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  1. #1
    Join Date
    Mar 2010
    Posts
    43

    Blower speeds, call to dehumidify

    I have a 3 ton AC with a variable speed blower furnace in central Ohio.
    G71MPP-60C-090
    XC14-036-230
    C33-49C-2-6 .073 orifice

    Beenthere and others have largely stated ~350/ton is a good max AC speed to control humidity. What is a safe speed on a call to dehumidify? 756 as option 1 below seems low volume.

    Option 1:
    885 CFM first 7.5 minutes
    1080 CFM max speed to end of cooling call
    756 CFM call to dehumidify

    Option 2:
    984 CFM first 7.5 minutes
    1200 CFM max speed to end of cooling call
    840 CFM call to dehumidify

  2. #2
    Join Date
    Jul 2004
    Location
    Massachusetts
    Posts
    6,837
    The manufacturer has the info to most accurately answer your question but normally I'd look for a minimum airflow on a continuous basis, such as dehumidifying, as 80% of the minimum cooling speed. So at 350-CFM/ton = 1050 x .8 = 840 CFM. Any slower and I'd be concerned about ice forming on the evaporator coil.
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  3. #3
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    I don't see an option 3... get a thermostat that has humidify control and use the Dehumdifying terminal on the control board that pretty mcuh all vairable speed furnaces and air handlers have. Then you jsut dehumidify on-demand only. IF humidity is high, it slows to 80% airlfow. All other times, you get normal airflow for better efficiency and full system capacity.

    So for example, lets say you have a setback in the evening before bedtime. The unit might comes on and run for lets says 6 minutes to pull down the humidity then continue to run for 10 more minutes ot dorp the temprature maybe 2F.

    In hotter weather when you have lots of heat load, you migh not even need to slow the fan speed. It might run at normal the whole time or slow the fan for jsut a few minutes at the beginning.

  4. #4
    Join Date
    Mar 2010
    Posts
    43
    Quote Originally Posted by motoguy128 View Post
    I don't see an option 3... get a thermostat that has humidify control and use the Dehumdifying terminal on the control board that pretty mcuh all vairable speed furnaces and air handlers have. Then you jsut dehumidify on-demand only. IF humidity is high, it slows to 80% airlfow. All other times, you get normal airflow for better efficiency and full system capacity.
    The two examples given include a Comfort Sense 7000 slowing the blower on a call to dehumidify. That call is 70% of max cooling speed.

  5. #5
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,537
    Quote Originally Posted by motoguy128 View Post
    I don't see an option 3... get a thermostat that has humidify control and use the Dehumdifying terminal on the control board that pretty mcuh all vairable speed furnaces and air handlers have. Then you jsut dehumidify on-demand only. IF humidity is high, it slows to 80% airlfow. All other times, you get normal airflow for better efficiency and full system capacity.

    So for example, lets say you have a setback in the evening before bedtime. The unit might comes on and run for lets says 6 minutes to pull down the humidity then continue to run for 10 more minutes ot dorp the temprature maybe 2F.

    In hotter weather when you have lots of heat load, you migh not even need to slow the fan speed. It might run at normal the whole time or slow the fan for jsut a few minutes at the beginning.

    Lets get serious, 20-30 mins. of cooling is required to load the cooling coil/pan to the point that moisture starts going down the drain. At the end of the cooling cycle, all the moisture on the coil/pan evaporates back into the home. We need a good hour run to remove 2 lb. of moisture per ton of capacity from the space. I suggest setting the air flow to max the latent removal. So during a long run, you get the house dried down a little. During the evenings the runs are short and do not remove much moisture.
    Seldom is the home too dry. During low/no cooling loads and outdoor dew points +60^F, supplemental dehumidification is required to maintain <50%RH in a home.
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  6. #6
    Join Date
    Mar 2010
    Posts
    43
    Crickets? No opinion on a safe blower speed at 70% call to dehumidify?

    Would slowing the blower via call to dehumidify be best left off in favor of a constant lower speed of say 1050 - 1080 CFM?

    Option 3:
    947 CFM first 7.5 minutes
    1155 CFM max speed to end of cooling call
    808 CFM call to dehumidify

  7. #7
    Join Date
    Aug 2009
    Location
    Jurupa Valley, CA
    Posts
    1,781
    Too many variables to say. Ideally, you run it as low as you can and still keep the evaporator from icing up. This ideal speed is going to vary based on the evaporator itself, and incoming air temp.

  8. #8
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,324
    If your going to use a thermostat that can slow the blower. I would have them set the blower to a CFM that matches fairly close the the SHR that the home has(probably the 1155 you posted). Then set the blower ramp profile to go to full speed right a way. And let the thermostat decide if the blower needs slowed down or not.

    Its more efficient that way on the warmer days when it runs long enough that a slow ramp up is not needed because the humidity is already low enough.
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  9. #9
    Join Date
    Mar 2010
    Posts
    43
    I'm considering using a different company to install a txv, set airflow and charge the way it is supposed to be done. We'll see. I had a humidity problem last year but that was largely related to a major leak in the supply trunk left by the company I'm going to fire.

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