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  1. #1
    Join Date
    Jan 2014
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    98
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    Confused Why is the RH high at the return vents?

    For the most part, our house stays between 48% and 55% RH most of the time but does get over 60% more than I'd like. I have several Sensorpush sensors that track temp and RH. With that said, I have taped the sensors to the return vents and of course the RH goes up when the AC compressor turns off and the RH goes down when the AC compressor turns on. I can see them both oscillating in the graphs that tracks temp and RH.

    The odd thing is I'm noticing the RH gets close to 70% when the AC turns off on a couple returns. The other returns don't get quiet as high but still gets in the 60s.

    Is it normal to have an RH of 70% at the return vents when the AC is off???

    If this isn't normal then maybe I've found part of the source of the humidity problem.

    Thanks guys!


  2. #2
    Join Date
    Mar 2018
    Location
    Chico, Ca #StateofJefferson
    Posts
    650
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    You could have outside air leaking into the return duct or from around the hole the return grille is fitted into. Also could just be reading the moisture on the coil re-evaporating into the air depending on your layout. Dont think Ive ever measured it at the grille unless it was running, if off I usually check by the tstat

  3. #3
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    9,367
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    Ok then, I will open this can of worms.

    The source of all the moisture in your home is the life saving infiltrating fresh air and the occupants, Usually. If you are lucky you get a fresh air change every 4-5 hours to purge indoor pollutants and renew oxygen. Low outdoor dew points dries your home. High outdoor dew point air make your home damp.

    The a/c cools your home by lowering the air temperature as the air flows through a cold coil. If the coil is cold enough moisture is removed also. The moisture collects on the cold coil. After the coil collects several lbs. water drips into a pan and goes down the drain. When the cold air leaves the it is 90%RH entering your home cold and damp. 55^F, 80-90%RH. High %RH but drier than when it entered.
    Next the a/c shuts off and the 2 lbs. moisture left on the cold coil and slowly evaporates back into the home. Until the coil is dried, high %RH slowly moves back to home, this takes a couple hours.
    How is that for a quick simple explanation of were is all that moisture coming from or going.
    If that does not explain it, come with a question.
    If you live in a green grass climate, expect a damp home whenever the a/c does not run much and the outdoor dew points are +55^F for an time.
    Regards Teddy Bear
    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"

  4. #4
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    9,075
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    There are RETURN GRILLES, DIFFUSERS ( Supply)
    and Vents ( Supply).

    I don't understand the term ' Return Vents'.

    Generally, there are One to Four RETURN(s)
    & A DOZEN or more Vents / Diffusers.

    Supply Air = ~90% at 55'F close to the Air Handler.

    ______ _ _ _ _ _ _ _ _ _____
    The conditions of the Returns should be the same as/ similar to Room Air.
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  5. #5
    Join Date
    Jan 2014
    Posts
    98
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    Thread Starter
    I was talking about the return grills. When the AC compressor turns off the humidity at the surface of the return grills goes up. Maybe it's the water in the pan evaporating and oozing back into the room via the return grills and supply vents.

  6. #6
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    9,367
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    well ya!
    That is what happens, the moisture on the coil/pan evaporates back into the home however it can.
    Regards Teddy Bear
    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"

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