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  1. #14
    Join Date
    Aug 2008
    Posts
    9
    So then I assume the low setting (stage 1) on the furnace could not / should not be set on the 50% setting or whatever runs once the outdoor AC unit shuts down then the AC is running?

  2. #15
    Join Date
    Dec 2002
    Location
    Rochester, MN
    Posts
    5,304
    It depends on the A/C size, and furnace size. Some A/C will move more air than furnace's 1st stage, and some furnace will move more air than the A/C.

    Here is what I got.

    XV90 60k
    XR11 2ton

    My air flow in A/C

    50% = 400 cfm
    80% = 650 cfm
    100% = 800 cfm

    Air flow of the furnace.

    1st stage =775 cfm
    2ns stage = 1,126 cm

  3. #16
    Join Date
    Apr 2008
    Location
    Princeton, NJ
    Posts
    581
    Quote Originally Posted by mayguy View Post
    It depends on the A/C size, and furnace size. Some A/C will move more air than furnace's 1st stage, and some furnace will move more air than the A/C.

    Here is what I got.

    XV90 60k
    XR11 2ton

    My air flow in A/C

    50% = 400 cfm
    80% = 650 cfm
    100% = 800 cfm

    Air flow of the furnace.

    1st stage =775 cfm
    2ns stage = 1,126 cm
    Is it normal for the CFM's in cooling mode to be lower than the CFM's in heating mode? I'm looking at the 1126 compared to the 800. I would think that since the house size does not change with the season, the required CFM air flow would be close to the same. Is that not the case?
    Key1
    In the land of the blind.....the man with one eye is king....

  4. #17
    Join Date
    Dec 2002
    Location
    Rochester, MN
    Posts
    5,304
    Quote Originally Posted by key1cc View Post
    Is it normal for the CFM's in cooling mode to be lower than the CFM's in heating mode?
    Again, it depends on what the Manual J comes back for the home itself. Rule of thumb is about 400 cfm per ton, and If I was down say AZ, my home may call for 4tons of A/C, now that becomes 1,600 cfm of air for A/C, and may only need 45,000 BTU of heating, now my heating speed is less than my cooling speed.


    I'm looking at the 1126 compared to the 800.
    Where are you getting this number from?

    I would think that since the house size does not change with the season, the required CFM air flow would be close to the same. Is that not the case?
    Your size of the home don't change, but the heating or cooling load does.

  5. #18
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,174
    Quote Originally Posted by key1cc View Post
    Is it normal for the CFM's in cooling mode to be lower than the CFM's in heating mode? I'm looking at the 1126 compared to the 800. I would think that since the house size does not change with the season, the required CFM air flow would be close to the same. Is that not the case?
    Key1
    You can add more BTUs per CFM then you can remove.

    So it varies by the area, which mode needs to move more air.
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    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

  6. #19
    Join Date
    Apr 2008
    Location
    Princeton, NJ
    Posts
    581
    Quote Originally Posted by mayguy View Post
    .



    Where are you getting this number from?

    In your post you listed heating stage-2 airflow as 1126 CFM and your 100% cooling airflow you listed as 800 CFM.

    Key1
    In the land of the blind.....the man with one eye is king....

  7. #20
    Join Date
    Dec 2002
    Location
    Rochester, MN
    Posts
    5,304
    Quote Originally Posted by key1cc View Post
    In your post you listed heating stage-2 airflow as 1126 CFM and your 100% cooling airflow you listed as 800 CFM.
    Oh ok. Yes, will need more air flow in heating than you do for cooling since cooling demand is not high up here in the northern part of the USA vs southern.

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