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  1. #1
    Join Date
    Oct 2006
    Posts
    8

    Downsizing evaporators

    I have always heard that for greater effeciency, the factories will up size the evaporator btu over the rated btu of the condensing unit.
    I have a person who says in Houston, to alleviate the humidity problem, you should downsize an evaporator 1/2 ton below what the btu rating is on the condensing unit to remove more moisture.
    Maybe I am learning something new.
    Is this true and why? Thanks, for the help.

  2. #2
    Join Date
    Feb 2008
    Location
    Florida
    Posts
    146
    I've never hurd that. I know if you put a smaller coil in, you can kill the comp. Plus there are other ways to control humidity with out installing a un matched system.

  3. #3
    Join Date
    May 2007
    Location
    somewhere between heaven and hell
    Posts
    957
    Long ago I use to work for a contractor who oversized evaporator coil by 1/2 a ton from the condenser. I personally recommend exact size for both. Long run by oversizing indoor coil by 1/2 ton I don't think there ever was a problem while I was at this company and I was there for like 7 years. Again though I don't recommend it.

  4. #4
    Join Date
    Jun 2001
    Location
    Louisville, KY
    Posts
    12,189
    Perhaps you should have read the instructions before calling.

  5. #5
    Join Date
    Dec 2002
    Location
    Houston,Tx.
    Posts
    16,115
    Quote Originally Posted by toptek View Post
    I've never heard that. I know if you put a smaller coil in, you can kill the comp.

    Well there sure are going to be a lot of dead compressors in Houston one day.


    I don't know were you heard that but it must have been from a Plumber.
    __________________________________________________ _______________________
    “Experience is a hard teacher because she gives the test first, the lesson afterwards". - Vernon Law

    "Never let success go to your head, and never let failure go to your heart". - Unknown

  6. #6
    Join Date
    Jun 2001
    Location
    Louisville, KY
    Posts
    12,189
    LOL...

    dats why i posted da link
    Perhaps you should have read the instructions before calling.

  7. #7
    Join Date
    Dec 2007
    Location
    Ohio
    Posts
    2,876
    Quote Originally Posted by jrbenny View Post
    That's an interesting read....I lived in Mobile AL for a few years....everything down there was undersized. It was interesting to see the differences between there and here (Ohio).
    I need a new signature.....

  8. #8
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,597
    Big coils, medium, or small coils can be desighed to work with any size compressor. Cap tubes, orfices, or expansion valves can be used with any of the above combinations. Specific design includes the corrert amount of refigerant matching all the devices. Most techs should avoid mixing components because of lack of design expertise.
    Regarding moisture removal, the larger the coil, the lower the amount of air flow. Large coils remove more moisture without freezing up as quick. Large coils retain more moisture on the coil between cycles (problem). Large coils remove more btus of sensible and latent heat per watt of energy.
    Use the coil desighed for the system unless you are really good. With the current +13 SEER systems, all the coils are large. Slow the air flow to remove as much moisture as reasonable while avoiding sweatting ducts, while getting adequate air distribution throughout the home.
    In a home with adequate fresh air ventilation, normal occupancy, and optimum moisture removal by the a/c, supplemental dehumidification is required during wet cool weather. In green grass climates during wet weather, the moisture load is 3-5 lbs. per hour of latent load without any sensible load. Homes that are dry during this condition are not getting the proper amount of fresh to purge the pollutants and renew the oxygen. I am interested in any discussion of 75 cfm of fresh air during low/no sensible and high latent load and maintaining <50%RH. Most home are not getting adequate fresh air. Regards TB
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

  9. #9
    Join Date
    Oct 2006
    Posts
    8
    The technician did install a TXV with the smaller coil. He said the smaller coil with the TXV would remove more moisture than the larger coil.
    Still seems strange. Correct me if I am wrong.

  10. #10
    Join Date
    Mar 2005
    Location
    burlington county n.j.
    Posts
    9,761
    as long as system is set up right with txv and proper charge you're wrong......


    around here we usually go with the larger coil thats an ari match for highest efficiency.

    i have read a lot about using the undersized coils down south and never heard of a problem.

  11. #11
    Join Date
    Dec 2002
    Location
    Houston,Tx.
    Posts
    16,115
    Quote Originally Posted by aerod1 View Post
    The technician did install a TXV with the smaller coil. He said the smaller coil with the TXV would remove more moisture than the larger coil.
    Still seems strange. Correct me if I am wrong.
    Heard this said once, that every manufacturer creates and publishes "expanded" ratings. They may go by different names, and contain varying amounts of data, but they comprise all the different sensible and latent capacities at different air flows, different coil and condenser combinations, and different super heat settings (for TXV systems).
    This data will be the best source of the answer for your question. I can tell you that a larger coil, with higher airflow, will get you more sensible and less latent, but I can't tell you how much for each different combination. Only the manufacturer can do that; by way of their published data, hope this helps.
    __________________________________________________ _______________________
    “Experience is a hard teacher because she gives the test first, the lesson afterwards". - Vernon Law

    "Never let success go to your head, and never let failure go to your heart". - Unknown

  12. #12
    Join Date
    Apr 2002
    Posts
    11,808
    The smaller coil runs colder, loses total capacity

    The latent capacity increases at the expense of the sensible capacity. So when the system runs you have more moisture removed, and because you have reduced sensible capacity the system runs longer

  13. #13
    Join Date
    Nov 2000
    Location
    Eastern PA
    Posts
    68,964
    Quote Originally Posted by Carnak View Post
    The smaller coil runs colder, loses total capacity

    The latent capacity increases at the expense of the sensible capacity. So when the system runs you have more moisture removed, and because you have reduced sensible capacity the system runs longer
    Most concise way to state this. Teddy Bear's post is also correctly informative. There is nothing new here and not every application will benefit from the same coil sizing.

    Smaller coil decreases efficiency while increasing comfort in humid areas.
    lower air flow can do the same.
    Larger coils can increase efficiency while decreasing moisture control.

    In a very dry, hot region, a larger evap coil with higher amount of air will provide great cooling at higher efficiencies. This same set up will cause uncomfortable air conditions in moderate temperature areas with heavy humidity. You have to consider the conditions of the application before deciding which way to design the system.
    Government is a disease...
    ...masquerading as its own cure…
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