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Thread: a/c cycle times

  1. #1
    Join Date
    Jun 2005
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    32
    After earlier discussions I lowered my blower speed from "high" setting to "medium/low" setting and getting longer run times 15-20 mins versus 8-10 mins. I was able to find specs on furnace blower speeds via Carrier website. According to their site the "high" blower setting is 2400 CFM, "med/low" 2200 CFM, and "low" 1700 CFM (5 ton/12 seer unit). My question is if 400 CFM per ton is the ideal fan setting then why in the world would they default set the cooling fan speed on "high" which is 2400 CFM (6x what you need)!
    Does this sound like accurate information or is there much more to it?

    Thanks for all the help.

  2. #2
    you have to consider more than fan speed

    duct size supply and return
    type of filter
    type of A coil

    everything that put into the air supply cause restriction.

    Put it back to high leave it alone..

    At least with more cfm's you sure that the liquid is boiling off the a coil.

    regards,
    kelvin

  3. #3
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    Jan 2004
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    Lancaster PA
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    Thats 2400 cfm max for the unit.

    Theres awhole lot more to it.
    Doubtfull you were getting 2400 cfm.

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  4. #4
    Join Date
    Jun 2005
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    32
    I was initially having humidity problem which I could only attribute to short cycles. I've had unit checked twice in 6 weeks and everything is to spec. I was hoping that setting back to "medium" speed would increase humidity removal by extending run times. Do you guys think I'm harming the system by lowering to "medium" speed ? According to Carrier's chart thats only 200 CFM less than max speed. If I do need to switch back to "high" setting do you guys have any suggestions to remedy short cycles/humidity problems?

    Thanks,
    Mike

  5. #5
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    Jan 2004
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    A 5 ton unit can have a cfm as low as 1750 on a psc motor, and not harm the unit.
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  6. #6
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    Jun 2005
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    I definitely do not want to harm the compressor but the high setting is just not allowing the evaporator to pull the moisture out of the air. I think the unit is oversized anyways (5 tons for 2400 sq ft, 2 story). The "high" speed is blasting air out of the registers and satifying the thermostat quickly. I have even partially closed off all of the downstairs registers to avoid that problem. If lowering the blower to medium setting is going to harm the compressor then I guess I will have to live with the humidity for now. The humidity consistently runs in the low 60's regardless of how low I set the thermostat.

    Thanks for the replies!

  7. #7
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    Try it on low, go out and listen to the outdoor unit, if it changes its sound, then go back to med.

    How old is your system again.
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  8. #8
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    Jun 2005
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    Less than two years old

  9. #9
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    Short of changing out the entire system, or having the air handler replaced with a VS blower.

    You can have an installer do this.

    Run a 8" bypass with motorized damper, power open, spring close.

    Wire it to open on a cooling call, and wire it through a freeze stat, so that if the coil starts to get too cold the damper closes.

    I've done this to a couple over sized systems, and it helps.

    Might get you a 6 to 8% drop in RH.


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  10. #10
    Join Date
    Jun 2005
    Posts
    32
    That seems to be a better option than installing damper/baffle system throughout. Got a bid last year that was $2300.00. I will try to "low" setting tomorrow. Do I just listen for louder humming coming from compressor?

  11. #11
    Join Date
    Dec 2002
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    174

    just...

    ....make sure that compressor isnt sweating. What is your temp difference on supply vs return air?

  12. #12
    Join Date
    Jun 2005
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    32
    temp difference is 15 degrees....I'm pretty much a novice at this so I'm assuming that a sweating compressor is bad. Last summer the compressor was sweating so badly that a stream of water was coming out from underneath the condensing unit. It was under warranty at the time so I had install company come out to check. They told me that it was normal for compressor to sweat on very hot/humid day. Was I sold a bill of goods?

  13. #13
    Join Date
    Nov 2004
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    SW FL
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    Originally posted by mrb8828

    "high" blower setting is 2400 CFM
    "med/low" 2200 CFM
    "low" 1700 CFM (5 ton/12 seer unit).

    My question is if 400 CFM per ton is the ideal fan setting then why in the world would they default set the cooling fan speed on "high" which is 2400 CFM (6x what you need)!
    2,400 High / 2,000 CFM Nominal = 1.2
    ..OR.. only 20% over Nominal

    WHO stated that 400 CFM / ton is Ideal setting?
    400 CFM is a Nominal flow rate for measuring Air Handler performance in 'test' conditions or Average so to speak.

    15' F delta T, you should have ~20'F for improved humidity control ( 50% R.H.).
    So Medium setting will likely increase dT to ~17' or 18'F. RESULT ~ 55% R.H.

    AND You never mentioned your Location ...
    _ Maine or Texas _

    5-ton is just about right size ..
    but depends on Location, House / Window Orientation,
    Insulation, Tightness, amount of duct in the Attic and WINDOWS .. Glass Tinting / # / Area.
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

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