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How do you know the best fan speed for A.C .

Hello : How do you know the best fan speed for A.C . I have a 2.5 ton ac unit connected to a hile 45,000 - 70000 btu 2 stage furnace . I can adjust the d.c. fan motor from 1392 -573 CFM . To remove humidity a lower speed will work better so I have read , presently I have the cooling speed to be around 924 CFM . Ideas people , and thanks .

Comments

  • JUGHNEJUGHNE Posts: 6,431Member
    Some air handlers and furnaces with variable speed motors have a feature that runs the motor at a reduced speed for the first 10 to 15 minutes of the cooling cycle. This is for humidify control, it ramps up to your normal cooling speed after the time elapses.

    Real old school with only 3-4 fixed speeds was to observe/measure the water draining off of the coil. Just monitor the air temp leaving the coil so it does not freeze.
    Many factors involved and some hands on observations.
    I have two 2 ton AC's in my house and run the little 1/4 HP motors on low speed. Between the two zones there are at least 25 supply runs, some 7 and 8".
    However, the return from zone A feeds the AH for zone B and then the return from B feeds the AH for zone A. Having these cross connected I keep both blowers running while cooling.
    After initial cool down with 4 tons one unit is switched off. The active cooling coil is receiving warmer air because of the cross connection......no fear of freeze up .......so far.
    So I can cool 2366 sq feet plus basement (a lot of ductwork) with 2 ton AC.
    It seems my weird madness has paid off. I feel good about it and wonder if maybe 2 1.5 tons would have been better. But we go from low 80's with no AC and then pop up to 95 for a few days. Most here would run AC all summer. But with windows closed you can't hear the Martins sing. :)
  • aircooled81aircooled81 Posts: 194Member
    The rule of thumb (around the bay area) is either 350, 400 or 450 cfm per ton.
    So if you had 2.5 tons, multiply that by 350cfm and you shoud be at about 775 for some dehumidification, and about 900 cfm for a comfortable cool.
    the faster the fan spins, the more air you move, but the less time the air spends on the cold evaporator coil surface.
    Since the latent heat is the money saver, when you are removing more moisture, or basically changing moist humid air to condensate, you are more efficient.
    Some folks cant stand a dry house, so if your sinuses are bothering you after 1/2 hour of running the ac, might be time to speed the fan up.
  • Total1Total1 Posts: 44Member
    thanks to everyone that has contributed to this thread . Appreciate the knowledge . thanks again ..
  • Total1Total1 Posts: 44Member
    Thanks . so the slower the speed the more efficient the ac operates ? my speed options are 593 , 670 , 743 ,836 ,914,1015,1117 and 1212 CFM . I was thinking of setting the fan speed to 836 CFM , kind of split the difference . A good plan ?

  • Total1Total1 Posts: 44Member
    or maybe 743 CFM ??
  • IronmanIronman Posts: 5,401Member
    I'd leave it where it is. 400 cfm per ton is the normal design. You can go up or down by about 10% from that, but lowering it too much may cause the coil to ice up which in turn will cause liquid refrigerant to flood back to the compressor and damage it.
    Bob Boan


    You can choose to do what you want, but you cannot choose the consequences.
  • Total1Total1 Posts: 44Member
    Ironman said:

    I'd leave it where it is. 400 cfm per ton is the normal design. You can go up or down by about 10% from that, but lowering it too much may cause the coil to ice up which in turn will cause liquid refrigerant to flood back to the compressor and damage it.

    right now it is set at 1212 CFM ..
  • IronmanIronman Posts: 5,401Member
    You said 924 cfm in your first post. 400 x 2.5 = 1000.
    1212 is too much; it won't dehumidify properly.
    Bob Boan


    You can choose to do what you want, but you cannot choose the consequences.
  • Total1Total1 Posts: 44Member
    sorry - 1212 was what it was set from factory ..
  • Total1Total1 Posts: 44Member
    I have it set now for 1015 :) Seems nice . thanks again people
  • IronmanIronman Posts: 5,401Member
    The 924 setting would be fine if you want a little better dehumidification.
    Bob Boan


    You can choose to do what you want, but you cannot choose the consequences.
  • GWGW Posts: 3,488Member
    You may wish to throw your gauges on now to check superheat and subcool
    Gary Wilson
    Wilson Services, Inc
    Northampton, MA
    www.wilsonph.com
    [email protected]
  • MikeMike Posts: 94Member
    You really need to know the total static pressure of the furnace, to properly adjust the blower speed. The cfm numbers you're stating are from the install manual, yes? These numbers are in reference to the duct pressure, as in a static pressure of .4 in of water column, the blower may give you the proper cfm on tap 3. A poorly designed duct will give you less cfm than the manual states. Without the proper amount of air passing through the furnace and coil, you're jeopardizing your equipment. This should have been done, when the equipment was installed. Also, you have to check the temp difference across the coil. 20 for r22, 25 for r410, are ideal. And don't forget to check your superheat!!!!
  • Harvey RamerHarvey Ramer Posts: 2,202Member
    Mike said:

    You really need to know the total static pressure of the furnace, to properly adjust the blower speed. The cfm numbers you're stating are from the install manual, yes? These numbers are in reference to the duct pressure, as in a static pressure of .4 in of water column, the blower may give you the proper cfm on tap 3. A poorly designed duct will give you less cfm than the manual states. Without the proper amount of air passing through the furnace and coil, you're jeopardizing your equipment. This should have been done, when the equipment was installed. Also, you have to check the temp difference across the coil. 20 for r22, 25 for r410, are ideal. And don't forget to check your superheat!!!!

    I have to ask, why are 20° dt for r22 and 25° dt for 410a ideal? I'm not trying to be smart, just learn.
    Ramer Mechanical
    ramermechanical.com
    To learn more about this professional, click here to visit their ad in Find A Contractor.
  • MikeMike Posts: 94Member
    The boiling point of r22 is lower than 410. Sorry, but I can't think of the exact temps at this moment. But that's the reason, the manufactures lowered the cfm per ton rating to 350. That gives the 410 systems, a better chance to dehumidify.
  • Total1Total1 Posts: 44Member
    I will check tomorrow what freon is in the system . thanks
  • IronmanIronman Posts: 5,401Member
    Mike said:

    You really need to know the total static pressure of the furnace, to properly adjust the blower speed. The cfm numbers you're stating are from the install manual, yes? These numbers are in reference to the duct pressure, as in a static pressure of .4 in of water column, the blower may give you the proper cfm on tap 3. A poorly designed duct will give you less cfm than the manual states. Without the proper amount of air passing through the furnace and coil, you're jeopardizing your equipment. This should have been done, when the equipment was installed. Also, you have to check the temp difference across the coil. 20 for r22, 25 for r410, are ideal. And don't forget to check your superheat!!!!

    Mike said:

    The boiling point of r22 is lower than 410. Sorry, but I can't think of the exact temps at this moment. But that's the reason, the manufactures lowered the cfm per ton rating to 350. That gives the 410 systems, a better chance to dehumidify.

    I'm sorry, but I've never heard of this. Can you provide some documentation to corroborate this?
    Bob Boan


    You can choose to do what you want, but you cannot choose the consequences.
  • BobCBobC Posts: 5,056Member
    Smith G8-3 with EZ Gas @ 90,000 BTU, Single pipe steam
    Vaporstat with a 12oz cut-out and 4oz cut-in
    3PSI gauge
  • Total1Total1 Posts: 44Member
    Freon type = r 22
  • Harvey RamerHarvey Ramer Posts: 2,202Member
    I'm familiar with the boiling points of freon. What i'm after is how or why should 22 have a different dt than 410a?
    In my experience the type of freon is irrevelant to the dt. The dt is a product of evaporator temp, evaporator design (fin spacing, coil depth), cfm, wet bulb temp of entering air, and outdoor conditions affecting the refrigerant mass flow rate.
    Oh and the system's design sensible heat ratio also affects the dt across the evap. AKA Manual-S

    Its a moving target.
    Ramer Mechanical
    ramermechanical.com
    To learn more about this professional, click here to visit their ad in Find A Contractor.
  • GWGW Posts: 3,488Member
    There is no set split for ac, no matter what juice you're running. The more humid the return air the less split you will have. The hard core ac guys will talk in "change in Enthalpy".

    We stated off as a simple beginner question, now we are getting into finer details. I surely am not an AC Guru, but understand some basics.

    We haven't discussed with the metering device is yet. Is it a TXV or is it a cheapie piston?

    Ironically, as much as Goodman gets poo poo'd, I absolutely get giggles over the fact that they give you a TX with adjustable superheat. I wish the other manufacturers would follow suit. I install high-dollar Lenox stuff and it's just a basic non-adjustable TX.
    Gary Wilson
    Wilson Services, Inc
    Northampton, MA
    www.wilsonph.com
    [email protected]
  • Harvey RamerHarvey Ramer Posts: 2,202Member
    I like tracking the change in enthalpy. Doesn't make me a hard core ac guy though. It is interesting watch8ng a system go through it's paces though.Doing regular maintenance, I usually see a 7.5 ish enthalpy delta across the evaporator. An inefficient compressor can bring that back to 7 and below. Just today I started up a new system in a hot house. Enthalpy change was over 8 and the 3 ton unit was moving over 40,000 btu's. Return air wet temp was slightly over 68. Dehumidification was epic at 1.9 gallons per hour.
    Ramer Mechanical
    ramermechanical.com
    To learn more about this professional, click here to visit their ad in Find A Contractor.
  • Total1Total1 Posts: 44Member
    I set the fan to 924 CFM for the a/c . Just today the humidity dropped 4 % to 40 % . thanks to all the people that responded to this thread .
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