Question about fan speed settings
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  1. #1

    Question about fan speed settings

    Greetings,

    I've got a 5-ton unit with adjustable fan speeds and trim adjustments. Given that a 5-ton unit delivers 2000 CFM, why are there options to adjust the fan speed to 1200, 1600, 1800 and 2000 CFM speeds?

  2. #2
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    The indoor unit manufacturer allows the unit to operate at 3-5 tons of cooling. Each ton of cooling requires a different airflow thus 1 unit can be used in several applications
    ...

  3. #3
    That's what I thought, but the unit is referenced against 3, 4 and 5 ton systems....

    The 3 ton system has a fan speed range of 600-1200 CFM depending on the dip switch settings, with a +/-10% trim adjustment to make finer adjustments
    The 4 ton system has a fan speed range of 1000-1600 CFM
    The 5 ton system has a fan speed range of 1200-2000 CFM

    Could, for example, the 5 ton system be run at 1600 CFM to satifsy airflow requirements, or must the fan deliver 2000 CFM for the system to operate properly?

  4. #4
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    A general rule of thumb is 400cfm per ton. In cooling there are 2 types of "heat" the temperature and the moisture. By lowering to say 350cfm per ton you will receive better moisture removal and poorer temperature removal (the system will in longer to satisfy the temperature requirements but the relative humidity will be lower), conversely if 450cfm per ton were used, you would receive better temperature removal and less moisture removal per hour of runtime. It depends primarily on your climate. Some systems will allow you to set a relative humidity on the thermostat and will ramp the blower up and down as needed to maintain a required humidity.
    ...

  5. #5
    Quote Originally Posted by kangaroogod View Post
    By lowering to say 350cfm per ton you will receive better moisture removal and poorer temperature removal (the system will in longer to satisfy the temperature requirements but the relative humidity will be lower), conversely if 450cfm per ton were used, you would receive better temperature removal and less moisture removal per hour of runtime.
    Thanks for the clarification....Actually, you may have helped me determine what might be causing an issue we're having. The humidity in the house is high, so even by setting the AC at 73 degrees, we wake up in a sweat at night. I guess you could say the house is cold but clammy.

    How low a fan speed setting is safe with a 5 ton unit, before you risk icing the evaporator, and/or causing a liquid flooding condition in the compressor? We could certainly benefit from more moisture removal, as we have no issues with the temperature removal side of the equation.

  6. #6
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    Quote Originally Posted by seadweller View Post
    Thanks for the clarification....Actually, you may have helped me determine what might be causing an issue we're having. The humidity in the house is high, so even by setting the AC at 73 degrees, we wake up in a sweat at night. I guess you could say the house is cold but clammy.

    How low a fan speed setting is safe with a 5 ton unit, before you risk icing the evaporator, and/or causing a liquid flooding condition in the compressor? We could certainly benefit from more moisture removal, as we have no issues with the temperature removal side of the equation.
    This is a symptom of an over sized system. Or high infiltration and or duct leakage!
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  7. #7
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    Quote Originally Posted by seadweller View Post
    Thanks for the clarification....Actually, you may have helped me determine what might be causing an issue we're having. The humidity in the house is high, so even by setting the AC at 73 degrees, we wake up in a sweat at night. I guess you could say the house is cold but clammy.

    How low a fan speed setting is safe with a 5 ton unit, before you risk icing the evaporator, and/or causing a liquid flooding condition in the compressor? We could certainly benefit from more moisture removal, as we have no issues with the temperature removal side of the equation.
    Just above 1600 CFM.
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  8. #8
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    Quote Originally Posted by beenthere View Post
    Just above 1600 CFM.
    As Beenthere said above 1600. Where do you live? What's your homes size etc???

    Might want to invest in t-stat that reads humdity levels and or one you but at Walmart for the same effect. This will help you to determine two things temp in the home and humdity which can tell you a lot.

    If the blower is set up to another tonage for its size then it would cool but never deliver the rated cfm for the system to work right, would remove more humdity as it would just run to try to stay up, and could freeze the coil up as its not moving enough air for the real size of your condenser.

  9. #9
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    IF the AC isn't running constantly for at least about 2-3 hours in the late afternoon on a sunny humid day at or above design temp in summer ot hold 75F, then it's likely oversized. IF your lieking the mid Atlantic for example, that might only be 89-90F. So in those temps, it should run almost continously at 90F outside from around 3PM-9PM.

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