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  1. #1
    Join Date
    Aug 2010
    Location
    medina ohio
    Posts
    7

    Central Air blower speed???

    I have a carrier weathermaker 8000 58tua furnace. I believe the blower speed for the ac is set incorrect. I have read that you set blower speed to 400 cfm per ton of cooling and I have a 2 ton system. Right now cooling is set to high speed on the blower which is 1195 cfm. Duct noise has always been an issue with both the returns and supply vents. The heat side is currently using the low and med-low, so the only available speed setting on the blower motor is the med-high. According to the installation manual the med-high setting looks to be about 100 cfm less depending on the external static pressure? Is it worth slowing down the blower to the med-high setting? Other than noise is there anything wrong with having the blower speed set so high? If I use the low speed setting for cooling am I going to have to move the wire back and forth between heating and cooling seasons? Also, I am not going to change the blower speed myself because I know the charge on the ac will need to be adjusted. Thanks for you help!

  2. #2
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    Having blower set too high uses more energy (espcially with undersides ductwork) and you won't remove humidity as well if at all. This makes it a low less comfortable.

    The speed range on PSC motors is somewhat limited. Depending on your static pressure, you may be able to go to med-lo or even lo to get to 800CFM.

    One fairly easy way ot determine the airlfow (when it't not hot inside and outside) is to run the furnace and then measure the temperature rise as close to the furnace as possible after it's run for at least 5 minutes. Then take the BTU's, divide by 1.08 and then by the temp rise and you have CFM. Compare that to the blower speed charts in the data sheets for the furnace. Then assume your static rises a little as the blower speed increases and goes down as speed decreases.

    I don;t see any hardm in Med-hi unless you have extremely restricted ducts. But again, it's possible you my be able to go down to lo speed.

  3. #3
    Join Date
    Jul 2004
    Location
    Massachusetts
    Posts
    6,837
    It sounds like the blower is inappropriately sized for your cooling needs. However, a good service tech can slow the blower down using low or medium low without any problem, even though the heating uses the same speeds. Carrier/Bryant actually sizes the airflow for their systems at a factory default of 350 CFM/ton, not 400 CFM/ton. So you'd be looking for 700 CFM for your system in an ideal world but 800 CFM would also work. If you have the installation manual for the unit, the fan curve would tell you exactly what speed tap to use to get closest to the desired CFM, based on TESP. Your installing contractor should be able to determine all of those things for you. Or you've got the wrong contractor. That could be a possibility, eh?
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  4. #4
    Join Date
    Aug 2010
    Location
    medina ohio
    Posts
    7
    Here is a chart of blower speeds for my furnace. How can the low blower speed be both used for heating and cooling?
    Attached Images Attached Images  
    Attached Files Attached Files

  5. #5
    Join Date
    Aug 2010
    Location
    medina ohio
    Posts
    7
    Also, this is an existing install in a house that I bought. House has always seemed humid even with the ac set at 72. I have never brought this issue up to my tech. I just want to get all my info together before I give him a call.

  6. #6
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,112
    Any good service tech will know how to make it use the same speed for both heating and cooling.
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