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  1. #1
    Join Date
    Jan 2019
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    Dehumidifier in conjunction with HVAC

    Humidity question. Need advice!
    (Quick summary: House humidity was fine, but air handler closet was 75%RH, growing mold. After putting small dehumidifier in A/C closet, the closet is better, but the rest of the house is 70%RH)

    My air handler is in a closet in the hallway. We noticed the air handler closet would stay around 75% humidity (even if the rest of the house was consistently around 50%). The RH in the closet went even higher when the A/C was running--we were getting mold on the wall inside that closet.

    We got all the ducting replaced last month (for different reasons), and while everything was opened up, we discovered the old furnace flue was never sealed---so it was allowing tons of outside/attic air to be pulled into the air handler. We got that sealed up, which we thought would fix the humidity issue. But the closet stayed humid.

    This week I put a small, standalone dehumidifier in the A/C closet (I had discussed the idea previously with the HVAC guy who did our ductwork). The dehumidifier is successfully keeping that closet around 50%RH (it is running nearly constantly, but the volume of water seems to be low). But now the rest of the house is at around 70%RH! So the closet is better, but now the house is worse than ever! I'm wondering if the dehumidifier is changing the air going into the air handler, which is affecting the efficiency of the A/C. Could it be that the A/C is not working correctly (the air is sure cold)?

    Is a dehumidifier near the air intake a good idea? Are there any other variables I could change to help narrow down the root issue?

    I'm not sure who to call. Some friends recommended an energy efficiency specialist to look at the whole house---or should I call an HVAC pro first?

    Notes & extra info:
    1-story, 1400 sq ft house; no subfloor; flat roof with very little attic space (no crawl-space); ducting is above a dropped ceiling. Located in Florida (very humid outside year-round). The house was built in 1950s originally without HVAC. The air handler closet is in the hallway, and that closet door is the only return in the house. There are no vents in the hallway (so the hallway air is theoretically more stale). The whole HVAC system was replaced June 2017. Then we had all the ducting replaced (with flex duct) in December 2018 and made sure to seal off spots where outside air was coming in. There is no room in that small closet nor in the attic for a whole-house dehumidifier.

    Not sure what to do from here. My wife has health problems, and the humidity/mold-risk needs to be resolved.

    Thanks for your time!

  2. #2
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
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    Putting a dehumidifier in the air stream before the cooling coil adds dry air and heat to the air stream entering the cooling coil. the net result is that the a/c coil removes less moisture when the dehumidifier is running. You gain nothing with the dehumidifier in that air stream. Adding the dry warm air down stream of the cooling coil adds to the a/c's drying effect. So the dry air from the whole house dehumidifier should be added to the air cold air supply.
    back to basics, get your a/c setup correctly. The well setup a/c will maintain <50%RH during peak cooling loads. If not get the a/c checked.
    During evenings and rainy day with high outdoor dew points, you will need supplimental dehumidification. A small whole house dehumidifier is probably the best choice to maintain low humidity during the hours of low sensible cooling.
    Check out the Ultra-Aire whole houses dehumidifiers with fresh air option.
    Keep us posted on your solution.
    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"

  3. #3
    Join Date
    Jan 2019
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    Thread Starter

    A/C closet RH

    Thank you for your response! I had a feeling the dehumidifier in the air stream could affect the way the A/C worked.

    Now that we've taken the dehumidifier away from the air handler, I think the humidity in the house will go back to normal, once the A/C system catches back up. The RH throughout the house was never a problem before. I'm not sure if a whole-house dehumidifier is necessary yet, but I will keep that in mind (thank you!).

    Assuming overall indoor RH does go back to normal, the problem we still need to solve is the humidity level in the A/C closet. Without the standalone dehumidifier, that closet stays consistently at 70-75%RH. In the summer, you can see sweat dripping from the duct exiting the air handler, which was growing mold on the closet wall.

    It seems like the most humid part of the closet is the upper half (that's where the mold was). The only ventilation in that closet is the bottom section of the door--and the air handler pulls air from the bottom, so I guess the upper air stays stale.
    I'm trying to brainstorm... maybe increasing airflow to that closet would allow the upper air to get pushed into the A/C system?

    What do you think about adding a couple very small fans in the top of the closet to push air down into the intake? Or maybe cutting another return on a different side of the closet?
    Any ideas to reduce RH% in the A/C closet?

    Thanks again.





    Quote Originally Posted by teddy bear View Post
    back to basics, get your a/c setup correctly. The well setup a/c will maintain <50%RH during peak cooling loads. If not get the a/c checked.
    During evenings and rainy day with high outdoor dew points, you will need supplimental dehumidification. A small whole house dehumidifier is probably the best choice to maintain low humidity during the hours of low sensible cooling.
    Check out the Ultra-Aire whole houses dehumidifiers with fresh air option.
    Keep us posted on your solution.
    Regards Teddy Bear

  4. #4
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
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    Quote Originally Posted by Jomo86 View Post
    Thank you for your response! I had a feeling the dehumidifier in the air stream could affect the way the A/C worked.

    Now that we've taken the dehumidifier away from the air handler, I think the humidity in the house will go back to normal, once the A/C system catches back up. The RH throughout the house was never a problem before. I'm not sure if a whole-house dehumidifier is necessary yet, but I will keep that in mind (thank you!).

    Assuming overall indoor RH does go back to normal, the problem we still need to solve is the humidity level in the A/C closet. Without the standalone dehumidifier, that closet stays consistently at 70-75%RH. In the summer, you can see sweat dripping from the duct exiting the air handler, which was growing mold on the closet wall.

    It seems like the most humid part of the closet is the upper half (that's where the mold was). The only ventilation in that closet is the bottom section of the door--and the air handler pulls air from the bottom, so I guess the upper air stays stale.
    I'm trying to brainstorm... maybe increasing airflow to that closet would allow the upper air to get pushed into the A/C system?

    What do you think about adding a couple very small fans in the top of the closet to push air down into the intake? Or maybe cutting another return on a different side of the closet?
    Any ideas to reduce RH% in the A/C closet?

    Thanks again.
    You misunderstand my comments. The dehumidifier in the return air does not decrease the amount of moisture that is removes by both running. It is about the same. When the a/c is off, the dehumidifier removes moisture, but probably not enough because of it's location and low capacity.
    The location of condensation is because of the temperature of the surface. Insulating the surface with a closed cell foam thick enough to raise the surface temperature above the dew point of the air in the space. The temperature of the air coming from the a/c coil should be about 50-54^F max. The cooling coil should be <30F colder than the room temperature. All of this has to do with removing moisture while doing significant cooling. That is mission one. Make no mistake, you will need significant dehumidification during rainy weather and evenings when the sensible cooling load go to near zero and the outdoor dew points remain high.
    The moisture problem depends on weather and internal moisture generation.
    During low dew point weather warm weather, not much of a humidity problem.
    You need supplemental dehumidification during common weather in green grass climates. Near condensation on any surface for +24 hours start the mold growth cycle. +60% RH for weeks allows the dust mites to grow any place that skin scale accumulates.
    When you are able to maintain <50%RH, you will much more comfortable at warm temperatures, especially with some air move like from a ceiling fan.
    Most a/c contractors feel occasional high humidity is not the end of the world because a little mold and dust might does not effect all people. Adding VS equipment only slightly improves the problem. Getting the a/c setup ideally and addding a 3-4 lb. per hour whole house dehumidifier will maintain 50%RH even when the a/c is off. This means a much drier, healthier, and more comfortable home with less energy use.
    Keep us posted on what you do.
    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"

  5. #5
    Join Date
    Jan 2019
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    Thread Starter
    Thanks again for your insight.

    Unfortunately, a central dehumidification system isn't an option right now. But we took our current standalone dehumidifier out of the A/C closet, and bought a 2nd one. We are now running them throughout the day, and we are keeping the house below 50%RH for the first time. And so far that has helped the humidity in the A/C closet drop to the low 40s. We shall see how it performs in different seasons.

    I had thought the closet was a bigger issue than the house, but I guess I was wrong.





    Quote Originally Posted by teddy bear View Post

    Insulating the surface with a closed cell foam thick enough to raise the surface temperature above the dew point of the air in the space. The temperature of the air coming from the a/c coil should be about 50-54^F max. The cooling coil should be <30F colder than the room temperature. All of this has to do with removing moisture while doing significant cooling. That is mission one. Make no mistake, you will need significant dehumidification during rainy weather and evenings when the sensible cooling load go to near zero and the outdoor dew points remain high...When you are able to maintain <50%RH, you will much more comfortable at warm temperatures, especially with some air move like from a ceiling fan.
    Most a/c contractors feel occasional high humidity is not the end of the world because a little mold and dust might does not effect all people. Adding VS equipment only slightly improves the problem. Getting the a/c setup ideally and addding a 3-4 lb. per hour whole house dehumidifier will maintain 50%RH even when the a/c is off. This means a much drier, healthier, and more comfortable home with less energy use.
    Keep us posted on what you do.
    Regards Teddy Bear

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