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  1. #1
    Join Date
    Jul 2008
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    The Colony tx.
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    3

    Question R/A size not enough?

    This is a first for me, not very computer literate. Have been out of the business for 8 years due to being disabled. I want to catch up on the technology and to refresh old skills, so please bare with me.

    A friend ask me if he had enough return air and it caught me off guard. What do you think?

    He has a 4 ton matched system, 20x25 unit opening. His r/a grilles are as follows. 10x36, 10x24. he also has a 20x24 grille blocked off. My question is, is the first two sizes adequate without the 20x24 opening?

  2. #2
    Join Date
    Aug 2002
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    Office and warehouse in both Crystal River & New Port Richey ,FL
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    18,836
    You need all three ,IMHO.

  3. #3
    Join Date
    Apr 2008
    Location
    Princeton, NJ
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    581
    Quote Originally Posted by dash View Post
    You need all three ,IMHO.
    Not to hijack the thread but answers to my question may provide the OP with more insight also. On a very similar topic I have a 4-ton unit and am considering increasing the size of upsatirs return grilles. I have an 18x18 down stairs in the entry hall, a 9.5x6 in the living room, and two 9.5x6 grilles upstairs....for a 2800 sq. ft. house in NJ.

    I'm looking at increasing the size of the two upstairs to 14x14 each. Is that enough?

    The upstairs is typically ~10 degrees hotter (in summer) than downstairs.

    Key1

  4. #4
    Join Date
    Aug 2002
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    Office and warehouse in both Crystal River & New Port Richey ,FL
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    For standard return grilles,1.25 to 1.5 sq. ft. per ton.


    There are grilles that are more open ,less restrictve that can be smaller.

  5. #5
    Join Date
    Mar 2008
    Location
    Houston, Texas
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    352
    Quote Originally Posted by key1cc View Post
    Not to hijack the thread but answers to my question may provide the OP with more insight also. On a very similar topic I have a 4-ton unit and am considering increasing the size of upsatirs return grilles. I have an 18x18 down stairs in the entry hall, a 9.5x6 in the living room, and two 9.5x6 grilles upstairs....for a 2800 sq. ft. house in NJ.

    I'm looking at increasing the size of the two upstairs to 14x14 each. Is that enough?

    The upstairs is typically ~10 degrees hotter (in summer) than downstairs.

    Key1
    You do have one very good point in your post. The upstair is always warmer due to the heat rising, that is why it is important to have return at the highest point. While using the heat, it allows the warm air to be "re-used" and distributed to downstairs where it is needed. While cooling it takes the heat load off of the house.

    In most cases in a 2 story home with one system, you have to heat the downstairs and cool the upstairs.

    If the same house had 2 systems, if there were some sort of problem with the upstairs heat or the downstairs cool, you may not even realize it.

    You may have to read that one again.

    The sizing of the returns you gave aren't normal. 9.5 X 6? Make sure it is a return, also the opening isn't as important as the duct size going to it.

    If those are your actual sizes, that wouldn't be enough return.

    In a perfect world you would have a return in every bedroom upstairs and at least one at the highest point available upstairs. (of course some return would be needed d/s as well)

  6. #6
    Join Date
    Apr 2008
    Location
    Princeton, NJ
    Posts
    581
    Quote Originally Posted by Skip 2 my lou View Post
    The sizing of the returns you gave aren't normal. 9.5 X 6? Make sure it is a return, also the opening isn't as important as the duct size going to it.

    If those are your actual sizes, that wouldn't be enough return.

    In a perfect world you would have a return in every bedroom upstairs and at least one at the highest point available upstairs. (of course some return would be needed d/s as well)
    That is the actual size of the opening in the wall. The grill that covers it measures exactly 11.5 x 7.5 inches.

    There are no ducts for my returns....wall cavity is used by panning from the basement. I measured the available space between the studs and it is 14 iches wide.

    Ok Dash......
    If I use 1.2 sq. ft/ton I need 4.8 sq. ft.
    If I use 1.5 sq. ft/ton I need 6.0 sq. ft.

    I currently have 3.4 sq. ft. of grill now.....so I am low.

    If the 2 upstairs grills are replaced with 14x14 that will take me to 4.97 sq. ft
    If they are replaced with 14x20 that will take me to 6.1 sq. ft.

    This is just what I needed to know. Thanks guys. Sorry "OP". You can have your thread back.

    Key1

  7. #7
    Join Date
    Jul 2008
    Location
    The Colony tx.
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    3
    Thanks Dash. But not a consumer. Been disabled from job for eight years and rusty in the memory banks. {pain meds tend to do that} Just now able to set at computer for 30min. at a time Ran service for 28 years.

    The attitude from others doesn't bother me, you find elitism in any area of expertise.

    Tj

  8. #8
    Join Date
    Jul 2008
    Location
    The Colony tx.
    Posts
    3

    Red face

    Thanks Dash. But not a consumer. Been disabled from job for eight years and rusty in the memory banks. {pain meds tend to do that} Just now able to set at computer for 30min. at a time Ran service for 28 years.

    The attitude from others doesn't bother me, you find elitism in any area of expertise.

    Tj

  9. #9
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
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    18,836
    You're welcome ,that consumer notice is permanent,hope you stay around the site,twenty eight years as a tech,you'll have a lot to offer here.

  10. #10
    The 10x36 and 10x24 are good for 1500 cfm capacity on the r/a...this leaves you 100 cfm short of required for the 4ton...not a huge deal(may be a little loud and minimal increased amps on blower) The real question is the size of the ducts feeding the cans...should be 18" round or equivalent minimum

  11. #11
    Join Date
    Jul 2008
    Location
    North Dallas
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    18
    Quote Originally Posted by proairhvac View Post
    The 10x36 and 10x24 are good for 1500 cfm capacity on the r/a...this leaves you 100 cfm short of required for the 4ton...not a huge deal(may be a little loud and minimal increased amps on blower) The real question is the size of the ducts feeding the cans...should be 18" round or equivalent minimum
    Really not picking on anyone. I do believe we should all learn a thing or 2. If return is too small you'll draw less amps.

  12. #12
    Curious how a small return draws less amps...always assumed with more restriction the blower has to work harder to move air...always want to learn...could you explain?

  13. #13
    Join Date
    Mar 2008
    Location
    Houston, Texas
    Posts
    352
    Quote Originally Posted by proairhvac View Post
    Curious how a small return draws less amps...always assumed with more restriction the blower has to work harder to move air...always want to learn...could you explain?
    The blower deal more with moving the air once it is inside the blower chamber. Although the air is having a hard time getting to the blower chamber, the volume of air the blower is dealing with is relatively low for its design.

    If you increase the r/a the blower is then running fully loaded, and using more power, but it is being more productive what the power consumed. If you do a cfm / kw used evaluation this should be plain to see.

    If you wanted the blower to pull the same amperage as before adding the r/a's, simply lower the blower speed. Your cfm / kw ratio will still be better because the air has less restriction getting to the airhandler.

    Go out to your car, using a bottle jack lift the drive wheels so they aren't making contact with the surface. Put it in drive and watch it go. Your cars tire rotation / per gallon of gas likely increased off the charts. Your cars engine has a light load on it, fuel usage is very low. Performance is also very low.

    Same thing as before.

    Don't try any of this at home, unless you are paying me to show you the correct / safe way in person.

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