Page 1 of 3 123 LastLast
Results 1 to 13 of 28
  1. #1
    Join Date
    Jul 2012
    Posts
    20
    Post Likes

    Ac on Oversized Furnace

    Hi, I need to install air conditioning in a house that is 2500 sq ft (2 level, 1250 Sq Ft/Level), The furnace is a Lennox 80,000 BTUs with the "048" in the model number (up to 4 Ton Cooling). I did a manual J, turns out I need 2.5 Tons of cooling (new well insulated house). The furnace cfm chart on low will provide 1250 CFM min (too high for 2.5 Tons).

    Would it be better to install an oversized unit (3 Ton) with proper airflow or have too much airflow going through a 2.5 Ton system.

  2. #2
    Join Date
    Apr 2003
    Location
    St. Louis
    Posts
    1,041
    Post Likes
    Talk with lennox they will know how much air will flow through a 2.5 coil of theirs with that furnace.

  3. #3
    Join Date
    Jul 2012
    Posts
    20
    Post Likes
    Thread Starter
    I already know how much CFM will be going through the furnace as stated above.

  4. #4
    Join Date
    Nov 2006
    Location
    Athens, Ohio
    Posts
    6,433
    Post Likes
    That’s based on static pressure. Have you measured static pressure on it so you’ll know the actual air flow?

    If you’re in a green grass climate I would not upsize the cooling.
    AOP Rules: Rules For Equipment Owners.

    Free online load calculator: http://www.loadcalc.net/


    There = not here. Their = possessive pronoun. They're = they are
    It's = contraction of it is. Its = the possessive form of it
    Too = also. To = expressing motion. Two = 2
    Then = after that, next. Than = indicates a comparison.
    Questions should end with a question mark "?" Statements end with a period "."

  5. #5
    Join Date
    Jul 2012
    Posts
    20
    Post Likes
    Thread Starter
    According to their chart,

    At 0.1 "W.c, low speed will deliver 1360 CFM

    At 0.5" W.C, low speed will deliver 1285 CFM.

    At 0.9" W.C, low speed will deliver 901 CFM

    I have not measured static, but I would have to be highly restricted near 1" W.C to be at 400 CFM/Ton. This is in the north, doesn't get too humid or hot.

  6. #6
    Join Date
    Jul 2012
    Posts
    20
    Post Likes
    Thread Starter
    So, going with a 3 ton unit with matched 400 CFM/Ton, I know the unit will dehumidify. If I have say 600CFM/Ton, what will happen?

  7. #7
    Join Date
    Nov 2006
    Location
    Athens, Ohio
    Posts
    6,433
    Post Likes
    Poor dehumidification. Noisy blower depending on duct sizes.
    AOP Rules: Rules For Equipment Owners.

    Free online load calculator: http://www.loadcalc.net/


    There = not here. Their = possessive pronoun. They're = they are
    It's = contraction of it is. Its = the possessive form of it
    Too = also. To = expressing motion. Two = 2
    Then = after that, next. Than = indicates a comparison.
    Questions should end with a question mark "?" Statements end with a period "."

  8. #8
    Join Date
    Apr 2003
    Location
    St. Louis
    Posts
    1,041
    Post Likes
    I'll make a 20 minute phone call verses reinventing the wheel! Since manufacturers know exactly what coil and expansion device works best.

  9. #9
    Join Date
    Dec 2006
    Location
    Vancouver, B.C.
    Posts
    361
    Post Likes
    Too high of a CFM over the coil can blow the condensation off of it instead of allowing it to drain. This can lead to water leaks and related issues. Your unit might run with excessive superheat leaving less cooling for your compressor. You would get less temperature drop over your coil so your cooling would be more sensible than latent, so if you are in a humid climate it will do less to remove the moisture from the air.

    I’m curious if your unit can run at a low enough CFM to operate your AC properly without, at the same time, causing your heat exchanger to overheat in heating mode. Is it gas or electric heat? Could run into problems either way.

    As mentioned above, I would talk to the manufacture and see what they recommend. You are presenting a difficult situation though.

    I’m sorry, I’m struggling to recall why your furnace was so large when your heat load is so little, I’m sure you mentioned that in your first post but can’t recheck that right now. if I recall it was due to a recent renovation. If your Reno has reduced your heat load load that much you also should expect your heat loss to also drop. Aren’t you oversized for heating now? That can cause issues to your comfort and equipment also.


    Sent from my iPhone using Tapatalk
    Quickly, I must hurry, for there go my people and I am their leader!

  10. #10
    Join Date
    Jul 2012
    Posts
    20
    Post Likes
    Thread Starter
    This is for a new home, I agree that 80,000 BTUs is oversized for a 2500 sq ft home, especially because it's new and well insulated.

  11. #11
    Join Date
    Jul 2018
    Location
    Wisconsin
    Posts
    739
    Post Likes
    Where are you located?

    Sent from my SM-G960U using Tapatalk

  12. #12
    Join Date
    Jul 2012
    Posts
    20
    Post Likes
    Thread Starter
    Located in Canada, gets to 90-100F for a few months in the year, 35-45% humidity during those months.

  13. #13
    Join Date
    Dec 2006
    Location
    Vancouver, B.C.
    Posts
    361
    Post Likes
    Sorry, I should have re read the first post before making my last reply. Half ton difference is only off by 200 CFM.

    What temperature rise do you get on heating right now and what does the name plate say you are allowed to have. It will post your min and max temp rise on there.


    Sent from my iPhone using Tapatalk
    Quickly, I must hurry, for there go my people and I am their leader!

Page 1 of 3 123 LastLast

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •