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  1. #1
    Join Date
    Mar 2005
    Posts
    56

    I currently have a Carrier 58STA070-12 furnace and would like to convert it to a 58STA070-08 , the objective would be to get the lower blower CFMs.

    The guys that performed a load calc told me that the blower speed was to high and that it is unable to remove the hunmidy from the house.

    I looked at the specs between the 58STA070-12 (1/3hp) and the 58STA070-08 (1/5hp) and the only difference that I see is the HP of the blower motors.
    Would it be possible to simply have a technician change out the blower motors in order to get the reduced RPMs? ..or is there more to it

    Here is a link to the spec sheet for this model furnace

    http://tinyurl.com/7smru

  2. #2
    Join Date
    Jul 2005
    Posts
    88
    Pay the service charge, have a tech answer your questions. Not familiar with Carrier #s any more but you may be able to run a differant speed with the motor you have now.

  3. #3
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,007

    RE-Balancing

    Originally posted by eisenberg
    I currently have a Carrier 58STA070-12 furnace and would like to convert it to a 58STA070-08 ,
    the objective would be to get the lower blower CFMs.

    I looked at the specs between the 58STA070-12 (1/3hp) and the 58STA070-08 (1/5hp) and the only difference that I see is the HP of the blower motors. http://tinyurl.com/7smru
    1/3 HP Blower motor is Not large
    for a 3-Ton (1200 CFM) unit.

    After reviewing Blower Performance specs on 58STA070-12.
    This blower is a nominal 1,075 RPM direct-drive
    with 3 speeds.

    High ___ 1,200 CFM at 0.5"
    Med-High 1,140 CFM
    Med-Low _1,050 CFM

    What is the current blower set-up:
    1. Air flow ______ CFM
    2. Supply Air S.P. _____"
    3. Supplt Air Temp __ 'F
    4. Motor Amps ____
    5. Motor Speed ___ RPM or Setting ___
    6. Room Temp __ 'F
    7. R.H. __ %
    8. Filter S.P. 0.___ "

    How much should the air flow be reduced?
    __ < ~20%
    At this time, I do NOT recommend reducing air flow
    to an -08 blower.

    Blower speed can be reduced and filter added to effectively provide controlled, marginally reduced air flow.

    If blower is currently set on High,
    it is likely delivering ~1,200 CFM.

    First, simply ADD a MERV 8 or 10 air filter(s).
    RE-measure:
    1. Air flow ______ CFM
    2. Supply Air S.P. _____"
    3. Supply Air Temp __ F
    4. Motor Amps ____
    5. Motor Speed ___ RPM - Same.
    6. Room Temp __ 'F
    7. R.H. __ %
    8. Filter S.P. 0.__"

    What are the changes in S.P.'s?
    (You must have an accurate gauge for
    these comparisons to be meaningful!)

    Did adding a higher efficiency air filter(s)
    reduce the air flow by ~7%?

    Change Motor Speed to Med-Low. Measure again.

    With the speed re-set to Med-Low and
    high efficiency air filter(s) added,
    I would anticipate ~990 CFM ( 330 CFM/ton)
    or a >16% reduction from nominal High set-point and
    Hopefully performed in a Simple manner.

    Did the Supply Air temperature drop by ~4'F?
    i.e. 76'F Room Temp.
    Supply Air was 59'F, now it is 55'F.

    If the difference between return (room) air and
    supply air temperatures is > 22'F,
    your system is operating near perfect.
    R.H. should be slightly less than 50%.

    If the air handler Delta T is not > 20'F,
    the refrigerant charge needs to be investigated.

    [Edited by dan sw fl on 08-10-2005 at 08:01 AM]
    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

  4. #4
    Join Date
    Feb 2005
    Posts
    249
    Yes that will work in this case because the blower wheel is the same size in both furnaces.

  5. #5
    Join Date
    Mar 2005
    Posts
    56

    fl1


    fl1,

    Are you sure that I would be able to simply swap the motor and it would emulate the -08 furnace.I wouldn`t need to swap any electronic parts?
    I am very sure that too much air is my problem.. when comparing the -08 & -12 you will notice a drastic difference in CFMs.
    I currently have the speed of the -12 model set to the blue wire (medium-high) which is blowing 1075 (.06 ESP) to 1140 (.05 ESP) cfms across a 2ton coil.
    People keep asking if the house gets cold due to the higher CFMs... I tell them no, I can`t get the system to go below 74 degrees in a 920 square ft house... even when it is only 80 degrees out side!!!! It just runs and runs...cycling off and on every 10-12 minutes!!!

    Do I need refrigerant??? I measured 74 degrees at the furnace return and 63-64 at the register.

    BTW - I stuck a peice of card board in the return blocking a little more than half of the return and was able to get 58 degrees coming out of the registers.... what does that tell you???

    ,Ethan

  6. #6
    Join Date
    Aug 2005
    Posts
    829

    Smile

    That tells me you just fixed your problem. I think Dan is right - just go get one of those high efficiency nylon filters that we have to take out of everyones house because they are icing up the coil.
    eventu rerum stolidi didicere magistro

  7. #7
    Join Date
    Feb 2005
    Location
    Dothan, Al
    Posts
    3,453

    eisenberg

    No, emphatically, NO!!
    Changing Hp motors will not change speeds.
    If you try to put a 1/5 HP motor onto a 1/3 hp 'load' ( the fan blade ), you will load the 1/5 motor too much. If it runs at all, it won't last long.

    However, you can change a 1/3 HP. 1075 rpm motor with a 1/3 HP, 825 RPM motor and get what you are after. I am not sure how this will affect yur system though.

    I definitely recommend haveing a local tech perform any service so that he/she may monitor the system. It does sound like you have more problems than airflow being too high. You are not cooling as you should be. It needs to be repaired before changing speeds.

    Hope this helps,
    Richard

  8. #8
    Join Date
    Jun 2005
    Posts
    71
    Bornriding is correct....an AC motor will inherently try to run near the rated RPM, unless its under/over loaded: all the horsepower does is insure that the motor is big enough to handle the load on it....the correct way is to change the motor with a lower RPM, but leave the horsepower alone...

    [Edited by robnjr on 08-15-2005 at 10:26 PM]

  9. #9
    Join Date
    Sep 2004
    Posts
    265
    After looking at the specs for this furnace (pages 2 and 6) it seems the only difference between the -08 and -12 version is the blower motor. Both motors use the same size blower wheel, have the same RPM's, the only difference is the hp, RLA and the amount of CFM's each motor will deliver. The version of motor depends on the nominal cooling size of unit. I read it as the motor size is an option depending on your cooling capacity.
    Correct me if I am wrong but should not the unit have come with the -08 motor since the OP has a 2 ton coil?

  10. #10
    Join Date
    Mar 2005
    Posts
    56

    chilledcool



    Chilledcool,

    Correct me if I am wrong but should not the unit have come with the -08 motor since the OP has a 2 ton coil?

    Yes it should have come with the -08, but after the system was installed the guy told me he put the -12 in since I had ceiling registers and he wanted to get the heated air to the floor. He did not even consider how it would impact the cooling side with humidity removal.

    ..get this... he installed the -12 leaving the cooling side on the highest speed (red wire) while leaving the heating side on the medium-high speed (blue wire)...he only install 7 runs of 6 inch flexi duct. He as attempting to push 1200 cfm through that small amout of duct work. oh yeah he installed a return with a grill rated for a 1.5 ton system (12X24).

    I figure that I need to convert all of the 7 runs to 8inch flexi duct w/ 4x12 registers whichever way I go with the blower motor. The origional installer did converted 3 of them to 8 inch... I will do the remaining 4 myself.

    reply with your thughts on this...from reading many posts on this site I am sure that I am right simply by doing the math and going by the manufacturers specs.

    ,Ethan



    ,Ethan


  11. #11
    Join Date
    Feb 2005
    Location
    Dothan, Al
    Posts
    3,453

    Chilledcool

    I doubt that the 08 blower wheel is the same diameter as the 12, there would be no reason to install a 1/3 HP motor on the same wheel that a 1/5 HP motor could operate. The only reason that they would install the different HP motors is if they have different loads, ie - the wheel.

    BTW: The motor only runs RPM, it is the wheel ( or cage ) that determines CFM, at whatever RPM the motor turns

    Hope this helps, & makes sense,
    Richard

  12. #12
    Join Date
    Mar 2005
    Posts
    56

    bornriding & robnjr



    Thanks for the info on the HP vs RPM... I will pass this info onto my tech

  13. #13
    Join Date
    Mar 2005
    Posts
    56

    bornriding



    Here is the spec sheet for that family of furnaces which shows that both the -08 and -12 blower wheels are the same size along with all of the other specs

    http://tinyurl.com/7smru

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