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  1. #1
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    Jun 2008
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    Whole-house Dehumidifier Ducting Options

    I recently bought a Honeywell DH-90 dehumidifier and was wondering how the different ducting options affect the performance of the unit? The instructions say the preferred method is to install a dedicated return seperate from the existing HVAC and seperate supplies to bedrooms/isolated rooms. To keep it simple, I was thinking (I know, wrong thing to do) of ducting the DH90 return to the HVAC return and the DH90 supply also to the HVAC return but further downstream than the DH90 return. It seems to me this would "detour" some of the main HVAC return air into the DH90 to be dehumidified and dump it back into the main air stream. There is a similar setup in the installation instructions but they show the dehumidified air ducted to the HVAC supply air. To my brain, the DH90 fan has to fight the HVAC fan when they are both being operated. With both sides of the DH90 ducted to the HVAC return, the DH90 sees a net pressure difference of zero. Does this make sense or has anyone seen a setup like this? I read some posts and can't seem to get a definitive answer...

  2. #2
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    Best with a dedicated return from the open part of house to the dehu. Discharge the dry air from the dehu into the supply of a/c. The fan on the dehu is made to handle .5" WG pressure of a/c. Doing your way requires operating the a/c fan with the dehu. Keep us posted on your results. 50%
    RH is a good setting. 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
    Join Date
    Aug 2003
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    906
    It depends. This is my understanding:
    Bypass installation (like you describe) has for a plus that no new ductwork is necessary. However the furnace fan has to run when the dehumidifier runs. For my house, installing new ductwork was not a possibility. Discharging the dehu on the supply side of the furnace was dodgy because the existing ductwork was already a bit too small. Adding 200 cfm to the already too high A/C airflow was not a good idea.

    The advantages of the independent ductwork installation should be obvious.

    The installation with discharge on the supply side has for advantage that it doesn't interfere with the A/C operation (dehumidification in particular).

    If what you need to do isn't clear or if you don't know what your ductwork can handle and what's the ESP on your system, please hire a pro to do the install. You'll be glad if you need warranty support; nobody will be able to blame you for installing it incorrectly. Also, this site frowns on DIY help requests and this answer is already toeing the line. Besides, I'm not a pro so use the above at your own risk.

  4. #4
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    Jun 2004
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    4H: Hot, Humid Houston H.O.
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    "Bypass installation" -- let me clear up my own understanding of the terrm. That means the dehu return is connected to the central AC return, and the dehu supply goes to the central AC supply, right? If that is the case I am nearly certain many existing AC systems would present more than 0.5 inch (water column pressure) resistance to the dehu fan. It would be operating out of spec, possibly way out of spec.

    I believe the dehu airflow would be far healthier for the equipment, if one could arrange a dedicated air return for the dehu in the central part of the house. The benefit is to avoid working against the negative pressure in the return plenum, which commonly can be 0.5 inch all by itself (as mine used to be). This is a statement made to hone my understanding of this technical matter, I believe Teddy Bear will be tolerant and correct me if this is mistaken. Fortunately most of the pro curmudgeons could care less about dehumidifiers<g>.

    If you want a forum for uncensored discussion, we could move part of a thread to the Modern Texas Home Project, a similar board which is a ghost town at present. Same software, same look and feel so minimal learning curve.

    Hope this is right, and hope this helps -- Pstu

  5. #5
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    Aug 2003
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    906
    Quote Originally Posted by pstu View Post
    "Bypass installation" -- let me clear up my own understanding of the terrm. That means the dehu return is connected to the central AC return, and the dehu supply goes to the central AC supply, right?
    (...)
    No, I meant "bypass installation" as for HEPA filters, where the filter takes air from the return, filters it and puts it back into the return a bit further downstream. See for example these installation instructions:
    http://www.discountfurnacefilter.com...A%20System.pdf

    I agree that it could have meant what you described.

  6. #6
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    Thank you for helping me understand bypass installation better. If the main air handler were to run when the dehu runs, seems to me it would work OK -- I am speculating as to the answer here and expect to be corrected if wrong. Trying to work thru the answer together with the other thinkers on the board, as opposed to actually giving technical advice I could stand behind.

    If the dehu had its own return and supplied air into the central supply plenum, unless the central HVAC had exceptionally high back pressure it would be within the specs of the dehu fan. This is based on

    1) Thermastor documents this as a recommended method,
    2) assuming the central AC has static pressure in the supply plenum below 0.4-0.5 inch w.c.,
    3) assuming the ductwork has little problem handling an extra 200 CFM of airflow.

    I am particularly interested in working through this problem because I may want to do something like this in my own house. Do believe the Thermastor UA models can be configured to run only when the central AC is not running, which would seemingly eliminate the pitfalls of #2 and #3. No doubt that ability likely applies to Aprilaire too, if you don't need as much energy efficiency.

    Hope this helps -- Pstu

  7. #7
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    Quote Originally Posted by teddy bear View Post
    Best with a dedicated return from the open part of house to the dehu. Discharge the dry air from the dehu into the supply of a/c. The fan on the dehu is made to handle .5" WG pressure of a/c. Doing your way requires operating the a/c fan with the dehu. Keep us posted on your results. 50&#37;
    RH is a good setting. Regards TB
    I would agree with the dedicated return for the dehu. You go in full bypass mode with the AC and you end up having to run the ahu fan for the dehu. Get a problematic building where you are forced to use a dehu and the air handler blower will never shut off. Get a lot of mositure re-evaporating.

  8. #8
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    .................
    Last edited by Carnak; 07-01-2008 at 08:19 AM.

  9. #9
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    Jun 2008
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    Indiana
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    Yea, the constant running of the AHU fan would keep drying out the AC coil between calls for cooling. This would make the dehumidifier worthless as it would be drying the air then sending that nice dry air right over the soaking wet coil. That seems to be the biggest downfall. I've decided to re-route an existing return and dedicate it to the dehumidifier. Now I'm trying to decide if it would be worth it to run dedicated supply vents to each bedroom as Honeywell recomends... I know this is the best setup but it's a lot of extra work.

  10. #10
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    Quote Originally Posted by DrivenByDemons View Post
    Yea, the constant running of the AHU fan would keep drying out the AC coil between calls for cooling. This would make the dehumidifier worthless as it would be drying the air then sending that nice dry air right over the soaking wet coil. That seems to be the biggest downfall. I've decided to re-route an existing return and dedicate it to the dehumidifier. Now I'm trying to decide if it would be worth it to run dedicated supply vents to each bedroom as Honeywell recomends... I know this is the best setup but it's a lot of extra work.
    I wonder why Honeywell recommends bedrooms as the location of supply vents. Is there a well-considered reason, or is it just a convenient story?

  11. #11
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    Quote Originally Posted by DrivenByDemons View Post
    Yea, the constant running of the AHU fan would keep drying out the AC coil between calls for cooling. This would make the dehumidifier worthless as it would be drying the air then sending that nice dry air right over the soaking wet coil. That seems to be the biggest downfall. I've decided to re-route an existing return and dedicate it to the dehumidifier. Now I'm trying to decide if it would be worth it to run dedicated supply vents to each bedroom as Honeywell recomends... I know this is the best setup but it's a lot of extra work.
    Discharging the dry air into the supply side of a/c system pushes the bulk of the dry out the supply side of the ducts. A small amount of the dry air moves back through the coil/blower/air filter. It several hours to slowly dry the coil/return. It is worth the effort to dry out the coil/ducts/filter periodicaly to break the mold growth cycle. Yes, the dehu operates little longer. For people wanting the best IAQ or allergies, this is a big advantage. Fresh clean air and <50%RH is preferred. If you wish to avoid the a/c supply duct, a small supply duct system should be used. A 8" return from the open part of the home and 4" supply key separate areas like the Ms BD. basements etc. 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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    Apr 2002
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    11,808
    Quote Originally Posted by DrivenByDemons View Post
    Yea, the constant running of the AHU fan would keep drying out the AC coil between calls for cooling. This would make the dehumidifier worthless as it would be drying the air then sending that nice dry air right over the soaking wet coil. That seems to be the biggest downfall. I've decided to re-route an existing return and dedicate it to the dehumidifier. Now I'm trying to decide if it would be worth it to run dedicated supply vents to each bedroom as Honeywell recomends... I know this is the best setup but it's a lot of extra work.
    Not worthless but less effective.

    I got a real problem scenario here and the bypass system coupled to an oversized AC basically has the the AHU fan running steady. The dewpoint in the space fluctuates wildly between 55 and 60.

    If you plot the dewpoint out it looks like a thick line "5 degrees wide"

  13. #13
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    Aug 2003
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    Quote Originally Posted by Carnak View Post
    Not worthless but less effective.

    I got a real problem scenario here and the bypass system coupled to an oversized AC basically has the the AHU fan running steady. The dewpoint in the space fluctuates wildly between 55 and 60.

    If you plot the dewpoint out it looks like a thick line "5 degrees wide"
    It sounds like you are getting an oscillation between the AC, dehu and their controls. Is it possible that either the thermostat or dehumidistat (or both) is located too close to a supply?

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