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Thread: LOW CFM effect and limits on evaporator/AC system ??

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  1. #1
    DIY Junior Member Jin's Avatar
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    Default LOW CFM effect and limits on evaporator/AC system ??

    I am trying to figure out all possibilities for my installation,
    and i was wondering what implies to run lower cfm than recommended
    on an AC system evap ??

    problem is that i'd need something like 4ton
    but i've only got 6 fans of ~150cfm each ( rated ~180cfm i believe .. )

    and i wanted to have some of them run on lower speed when AC is not required
    so other could run full and balance temp for each section

    what do you think ?

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    If there isn't enough airflow over the evaporator coil when the a/c is running, you won't remove enough heat and the water will freeze as it condenses on the coils, blocking airflow. Lower airflow can help for awhile before that happens because it will extract more moisture as long as the fan speed increases before it starts to freeze up. On mult-speed fan systems, they generally start slow, then ramp up. This will be especially problematic with an oversized compressor since it will freeze up the coil much faster than one properly sized.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #3
    DIY Junior Member Jin's Avatar
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    Thanks JAD for the quick reply.

    What i thought ...

    Problem is that house has concrete walls and setup is in basement, almost impossible
    to add more ducts to get higher cfm ( i used 6 4"PVC runs for all different section ..could add 1 or 2 at basement if required..but it seems to stay within confortable range for now ..22-24c )

    Also i wanted not to run all fans on full speed all the time,
    were supposed to run on reduced speed ( let's say half speed..just enough so we can't hear in rooms )
    and go to full speed when AC is on because a specific room thermo asked for cooling

    so let's say 1 out of 3 fans is running at full on the 2nd floor setup
    that would make for something like 250-350cfm total at max ..not much

    house would need 4-5 tons ( 1600sqft on 2 floors + basement of same size )


    so now i'm wondering if i should only get a 4-5 ton unit and install it within my 1000liter water reservoir
    that i use for GEO cooling and let the fans ask for what they need



    or go with 2 smaller AC setup ..something like 2 2tons
    or 2 1.5ton

    what do you think ?

  4. #4
    Plumber jimbo's Avatar
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    You are getting into engineering calculations that are critical for proper operation. Get the install/spec sheet for the air handlers. They will lay out how many cfm you need. A typical dx air handler in the 4~5 ton range will typically use between 1500 and 2100 CFM, so I see problems with your numbers.

  5. #5
    In the trades Dana's Avatar
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    Unless this house has a lot of east & west facing windows with high solar gain glazing it's unlikely to need even 4 tons of cooling. Basements rarely have anything like even a quarter the cooling load of the top floor (with it's roof-gains), and the middle story will likely have half that of the top floor. The 1-ton/1000' rule of thumb is usually "close enough" for single-story houses, but not for a 3 story where the bottom is mostly below-grade.

    A real Manual-J type cooling load calculation is called for. It's possible that even a 2 ton unit could handle the whole thing if you don't get into mid-day thermostat setbacks, etc.

  6. #6
    DIY Junior Member Jin's Avatar
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    hi Dana,

    no roof ( flat roof with r40~56 insulation ) and a very reflective steel deck

    house as very large windows all around, but all 3pans double lowE double argon
    unfortunately not all sunshades are in place, so i do get some SHG from 8 to 11 am and at late evening
    but we have many many high trees all around the house ( trees on 2.5 sides of the house starting at 10ft from walls .. good deciduous coverage on south side )

    anyhow, it will all get better when i am finished with sunshades and real doors on all openings


    i read also a few moments ago that a 1 ton for 1000'sqft for a recent house is more realistic

    so i guess i will be looking for 2 1.5ton units
    which would only be a little more than what required
    but i really need 2 seperate systems

    i need you guys to tell me one thing

    i've been looking for units for some time,
    and i've only read bad reviews about all the large brands ( lenox,goodman/amana, comfortzone etc... )
    with many failures within the first years

    now i ma puzzled, my old house has a lenox system that has been in used ( heat pump complete system ) for heat and cold for about 11 years now
    ( parent's house..still running )
    never required ANY intervention ..worked 100% of time
    and they put the AC ON in may and take it off only in october
    ( lots of unshaded windows in the wrong direction ..but beautifull river view ...neway )


    Should i be looking for a more uptodate brand like Fujitsu, Mitsubishi etc ???

    Can we use the mini split systems AC condensor with a regular evaporator ????
    the price for the condensors are almost the same but they are seer 15-18 compared to 13-14
    also looks easier to install ? ( fitted valved lines?? )

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