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Thread: cfm

  1. #1
    Hello:


    I was wondering is there a guideline for checking for cfm for the various wires representing the various speeds attainable for residential furnaces ( heating and cooling speeds)

    eg: high ? cfm
    med high ? cfm
    med ? cfm
    med low ? cfm
    low ? cfm

    even if you have some ranges that these speeds should be in
    ball park speaking, as there are alot of homes out there that there furnaces and acs are so old that they dont have the book for it anymore

    any advise greatly appreciated

    thank-you

    don

  2. #2
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    Medium Standard

    Originally posted by theapprentice
    I was wondering is there a guideline for checking for cfm for the various wires representing the various speeds attainable for residential furnaces ( heating and cooling speeds)

    eg: high ? cfm
    med high ? cfm
    med ? cfm
    med low ? cfm
    low ? cfm

    even if you have some ranges that these speeds should be in
    ball park speaking, as there are alot of homes out there that there furnaces and acs are so old that they dont have the book for it anymore
    Residential A/C units have historically been set-up as
    400 CFM per Ton.

    Variable speed Norm for Humid climate is 350 CFM/ ton
    ( a.k.a. LOW Speed)
    ------------------------ Arid -------- is 450 CFM/ ton
    ( a.k.a. HIGH Speed)

    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

  3. #3
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    I think what you are asking is, what cfm is beeing delivered at what speed? The answer to that is not so easy as it varies according to many things.You go by the manufacturers blower chart and determine what speed at what external static pressure will give you the correct cooling flow usually 400cfm/ton.You need to use a manometer to determine what your esp is and adj. blower accordingly.Heating is adj. by recomended temp. rise.If your a DIY I'm not worried about telling you any of this because you won't understand anyway. A lot if not most duct systems have issues whith duct size,layout,filters and there is no out of the box answer without taking readings with proper diag. tools.
    Some people swear by me and some at me

  4. #4
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    Even if you don't have the book ,you can calculate the air volume with the tools I mensioned along with a ductulator.
    Some people swear by me and some at me

  5. #5
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    A very good question apprentice.

    If the unit has a multi speed motor, its not quite ancient for the most part. Prior to multi speed, they were often belt driven with an adjustable sheave.

    That said you want to know how to determine CFM. Yep, having the blower curves and some static pressure readings would be great but as you said they may be hard to come by. So you will need to be somewhat creative or at the very least learn how to read CFM.

    Since you mentioned furnaces, temperature rise at any given speed can be used to find airflow. Several things you need to know. First and formost is the BTU input to the furnace. This is found on the rating plate but unless you verify the orifaces and check the gas pressure you cannot be too sure. Your best approach would be to fire the furnace and clock the meter. I will not detail this, you should do some serches on the subject but here are some other ways.

    Lets say you have a 100,000 btuh furnace that is an 80% efficient model. Your input BTUhs are 100k. Your output should be close to 80k. (multiply input x efficiency).

    Now knowing the output, you can determine airflow at each speed tap. Lets use high as an example. So you fire the furnace, allow it to run for about 10 minutes. Take a temperature reading in the return plenum. Then take a supply air reading in the supply plenum (out of line of sight of the HX). So lets say you have a 40 degree temperature rise. (70 ret, 110 supply). Now, multiply your temperture rise x 1.08 (at altitudes below 2000). this would give you 43.2. Now devide the btuh output by the temperature rise with the correction factor.

    80,000 / 43.2 = 1851 CFM.

    Now if youw anted to test medium high the same formula would hold true. As you can imagine, the temperature rise would be higher. Lets say 50 degrees.

    80,000 / (50 x 1.08)
    80,000 / 54 = 1481 CFM

    And so on...

    Hope this helps.


    CFM = BTUh output / (temperture rise x 1.08)

    As tim mentioned, always set the furnace up to fall near midpoint of the rated temperature rise, not the max plenum temperature. (40-70 you would target 50-60 degree rise)

  6. #6
    this is very helpful.

    just to make sure I understand, after calculating the cfm at each tap, I can somewhat safely assume it is a correct setting if the temp rise falls within mid range of the rated temp range on the name plate of the furnace?

    thanks

    don

  7. #7
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    Great reply Doc. I wish I were better at explaining things. I attend a great deal of training seminars and read of the subject of HVAC extensively[ so much so that the people I work with think I'm nuts]but I'm awful with explanations.It's in my head but has a hard time getting out of my mouth. Any way good reply.
    Some people swear by me and some at me

  8. #8
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    If all you want is to set up the furnace speed, just test them to get it to fall within the rise. The above method, would allow you to know which speed is correct for cooling mode. It amazes me how many techs think its always low speed for heat and high for cooling or better yet the ones that think the factory setting is correct. No, they just dont want to ship it with loose wires and have to connect it one way or another.

    I can tell you this, if you were to call me with a heating or cooling problem, I would ask you what the actual airflow is before I offered much else in the way of providing any assistance.

  9. #9
    Residential A/C units have historically been set-up as
    400 CFM per Ton.

    Variable speed Norm for Humid climate is 350 CFM/ ton
    ( a.k.a. LOW Speed)
    ------------------------ Arid -------- is 450 CFM/ ton
    ( a.k.a. HIGH Speed)


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    Designer Dan
    These Guys Are Good
    It's ALL About Windows

    the above figures are per ton
    400cfm/ton

    is the answer in 1851cfm per ton?
    in other words, if i divide 80000 by 12000 i get
    roughly 6.6

    do if i divide 1851 cfm by 6.6 would this give me cfm per ton?

  10. #10
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    Location
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    Thumbs down Heating and Cooling

    Originally posted by theapprentice
    Residential A/C units have historically been set-up as
    400 CFM per Ton.

    Variable speed Norm for Humid climate is 350 CFM/ ton
    ( a.k.a. LOW Speed)
    ------------------------ Arid -------- is 450 CFM/ ton
    ( a.k.a. HIGH Speed)

    is the answer in 1851cfm per ton?
    in other words, if i divide 80000 by 12000 i get
    roughly 6.6

    do if i divide 1851 cfm by 6.6 would this give me cfm per ton?
    DocH said It All for Heating.

    I mentioned general Cooling air flow set-up.
    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

  11. #11
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    Originally posted by theapprentice


    is the answer in 1851cfm per ton?
    in other words, if i divide 80000 by 12000 i get
    roughly 6.6

    do if i divide 1851 cfm by 6.6 would this give me cfm per ton?

    huh?

  12. #12
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    Originally posted by docholiday
    Originally posted by theapprentice


    is the answer in 1851cfm per ton?
    in other words, if i divide 80000 by 12000 i get
    roughly 6.6

    do if i divide 1851 cfm by 6.6 would this give me cfm per ton?

    huh?

    Ditto and then some???

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