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  1. #1
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    Adding a return air duct - should I go high or low?

    I am adding a return air duct. The existing duct is high on the wall, as are all the supply ducts. I have identified two locations where I could add a return air duct, one is high and one is low. Should I go high or low, and why? The contractors say it's up to me where they will put it. All things being equal, I would go with the low one as it's less costly to do in this case.

  2. #2
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    If it cost less low, its a good way to go.
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  3. #3
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    Jun 2009
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    Quote Originally Posted by beenthere View Post
    If it cost less low, its a good way to go.
    Yeah, I was thinking the low one is so simple I could do it myself, but of course I'm not allowed to talk about that here.

  4. #4
    Join Date
    Nov 2008
    Location
    Ottawa Valley, Eastern Ontario, Canada
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    170
    This subject has been beaten to death on these forums.
    I don't have any scientific explanation.
    All I know is I must provide enough return air for the air handler in question.
    In my experience, in residential work, it really doesn't seem to matter.
    Occasionally, the homeowner will request high wall returns.
    You want it, you got it.

  5. #5
    Join Date
    Jun 2009
    Location
    massachusetts
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    22
    We do supply low, return high, this keeps the air circulating better and keeps any dirt, dust or, animal hair from clogging filters. I know someone is going to say that's what there for but I've seen filters that have to be changed every two weeks because of animal hair.
    All that said air circulation is key. Cost vs. Efficiency

  6. #6
    Join Date
    May 2009
    Location
    Indiana
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    77
    Low supplies & High returns warm air rises suck the stratified heat off the ceiling -

    Is your home two story with Low Supplies (in the floor) down stairs & High Supplies (in the ceiling) up stairs? High returns upstairs (suck the stratified heat off the ceiling) & low return down stairs (suck the cool puddled air off the floor).

  7. #7
    Join Date
    Jan 2009
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    Keokuk, IA
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    5,520
    If supplies are located and designed correctly, they will mix the air so that stratification is not an issue, so return location is mostly irrelevant.

    One advantage of high returns is that they are less noticeable and you don;t have ot worry abotu placing furniture in front of them.


    But as mentioned, for better or worse, you'll suck in more dust closer to the floor, where most of the dust is located. But it's the function of a vacuum cleaner to clean the floors, not the HVAC system. Based on what I dump out of my vacuum once a week... I would be changing filters almost weekly if the returns were near the floor.

  8. #8
    Join Date
    Jun 2009
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    Albuquerque NM
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    Quote Originally Posted by Mark T View Post
    Low supplies & High returns warm air rises suck the stratified heat off the ceiling -

    Is your home two story with Low Supplies (in the floor) down stairs & High Supplies (in the ceiling) up stairs? High returns upstairs (suck the stratified heat off the ceiling) & low return down stairs (suck the cool puddled air off the floor).
    It's one story. All existing ducts are high. Adding a low return air duct would give me one high and one low return air duct, so some of each.

  9. #9
    Join Date
    May 2007
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    Dry as a bone Tucson
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    4,344

    returns

    Quote Originally Posted by garya505 View Post
    I am adding a return air duct. The existing duct is high on the wall, as are all the supply ducts. I have identified two locations where I could add a return air duct, one is high and one is low. Should I go high or low, and why? The contractors say it's up to me where they will put it. All things being equal, I would go with the low one as it's less costly to do in this case.
    If you are in a mostly heating climate you want low returns to get the cold air. If your climate is mostly cooling you want high returns to get the hot air.
    Last edited by ACFIXR; 06-19-2009 at 08:03 PM. Reason: spelling, of course
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  10. #10
    Join Date
    Nov 2008
    Location
    Ottawa Valley, Eastern Ontario, Canada
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    170
    Quote Originally Posted by ACFIXR View Post
    If you are in a mostly heating climate you want low returns to get the cold air. If your climate is mostly cooling you want high returns to get the hot air.
    Thank you for that.
    Around here (Eastern Ontario) it is a generally accepted practice to install low returns.
    We are in a 'mostly heating' climate.
    It is near the end of June, and I haven't turned on the cooling yet.
    As a matter of fact, the furnace is still running most nights.
    Could someone send some 'global warming' my way?

  11. #11
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    Jun 2009
    Location
    Albuquerque NM
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    OK, so it's going in low. Now here's the next question. I'm putting in 3-ton AC for my 1800 sq ft house. The current R/A duct is 20x10, which most would agree at 200 square inches is too small. The location I have selected to add the second R/A will easily take a 20x10. Anything bigger than that would involve considerably more cost. With two 20x10 R/A ducts that gives me 400 square inches. Since I ran all last winter with a 5-ton furnace pulling air in through 200 square inches (at high velocity, yikes), and given that I'm going to have them slow the blower down to 1600 cfm (4 ton) for heating, I'm sure the furnace will do fine with 400 square inches. I can have them set the blower for a 3-ton (1200 cfm) or 3.5 ton (1400 cfm) cooling, so for cooling I would have 1200 to 1400 cfm coming in through 400 square inches. Now, I know this isn't perfect and I know more R/A area is better, but how much trouble should I go through to get it larger than 400 square inches. I think the air velocity will be under the max (700 fps) for cooling and maybe even for heating as well. Any thoughts?

  12. #12
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    Lancaster PA
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    Should be ok.

    The velocity won't be high.
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  13. #13
    Join Date
    May 2007
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    Dry as a bone Tucson
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    temperature rise

    Quote Originally Posted by garya505 View Post
    OK, so it's going in low. Now here's the next question. I'm putting in 3-ton AC for my 1800 sq ft house. The current R/A duct is 20x10, which most would agree at 200 square inches is too small. The location I have selected to add the second R/A will easily take a 20x10. Anything bigger than that would involve considerably more cost. With two 20x10 R/A ducts that gives me 400 square inches. Since I ran all last winter with a 5-ton furnace pulling air in through 200 square inches (at high velocity, yikes), and given that I'm going to have them slow the blower down to 1600 cfm (4 ton) for heating, I'm sure the furnace will do fine with 400 square inches. I can have them set the blower for a 3-ton (1200 cfm) or 3.5 ton (1400 cfm) cooling, so for cooling I would have 1200 to 1400 cfm coming in through 400 square inches. Now, I know this isn't perfect and I know more R/A area is better, but how much trouble should I go through to get it larger than 400 square inches. I think the air velocity will be under the max (700 fps) for cooling and maybe even for heating as well. Any thoughts?
    Your airflow should be adjusted in heating by the temperature rise method not by the stated tonnage method. That is why there is more than one speed on your blower motor.
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