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  1. #1
    Join Date
    Aug 2018
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    Why is fan speed different in heating vs. cooling mode?

    Hello - Curious why the fan speed tends to be different in heating or cooling mode in a gas furnace?CAC system, generally speaking.
    That is, on a "normal" heating day vs. a "normal" cooling day, my understanding is that the fan will run at either a higher or lower speed (not sure which is which) depending on whether the system is in heating or cooling mode.
    Why is that?
    Thanks

  2. #2
    Join Date
    Nov 2006
    Location
    Athens, Ohio
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    Factory settings typically are high for cooling and low for heating. That is not necessarily what is needed.
    Cooling systems need between 350 and 400 CFM. Furnaces need between 100 and 150 CFM per 10,000 btu capacity. In my region, heating capacity is greater than cooling capacity. The duct system effects how hard the blower must work to deliver that amount of air flow.

    There. That was easy, wasn’t it?
    AOP Rules: Rules For Equipment Owners.

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


    There = not here. Their = possessive pronoun. They're = they are
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  3. #3
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
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    Air flow rates are critical for providing the %RH level in your home during the cooling season. If you want 50%RH during a/c operation, the air flow needs to be adjusted to provide dry air to the home. Adjust the air flow to have an a/c coil temperature 30^F lower than the supply air temperature.
    Keep us posted.
    Regards Teddy Bear
    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"

  4. #4
    Join Date
    Nov 2006
    Location
    Athens, Ohio
    Posts
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    Many owners want to set blower speeds to reduce noise. Sound is not a basis for adjusting blower settings. Noise often indicates that the duct system is undersizedfor the equipment.
    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
    Jan 2004
    Location
    PA
    Posts
    75,874
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    In heat mode your furnace will need a min temp rise, so the air flow has to slow enough to have that min rise. On an A/C the temp change is only gonna be 20 plus or minus degrees, so it needs to move a lot more air per BTU.

    A 36,000 BTU output furnace with a 50 degree temp rise needs to move 667 CFM
    A 3 ton/36,000 BTU A/C needs to move 1203 CFM for a 20 degree temp change(allowing 10,000 BTUs for latent heat).
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