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  1. #1

    whole house dehumidifier install question

    First post here. I appreciate all the great info.
    Just bought a house and once moved in instantly had allergy issues. The AC was shot anyway so we replaced it with a new Trane unit. Despite this we are still having issues with high humidity peaking at almost 70 and down to 55 or so when the unit has been running. We are looking at adding a whole house dehumidifier with outside air ventallation. The question is what configuration to install it in. The first option pulls air from the return duct, mixes with outside air, filters and returns it to the return duct before entering the ac unit. I like this option because it will go thru the main units filter. My problem is that I dont know if it is a good idea to blow air over the coils when the AC unit is not cooling and redistribute the moisture off the coils back into the house. The second option is to pull out of the return and bypass the AC unit and feed air back into the supply duct. The problem with this option is I do not get the pass thru the AC filter and only the dehumifiers filter would filter the outside air mixed with the return air. Additionally I would need to add a damper on the supply so that when the ac was running it does not backfeed air thru the dehumidifier supply side . Can someone advise which option is better? The unit we are looking at is the true dry DR90 thanks in advance for any feed back.

  2. #2
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,073
    Quote Originally Posted by chris311 View Post
    First post here. I appreciate all the great info.
    Just bought a house and once moved in instantly had allergy issues. The AC was shot anyway so we replaced it with a new Trane unit. Despite this we are still having issues with high humidity peaking at almost 70 and down to 55 or so when the unit has been running. We are looking at adding a whole house dehumidifier with outside air ventallation. The question is what configuration to install it in. The first option pulls air from the return duct, mixes with outside air, filters and returns it to the return duct before entering the ac unit. I like this option because it will go thru the main units filter. My problem is that I dont know if it is a good idea to blow air over the coils when the AC unit is not cooling and redistribute the moisture off the coils back into the house. The second option is to pull out of the return and bypass the AC unit and feed air back into the supply duct. The problem with this option is I do not get the pass thru the AC filter and only the dehumifiers filter would filter the outside air mixed with the return air. Additionally I would need to add a damper on the supply so that when the ac was running it does not backfeed air thru the dehumidifier supply side . Can someone advise which option is better? The unit we are looking at is the true dry DR90 thanks in advance for any feed back.
    The True dry 90 has a merv 11 air filter. This is better than most air handler filters. If a/c blower is a VS fan, cost of fan operation is minimal and the only negative is the rapid re-evaporation of moisture from a/c coil.

    If the fan is psc motor, I suggest a 3rd way of installing the dehu. The objective is be able to operate the dehu without the air handler blower. The return to the dehu is independent from the a/c. So an independent return from the open part of the home and from outside. The supply from the dehu goes to the a/c supply. This allows operating the dehu without the a/c blower. A small amount of the dehu output return back through the a/c to the home. All of the air passing through dehu sees the home before returning back to the dehu. Make sure to provide fresh air when the home is occupied. To minimize re-evaporation from the a/c coil, go to +3^F deadband and use t-stat setup when the home is not occupied for +4 hours during the day. The dehu will maintain <50%RH with any a/c operation.
    If your fresh air inlet duct is +10 ft. consider 8" fresh air duct to avoid any restriction of fresh air. You need an air change in 4 hours when occupied.
    Keep us posted on your question and results.
    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"

  3. #3
    Quote Originally Posted by teddy bear View Post
    The True dry 90 has a merv 11 air filter. This is better than most air handler filters. If a/c blower is a VS fan, cost of fan operation is minimal and the only negative is the rapid re-evaporation of moisture from a/c coil.

    If the fan is psc motor, I suggest a 3rd way of installing the dehu. The objective is be able to operate the dehu without the air handler blower. The return to the dehu is independent from the a/c. So an independent return from the open part of the home and from outside. The supply from the dehu goes to the a/c supply. This allows operating the dehu without the a/c blower. A small amount of the dehu output return back through the a/c to the home. All of the air passing through dehu sees the home before returning back to the dehu. Make sure to provide fresh air when the home is occupied. To minimize re-evaporation from the a/c coil, go to +3^F deadband and use t-stat setup when the home is not occupied for +4 hours during the day. The dehu will maintain <50%RH with any a/c operation.
    If your fresh air inlet duct is +10 ft. consider 8" fresh air duct to avoid any restriction of fresh air. You need an air change in 4 hours when occupied.
    Keep us posted on your question and results.
    Regards TB

    Thanks TB. I should mention that my home is three split levels. The lowest level would be the easiest to add a return to. Would that still dehumidify the whole house? Wasn't sure if the circulating of low humidity air dehunidifies the house or if its the return pulling the air from a given area. Also, in regards to the first option. I do not have a VS fan but am ok with the fan running with dehumidifier. Is the re evaporation off the coils really a concern or should that have minimal effect on indoor humidity levels? I woukdnt mind keeping the coils dry anyway to prevent any growth on them. Thanks again for your help!

  4. #4
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,073
    Quote Originally Posted by chris311 View Post
    Thanks TB. I should mention that my home is three split levels. The lowest level would be the easiest to add a return to. Would that still dehumidify the whole house? Wasn't sure if the circulating of low humidity air dehunidifies the house or if its the return pulling the air from a given area. Also, in regards to the first option. I do not have a VS fan but am ok with the fan running with dehumidifier. Is the re evaporation off the coils really a concern or should that have minimal effect on indoor humidity levels? I woukdnt mind keeping the coils dry anyway to prevent any growth on them. Thanks again for your help!
    Pushing a small amount of dry out of the supply ducts and having it find its way back to the dehumidifier will dry the entire home. Moisture diffuses rapidly through the air in a home. Elevation of the return is not critical. The a/c fan uses a considerable amount of electricity. Almost as much as the dehumidifier. A/c coils dry in +1 hour without any air circulation. <45 mins with a/c fan on. Your choice.
    Might increase dehumidification cost 20-30%. The dehu will increase the drying of the a/c coil when circulating the dry air throughout the home.
    A/c coils will dry more rapidly with a <50%RH home than high %RH. The toughest time to control growth in a/c duct is when the a/c runs continuously for +days at a time.
    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"

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