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  1. #1
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    Clarity and understanding on returns

    I was reading a very old thread in regards to returns on here. Some information I believe I have correct but other aspects of this subject have me scratching my head. I would like some help further understanding returns please.

    Some of the information as I understand it calls for 1 sq inch of filter space for every 2 CFM. Does that rule of thumb account for the filter grill and the filter?

    I have seen many systems running with a lot less then the 2:1 rule and they seem to operating well. Could there really be that many incorrect return sized system? If there all operating well does the mean the 2:1 rule is incorrect?

    Also if a system is installed with the 2:1 rule and there are multiple returns, won't the return air still bottle neck at the air handler?

    Thanks all for the read and if you would to help out thank you in advance

  2. #2
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    Define runs well. Are you measuring air flow? Or at least static press. Or are you saying it runs well based on your gut feel and opinion? The only way you will know how well it runs is to measure and test. Once you do that you will have your answer.

  3. #3
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    I'm sorry i guess i mean the unit has ran for 15-20 years and is still sufficiently cooling the house.

  4. #4
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    It's amazing how many things can be wrong or a lot less than ideal and they will still run. When you test and measure you will see what I mean. Then when the problems are resolved you will see a clear difference as to how well it runs.

  5. Likes lkapigian liked this post.
  6. #5
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    Visalia California 93291
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    Quote Originally Posted by MikeSCV View Post
    I was reading a very old thread in regards to returns on here. Some information I believe I have correct but other aspects of this subject have me scratching my head. I would like some help further understanding returns please.

    Some of the information as I understand it calls for 1 sq inch of filter space for every 2 CFM. Does that rule of thumb account for the filter grill and the filter?

    I have seen many systems running with a lot less then the 2:1 rule and they seem to operating well. Could there really be that many incorrect return sized system? If there all operating well does the mean the 2:1 rule is incorrect?

    Also if a system is installed with the 2:1 rule and there are multiple returns, won't the return air still bottle neck at the air handler?

    Thanks all for the read and if you would to help out thank you in advance
    I see it all the time, running dozens of years with undersized ducting...the kicker is the unit is typically oversized and gives the appearance of "normal" all the while you are overpaying utility companies...that 4 tons is only delivering 3 tons of air but cools well as there is only a 3 to load on the home...correct duct errors put the proper sized unit deliver the proper amount if air save KWH and increase comfort.

    Perception and reality are 2 different things
    Carrier Presidents Award Winner 2013 and 2014 SCE QI (ACCA 5 Protocol) Contractor of the year 2014

    "If you think it is expensive to hire a professional to do the job, wait until you have hired an amateur"

    Red Adair

    Click "Like" if you like my post

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  8. #6
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    Thank you both for your replies. I would like to further understand returns and I had a few questions in my 1st post about return air. Did either one of you have any experience with return air ducts/filters/grills?

  9. #7
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    Visalia California 93291
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    Quote Originally Posted by MikeSCV View Post
    Thank you both for your replies. I would like to further understand returns and I had a few questions in my 1st post about return air. Did either one of you have any experience with return air ducts/filters/grills?
    the numbers in your first post are accurate and that is Fee Area...less is Not OK...in appearance, in your first post, it may Seem OK, but your oversized unit will mask duct deficiencies ..which includes filters and grills..it is problem plaguing the industry.

    the whole reason of my first post was that...your load is say 3 tons, you have say a 4 ton unit with undersized ducting grills filters ..,.on the surface an in informed "tech" thinks "nothing wrong here" where an informed tech checks things such as static and airflow, can inform the hone owner they are overpaying the utilities , size the system correct, smaller equipment more comfort and energy savings

  10. #8
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    Thank you very much. In regards to the the 1 sq inch of filter space per 2 CFM, does that equation account for filter and grill? There was some talk about 25% less for each?

    I see what our saying about the oversized units hiding incorrect duct sizing issues. I am very interested in learning about the proper way to design and install. I will read more about finding the correct amount of CFM a home needs, as this is the most logical 1st step in designing a home comfort system.

    In your experience, how well do customers respond to redoing there system if originally installed incorrectly? Do customers usually insist on swapping system for system or are they receptive to redoing there system if it is incorrect?

  11. #9
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    Dec 2011
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    For a filter you're not so much worried about cfm/in, you're more worried about the static pressure drop it causes, and if your fan can overcome the pressure drop.

    Example: A typical home filter will induce a "static pressure drop" of about 0.125", Your fan in the furnace needs to be able to produce an "external static pressure" (meaning the pressure drops of the system the fan can overcome to move the air) greater than that 0.125". In reality, it needs to overcome more than that filter, it needs to overcome all of the friction pressure drop caused by the ductwork***, static pressure drops caused by grilles, registers, and diffusers, etc.

    ***This concept of the fan overcoming the pressure drop of the ductwork is important because it was touched upon by lkapigian, he was mentioning how undersized ductwork causes you a higher utility bill because you'll likely have a larger than necessary fan coil. Smaller ductwork causes more of a pressure drop on the system, and thus the fan has to work harder to overcome that pressure drop. A normal 2-5 ton fan coil should have a fan in it that produces around 0.5-0.75" external static pressure, sometimes as much as 1" depending on where on the fan curve you're riding.


    If you're interested in the proper way to design the system, the easiest method would be to use the "rule of thumb" method, (which we engineers only use as a double check) which is essentially 450-500 square feet per ton of cooling. This is assuming your home has decent windows and wall insulation. You typically want to put around 0.8 cfm per sq. ft. on average in the home. 0.5 cfm/sqft in interior spaces, north and east facing exposures, and closer to 1cfm/sqft around south and west facing windows. Typically only air exhausting from restrooms unless they're exterior, then they can see about .5cfm/sqft. etc.

    With respect to ductwork, typically sizing a duct to only have a friction loss per 100ft of ductwork at about 0.08" should allow for proper air flow, and minimal static pressure loss. You can find calculators online to help you size ductwork. Essentially, an 8" round duct can move just over 200cfm, etc.

    This is a very BASIC method, and a proper engineer should be consulted if you're going for a full redesign so that load calcs can be run and equipment selected properly.

  12. #10
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    Quote Originally Posted by MikeSCV View Post
    Thank you very much. In regards to the the 1 sq inch of filter space per 2 CFM, does that equation account for filter and grill? There was some talk about 25% less for each?

    I see what our saying about the oversized units hiding incorrect duct sizing issues. I am very interested in learning about the proper way to design and install. I will read more about finding the correct amount of CFM a home needs, as this is the most logical 1st step in designing a home comfort system.

    In your experience, how well do customers respond to redoing there system if originally installed incorrectly? Do customers usually insist on swapping system for system or are they receptive to redoing there system if it is incorrect?
    Customers love that we re do there system, we are not into swapping out like for like nor will we--Don't need the practice and that is what A/C Gypsies are for--There is no uniqueness in sameness

    Attached is filter sizing proceedure
    Attached Images Attached Images
    Carrier Presidents Award Winner 2013 and 2014 SCE QI (ACCA 5 Protocol) Contractor of the year 2014

    "If you think it is expensive to hire a professional to do the job, wait until you have hired an amateur"

    Red Adair

    Click "Like" if you like my post

  13. #11
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    Wow, great post gentlemen. I will go through this information in depth when i get home today. Thank you both for sharing with me.

    Mike

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  15. #12
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    Quote Originally Posted by stevereigh View Post
    For a filter you're not so much worried about cfm/in, you're more worried about the static pressure drop it causes, and if your fan can overcome the pressure drop.

    Example: A typical home filter will induce a "static pressure drop" of about 0.125", Your fan in the furnace needs to be able to produce an "external static pressure" (meaning the pressure drops of the system the fan can overcome to move the air) greater than that 0.125". In reality, it needs to overcome more than that filter, it needs to overcome all of the friction pressure drop caused by the ductwork***, static pressure drops caused by grilles, registers, and diffusers, etc.

    ***This concept of the fan overcoming the pressure drop of the ductwork is important because it was touched upon by lkapigian, he was mentioning how undersized ductwork causes you a higher utility bill because you'll likely have a larger than necessary fan coil. Smaller ductwork causes more of a pressure drop on the system, and thus the fan has to work harder to overcome that pressure drop. A normal 2-5 ton fan coil should have a fan in it that produces around 0.5-0.75" external static pressure, sometimes as much as 1" depending on where on the fan curve you're riding.


    If you're interested in the proper way to design the system, the easiest method would be to use the "rule of thumb" method, (which we engineers only use as a double check) which is essentially 450-500 square feet per ton of cooling. This is assuming your home has decent windows and wall insulation. You typically want to put around 0.8 cfm per sq. ft. on average in the home. 0.5 cfm/sqft in interior spaces, north and east facing exposures, and closer to 1cfm/sqft around south and west facing windows. Typically only air exhausting from restrooms unless they're exterior, then they can see about .5cfm/sqft. etc.

    With respect to ductwork, typically sizing a duct to only have a friction loss per 100ft of ductwork at about 0.08" should allow for proper air flow, and minimal static pressure loss. You can find calculators online to help you size ductwork. Essentially, an 8" round duct can move just over 200cfm, etc.

    This is a very BASIC method, and a proper engineer should be consulted if you're going for a full redesign so that load calcs can be run and equipment selected properly.
    Attached is a Free Sizing Chart
    Attached Images Attached Images
    Carrier Presidents Award Winner 2013 and 2014 SCE QI (ACCA 5 Protocol) Contractor of the year 2014

    "If you think it is expensive to hire a professional to do the job, wait until you have hired an amateur"

    Red Adair

    Click "Like" if you like my post

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  17. #13
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    Seattle, WA
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    This is excellent. Will forward this on to my boss and tell him we can stop subscribing to Trane Trace.

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