location
    The Quad-Cities area (midwest).
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    If you're only looking for a low SEER, you can match the coil size to the condenser.

    If a higher SEER is desired, then normally a larger coil is necessary. A larger coil will not dehumidify the home as well, however.

    It just depends on your goal.

  4. #4
    Quote Originally Posted by George2 View Post
    If you're only looking for a low SEER, you can match the coil size to the condenser.

    If a higher SEER is desired, then normally a larger coil is necessary. A larger coil will not dehumidify the home as well, however.

    It just depends on your goal.
    We're way more concerned about humidity than temperature. We run a dehumidifier downstairs frequently to combat the sticky air. Can I make sure I'm understanding correctly? Larger coil means more cooling surface area which would mean more cold air in a shorter amount of time, right? So the AC wouldn't run as much. But that would also mean the big coil wouldn't ever get as cold or stay cold as long which wouldn't let it suck as much moisture out of the air as if it was running colder and longer. Is that the general logic behind it? That makes sense to me if so, and then I think I'd rather the smaller coil. Our energy bills are not high with our 25 year old unit so increasing the efficiency isn't a giant priority for us.

  5. #5
    Join Date
    Aug 2007
    Location
    Northern VA 38 degrees N by 76 degrees W
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    5,046
    Quote Originally Posted by hitthewall55 View Post
    Hi all, I posted earlier about some estimates we got. We got another one on a Trane today but this contractor is suggesting a 2 ton outdoor unit (max our electrical system can handle) paired with a 2.5 ton coil. He says this would cool better. Until the current coil failed on our 25 year old 2 ton Ruud, we didn't have any trouble keeping the house at 80 which is fine for us while we're home. (we live in the DC suburbs, so summers aren't particularly brutal, just humid) Does it make any sense to mismatch like this? None of the other contractors suggested this. What would the benefits to this be? Possible drawbacks?
    Ask them to do a Manual J load calculation, this will let you know what your homes latent load and sensible load is. From this the correct coil can be determined. There is no set combination that is right for your home without determining the type of load that needs to be dealt with.

    Most contractors will attempt to sell you the highest SEER possible but if installed in the wrong application it will cost you more money to attempt to accomplish what is needed.

  6. #6
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    33,371
    Putting the 2 ton Trane on a 2 ton coil gives 23,000 BTUs. On a 2.5 ton coil (031) gives 23,000 BTUs. Ask him how that cools better? All it did is skew a few hundred BTU off latent (humidity removal) and move toward sensible. OK, he could say that cools better but could add a point or 2 to RH%.

    Larger coil has more cooling area, means warmer coil which is why the larger coil drops humidity removal a hair.

  7. #7
    Ok, thanks so much. I don't see how the bigger coil would make sense for us. I think he oversized the furnace too. He didn't do any real load calculation, just kind of asked about our use and how things were with the units we had. He wrote up for the same size furnace as our current one, and the 2 guys who did measurements and spent a long time doing calculations on what we needed both came up with far lower outputs. I think we will pick between them. It seems like they were more by the book in terms of coming up with products for us.

  8. #8
    Join Date
    Dec 2010
    Location
    The Quad-Cities area (midwest).
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    Quote Originally Posted by BaldLoonie View Post
    Putting the 2 ton Trane on a 2 ton coil gives 23,000 BTUs. On a 2.5 ton coil (031) gives 23,000 BTUs. Ask him how that cools better? All it did is skew a few hundred BTU off latent (humidity removal) and move toward sensible. OK, he could say that cools better but could add a point or 2 to RH%.

    Larger coil has more cooling area, means warmer coil which is why the larger coil drops humidity removal a hair.
    B.L.

    What happens to the humidity (removal) numbers if a 1 1/2 ton coil is used?

    Thanks

  9. #9
    Join Date
    Nov 2004
    Location
    SW FL
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    5,985
    Quote Originally Posted by George2 View Post
    What happens to the humidity ... if a 1 1/2 ton coil is used?
    I'd stick to use of a refrigerator for making ice.
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  10. #10
    Join Date
    Jul 2006
    Location
    Virginia
    Posts
    4,512
    Is guy proposing the larger furnace and coil cheaper ?

  11. #11
    Quote Originally Posted by catmanacman View Post
    Is guy proposing the larger furnace and coil cheaper ?
    Nope, not a bit. A little more than the other Trane quote. About the same as the Amana.

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