Will air bypass lower relative humidity?
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  1. #1

    Will air bypass lower relative humidity?

    If I bypass a portion of my A/C supply air back to the return to pass over the cooling coil again, will I remove more moisture and thus help to lower my overall indoor humidity levels?

  2. #2
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    You'll probably just freeze up your coil.

    If would make more sense to pass the high humidity air across the coil and not the air you just dehumidified already.

    But I'm going to add I might be speaking out of my butt here. =)

  3. #3
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    Quote Originally Posted by airitout View Post
    If I bypass a portion of my A/C supply air back to the return to pass over the cooling coil again, will I remove more moisture and thus help to lower my overall indoor humidity levels?
    It is more effective to slow the volume of air moving over the coil than what you propose.

    Even that must be done with care.

    Another option is to find how and where humidity is getting into your house, and reduce it. This works in your favor year round, and doesn't use any electricity to pull off, once completed.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  4. #4

    ok but can you confirm

    ok- but can you confirm that a small amount of bypass back over the coil will have some effect on reducing humidity?

  5. #5
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    Quote Originally Posted by airitout View Post
    ok- but can you confirm that a small amount of bypass back over the coil will have some effect on reducing humidity?
    There are a lot of variables that come in to play.

    What is the humidity of the incoming air, exiting air, coil temp.....

    I would say yes you will remove a small amount more moisture by passing it over the coil a second time, but I still think it's futile.

  6. #6
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    The recirculated cold air will reduce the amount of air delivered to the house. You dehumidify by bringing air to the cold coil, wringing out the moisture, blowing it back to the house, sucking it back in again, over and over until you meet temperature and humidity levels you want. By short circuiting a section of it you reduce overall air turnover in the house.

    AND...the cold air mixing with the warmer return air will lower return air temperature, which will reduce heat transfer and moisture wringing ability.

    It is psychrometrically not in your favor to do this.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  7. #7
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    Quote Originally Posted by airitout View Post
    ok- but can you confirm that a small amount of bypass back over the coil will have some effect on reducing humidity?
    What you are calling bypass should be called recirculation.

    One method of humidity control in commercial applications is cooling coil bypass. But the air that is bypassed is not recirculated, it bypasses the cooling coil and enters the supply air, so total airflow in the space remains constant. To do this correctly requires a motorized damper that will automatically adjust the amount of air bypassed to maintain a target coil leaving air temperature. The trouble and expense to retrofit a residential system with a correctly engineered bypass would be better spent on other things like a whole house dehumidifier when the existing system can't be re-controlled or adjusted to maintain humidity.

  8. #8
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    Quote Originally Posted by shophound View Post
    The recirculated cold air will reduce the amount of air delivered to the house. You dehumidify by bringing air to the cold coil, wringing out the moisture, blowing it back to the house, sucking it back in again, over and over until you meet temperature and humidity levels you want. By short circuiting a section of it you reduce overall air turnover in the house.

    AND...the cold air mixing with the warmer return air will lower return air temperature, which will reduce heat transfer and moisture wringing ability.

    It is psychrometrically not in your favor to do this.
    Brother, you could not have said it any better, I have been trying to preach on this to a couple "blue boys" that frequent the same supply house as I do forever. I just finally gave up, there blood bleeds blue, you will not tell them anything.
    __________________________________________________ _______________________
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  9. #9
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    cool air can't hold very much moisture. You already knew that though.

  10. #10
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    Quote Originally Posted by Mr Bill View Post
    Brother, you could not have said it any better, I have been trying to preach on this to a couple "blue boys" that frequent the same supply house as I do forever. I just finally gave up, there blood bleeds blue, you will not tell them anything.
    Goes to show ya, Bill...some folks are just flat out "unlearnable".

    Like in the movie "Inception", where the lead character speaks of an idea in the mind is stronger than any disease ever created. Once it sets in and starts to grow it can be next to impossible to kill off, even if it is a thoroughly bad idea.

    I think this explains well why some folk here on the board are VERY slow to grasp being told certain things...they just don't want to hear it; it doesn't square firmly with how they are convinced it SHOULD be. When I was younger I suffered from this malady...letting go to let myself learn has been one of the most liberating things I've ever let myself do.

    It's also much more satisfying than stupidly holding onto an idea that is just flat out wrong in the real world.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  11. #11
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    Quote Originally Posted by fxb80 View Post
    What you are calling bypass should be called recirculation.

    One method of humidity control in commercial applications is cooling coil bypass. But the air that is bypassed is not recirculated, it bypasses the cooling coil and enters the supply air, so total airflow in the space remains constant. To do this correctly requires a motorized damper that will automatically adjust the amount of air bypassed to maintain a target coil leaving air temperature. The trouble and expense to retrofit a residential system with a correctly engineered bypass would be better spent on other things like a whole house dehumidifier when the existing system can't be re-controlled or adjusted to maintain humidity.
    The bypass idea is complicated to use on residential equipment but has the advantage of maintaining good air flow for good air circulation throughout the structure. Also the ducts are warmer and less prone to sweating on the outside. Residentially during high cooling loads, we are stuck with slowing the air flow over the cooling coil to remove enough moisture to maintain <50%RH.
    It is important to set the a/c up to remove enough moisture during extended high cooling loads to maintain the desired %RH in the home. This means that the a/c cooling coil temperature must be cold enough to provide dew points below 50^F.
    Supplemental dehumidification is intended to provide low %RH when the a/c has no/low cooling loads. Using a whole house dehu to maintain low %RH during high cooling loads will increase the cooling cost of the home. Typically, 50-70 lbs. of moisture needs to be removed from the home to maintain <50%RH during summer months. Removing the moisture with the a/c is requires a 48-52^F cooling coil and with 8-12 hours of operation. If the a/c has no cooling load, the whole house dehu will remove enough moisture to maintain <50%RH.
    A home with an a/c that does not remove moisture but depends on a dehumidifier for humidity control will have an additional 75,000 btu per day cooling load from the heat of the dehumidifier.
    A/c keeps the home dry during hot weather while cooling the home. The WH dehu keeps the home dry when the a/c is off and when there is not enough cooling loads to remove the moisture.
    Regards TB
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

  12. #12
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    Quote Originally Posted by fxb80 View Post
    What you are calling bypass should be called recirculation.

    One method of humidity control in commercial applications is cooling coil bypass. But the air that is bypassed is not recirculated, it bypasses the cooling coil and enters the supply air, so total airflow in the space remains constant. To do this correctly requires a motorized damper that will automatically adjust the amount of air bypassed to maintain a target coil leaving air temperature. The trouble and expense to retrofit a residential system with a correctly engineered bypass would be better spent on other things like a whole house dehumidifier when the existing system can't be re-controlled or adjusted to maintain humidity.
    The bypass idea is complicated to use on residential equipment but has the advantage of maintaining good air flow for good air circulation throughout the structure. Also the ducts are warmer and less prone to sweating on the outside. Residentially during high cooling loads, we are stuck with slowing the air flow over the cooling coil to remove enough moisture to maintain <50%RH.
    It is important to set the a/c up to remove enough moisture during extended high cooling loads to maintain the desired %RH in the home. This means that the a/c cooling coil temperature must be cold enough to provide dew points below 50^F.
    Supplemental dehumidification is intended to provide low %RH when the a/c has no/low cooling loads. Using a whole house dehu to maintain low %RH during high cooling loads will increase the cooling cost of the home. Typically, 50-70 lbs. of moisture needs to be removed from the home to maintain <50%RH during summer months. Removing the moisture with the a/c is requires a 48-52^F cooling coil and with 8-12 hours of operation. If the a/c has no cooling load, the whole house dehu will remove enough moisture to maintain <50%RH.
    A home with an a/c that does not remove moisture but depends on a dehumidifier for humidity control will have an additional 75,000 btu per day cooling load from the heat of the dehumidifier.
    A/c keeps the home dry during hot weather while cooling the home. The WH dehu keeps the home dry when the a/c is off and when there is not enough cooling loads to remove the moisture. Do the best you can with a/c and then use the dehu. Few understand the relationship
    Regards TB
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

  13. #13
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    Quote Originally Posted by airitout View Post
    If I bypass a portion of my A/C supply air back to the return to pass over the cooling coil again, will I remove more moisture and thus help to lower my overall indoor humidity levels?
    Yes, this is roughly equivalent to reducing the amount of air flowing through the coil. If the coil doesn't get cold enough for the air to reach the desired dew point, and changing the speed of the blower isn't an option, recirculating the air will lower the coil temperature, getting closer to the desired dew point. This will be at the expense of longer operating times and power cost.

    Cooling coil bypass in a residential system isn't optimal when processing humid fresh air, because typically some of that humid air will bypass the coil. In commercial systems the bypass is setup so that it is the return air that bypasses the coil, and all the fresh air goes through it, and that is effective.

    Recirculating is best done with a motorized damper controlled with a dehumidistat. This is to allow recirculation only when the humidity is too high. This will help avoid coil freezing when there's no need for the extra dehumidification, i.e. don't do this when the latent load is small. Or, use a dehumidifier, as others have suggested. You'll need a dehu anyway when the cooling load is very small and it's raining outside, for example.
    -If you won't turn it on then nothing else matters.

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