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  • Mark_46
    Mark_46 Member Posts: 312
    Heat vs A/C

    Assume a duct/airhandler system provides both heat and A/C and all equipment is properly set up and sized...all is ideal.

    Does heating require less CFM than A/C? If so, what is the general rule of thumb? Heat requires half the CFM of A/C?
  • Brad White
    Brad White Member Posts: 2,393

    AC requires more air than heating. This is principally a function of Delta-T. The ratio "depends". It could be 1:1 down to 1:0.6

    If your supply air temperature is at 54 degrees and your space is to be maintained at 74 degrees, that is a 20 degree delta-T which will define your airflow rate.

    (Sensible heat BTUH / (Delta-T x 1.085) is the formula at sea level.

    Heating cfm is similar and depends on, you guessed it, your outlet temperature. Modern condensing furnaces and hydro-air coil systems may have outlet temperatures between 90 and 110 degrees, sometimes more. Conventional furnaces and high-temperature (180 degree water) hot water coils may have supply air in the 120 to 150 degree range. Egads.

    If you are dealing with a hydro-air system with a fixed cfm based on cooling, your heating temperature would be adjusted to suit. In other words, more air than normal but at a cooler temperature. If a lower speed is desired (less airflow) the temperature goes up, simple as that.

    Thus as an example you can see a 3-ton system with say 27,000 sensible heat gain would deliver about 1,250 CFM. If the same space has a heat loss of 45,000 BTUH, the supply air temperature at 1,250 cfm would need to be 103.3 degrees to heat a 70 degree room.

    Cut the airflow to 2/3 (833 CFM) your air would need to be 119.8 degrees, all other things being equal.

    Now, if you are using a Unico or other High-V system, your airflow is already limited thus your cooling supply air is in the high 30's/low 40's, call it a 30 degree delta-T. You will be forced to the higher heating air temperature.
    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"

    -Ernie White, my Dad
  • John Mills_5
    John Mills_5 Member Posts: 935

    Some furnace require a lot of airflow to keep them off limit. Take a Rheem RGRA-10 90+ furnace. We would run it around 1400 CFM for heating. But often around here it is matched with a 3 ton A/C. For proper dehumidification, we'd want 1200 CFM. So in this case, we'd run the heat speed at medium low (assuming around .5 static) and cool speed on low.
  • Ken D.
    Ken D. Member Posts: 836

    In general, hot air has molecules that are already in motion, so it needs less energy to move it. Cool air has less molecules in motion, thus more energy to move it.
  • Mark_46
    Mark_46 Member Posts: 312
    Hey Brad

    What can I say? That's a beautiful post :o).

    While the text is a great help, the formula is the pi
  • Eugene Silberstein
    Eugene Silberstein Member Posts: 1,380
    As Mr. White mentioned...

    As Brad mentioned, cooling TYPICALLY requires more CFM than heating. This definitely does not imply ALWAYS. You can take a below-grade basement as a good example. The cooling load is very low, but the heating load is much greater.

    Always be sure to size your branch ducts for the larger CFM requirement, be it in the heating or the cooling mode.

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