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  1. #1
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    Does friction loss = air loss?

    When measuring external static pressure I know that it is the measurement of the friction of the air against the ducting. It is directly proportional to velocity and referred to as friction loss and measured in inches of water column.

    Is there an actual loss of air? In other words, does less air come out of the supply registers than is taken in from the return side due to friction loss? If a well sealed supply duct were measure for air flow, would there be a difference between the measured return air and the measured supply outlets that would be attributable to friction loss?

  2. #2
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    Ever hear of the conservation of matter and energy? If the system isn't leaking air, ALL the air that goes in HAS to come out, with no loss in mass flow. Unless you've invented some interdimensional time warp that sends air into some unknown void in the universe!

    What gets spent is energy. It takes power to move air. And that power is comsumed by the friction of the ducting, and turned into heat. Again, even the energy is conserved.

    Quote Originally Posted by mchild View Post
    When measuring external static pressure I know that it is the measurement of the friction of the air against the ducting. It is directly proportional to velocity and referred to as friction loss and measured in inches of water column.

    Is there an actual loss of air? In other words, does less air come out of the supply registers than is taken in from the return side due to friction loss? If a well sealed supply duct were measure for air flow, would there be a difference between the measured return air and the measured supply outlets that would be attributable to friction loss?

  3. #3
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    Quote Originally Posted by kuryakin View Post
    Ever hear of the conservation of matter and energy? If the system isn't leaking air, ALL the air that goes in HAS to come out, with no loss in mass flow. Unless you've invented some interdimensional time warp that sends air into some unknown void in the universe!

    What gets spent is energy. It takes power to move air. And that power is comsumed by the friction of the ducting, and turned into heat. Again, even the energy is conserved.
    While I have not heard of your term, I instinctively assumed just that. What goes in has to come out somewhere. This topic came up with a HVAC guy while talking about reading static pressure. He insisted the air is lost. I just could not buy it.

    Many thanks.

  4. #4
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    Higher static then the fan motor/blower can handle results in less air flow.

  5. #5
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    Jun 2007
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    Wisconsin
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    The more resistance you have will make the fan work harder to push the same amount of air or the fan won't keep up and you will have a loss of air. Imagine blowing as hard as you can, like having no resistance, and blowing through a straw as hard as you can causing resistance.

  6. #6
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    Quote Originally Posted by mchild View Post
    When measuring external static pressure I know that it is the measurement of the friction of the air against the ducting. It is directly proportional to velocity and referred to as friction loss and measured in inches of water column.

    Is there an actual loss of air? In other words, does less air come out of the supply registers than is taken in from the return side due to friction loss? If a well sealed supply duct were measure for air flow, would there be a difference between the measured return air and the measured supply outlets that would be attributable to friction loss?
    Some hvac guys can violate conservation of mass and energy. Some can expell more hot air than they breathed in (referring to your hvac guy).

    There are different types of duct loss: Friction loss, which is just drag on the air, or backpressure, which slows the air down. Air leakage, which is the actual loss of air from the supply ducts. It doesn't disappear, it just leaves the duct through unsealed cracks or openings before it gets to the air vents.
    Heat loss, which is the loss of heat from the air through the duct and insulation.

    Higher friction means higher pressure, and that means both higher air loss and heat loss. So in that sense he was correct, but he seemed to have either misled you, or himself, or else you heard him wrong.
    Last edited by hvacrmedic; 03-04-2008 at 01:04 AM.

  7. #7
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    I think the error that's made is in terminology. Too many people say air loss when what they really mean is air FLOW loss.

    Quote Originally Posted by hvacrmedic View Post
    Some hvac guys can violate conservation of mass and energy. Some can expell more hot air than they breathed in (referring to your hvac guy).

    There are different types of duct loss: Friction loss, which is just drag on the air, or backpressure, which slows the air down. Air leakage, which is the actual loss of air from the supply ducts. It doesn't disappear, it just leaves the duct through unsealed cracks or openings before it gets to the air vents.
    Heat loss, which is the loss of heat from the air through the duct and insulation.

    Higher friction means higher pressure, and that means both higher air loss and heat loss. So in that sense he was correct, but he seemed to have either misled you, or himself, or else you heard him wrong.

  8. #8
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    Quote Originally Posted by hvamimi View Post
    The more resistance you have will make the fan work harder to push the same amount of air or the fan won't keep up and you will have a loss of air. Imagine blowing as hard as you can, like having no resistance, and blowing through a straw as hard as you can causing resistance.
    If the blower is a centrifugal squirrel cage type, the higher the static pressure, the less the blower is loaded. IOW, the blower will not "work harder" to move the same amount of air if static pressure is increased. It will move less air and draw less amperage as a result.

    This is what blower/fan curves represent. The blower wheel is designed to move a given amount of CFM at a given static pressure. Tweak the static either way and the amount of CFM the blower can produce changes.

  9. #9
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    Sep 2006
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    Thanks guys. We are all on the same page. Now I just need to get my "pro" on that page.

    The difference in the measured return air CFM of 1,202 and the total supply air of 1,048 CFM was explained as a result of .34 total ESP (.14 supply side). I used the same straw analogy hvamimi gave, but he insisted the loss was due to "friction loss" and not leakage of the duct work.

    I may print out and use your responses as he may accept it more coming from pro's than a lay person. I'll get him trained eventually.

    Thanks again.

  10. #10
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    Where are the supply ducts located?

    Have they checked where the ducts come into the space,behind the grilles??

  11. #11
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    Quote Originally Posted by dash View Post
    Where are the supply ducts located?

    Have they checked where the ducts come into the space,behind the grilles??
    Ducts are in the attic.

    You may remember the difficulty I have had finding a company to resolve problems. I have ended up hiring a small company to do specific work as I detail. I had them seal the ducts where they pass through the dry wall (could slide your entire hand into the attic), enlarge the return filter grill and return duct, and add balance dampers on each of the branches. You (and a few other pros here) contributed to the work order as I read every post you make on air flow issues.

    Once all this was done, he then balanced the branches and through that effort found the total supply side had 1,048 CFM while the return was 1,202. The good news there is less than 1/2 degree difference between rooms (had been about 8*), the bad news is I am convinced that it is leakage (may use Areoseal to resolve). He wasn't and thought it was friction loss.

    Again thanks for all your help.

  12. #12
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    Good chance, its leakage on both the return and supply.
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    How many times must one fix something before it is fixed?

  13. #13
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    Quote Originally Posted by beenthere View Post
    Good chance, its leakage on both the return and supply.
    You may be right, but I spec'ed and confirmed the sealing of the new return duct and filer box that replaced the leaky small one that was originally in place. If I get Areoseal out they will do both.

    The issue I am wrestling with on getting the ducts sealed is that although they are in the attic, most of it has been foamed. About 2/3 of the supply and all of the return are in the attic space that is foamed. Thus, using a very general assumption that the leaks are equally distributed along the ducts, and only one third of the ducts are in an open attic, then the leakage to the outside would be about 51 CFM or about 4% (1202-1048=154/3=51).

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