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  1. #1
    Join Date
    Apr 2002
    Location
    Houston
    Posts
    25

    Deluxe condenser+furnace, or whole-house dehumidifier?

    Hello all.

    I live in Houston, and am looking to replace both upstairs and downstairs systems in my 16yr old house. I tend to be very humidity-sensitive... 50% RH at 73F the other night, and I was still sweating in bed, under very light covers and a ceiling fan on high.

    I'm in the process of getting proposals from several local contractors who I think are pretty knowledgeable (and we ARE doing a Manual J), but I'm getting different opinions about humidity control.

    One school of thought seems to be to go for top-of-the-line AC equipment (say Trane XL20i & XC80, or Carrier Infinity) to get maximum humidity control from the HVAC. The other approach is to go with single-stage cooling + VS furnace (Trane XL15i & XV80) plus a whole-house dehumidifier such as the Ultra-Aire XT150H or equivalent Honeywell DH150.

    The cost to me is virtually the same either way (dehumidifier option maybe 5% more expensive). Going in, I was really expecting to go the whole-house dehumidifier route, but one of the contractors I'm talking with is pretty insistent that I'll be happy with the XL20i, and argues that it'll be cheaper to operate, more reliable in the long run (less stuff to break down), and quieter.

    So what do you guys think?? If the cost is ~equal, go for maximum humidity control via the HVAC system, or go with the VS furnace + whole house dehu?

    Should I be worried about noise with an attic-installed XT150H?

    Much thanks!

  2. #2
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,376
    Third option:

    Reduce the amount of humidity that enters your house. If it can't get in, you don't have to spend money to get rid of it.

    Fresh air ventilation can be accomplished with a whole house humidifier; it can also be done with central a/c and a fresh air intake on the return air side. Caveat for both for Houston climate is that any fresh air brought into the house via either system must be dehumidified when outdoor humidity levels are high.

    What often gets overlooked in looking for dehumidification solutions in humid climates like Houston is both the house and how the a/c system is installed. If ducts are in the attic, and if they are not sealed up well, you will always struggle to keep the house air dry enough for comfort. Same goes for where the ducts punch through the ceiling from the attic into the house. Often there's a decent gap between the duct sheet metal and the drywall of the ceiling. That is an air and humidity leakage point. Hardly anybody ever looks at this area and thinks it is a problem, but it is. A big problem when trying to keep a house in a humid climate dry.

    I can't advise you on system choice, but I can say that no matter what system you ultimately go with, demand that all the duct fittings, joints, take-offs, plenums, ceiling penetrations, etc be sealed with duct mastic. Not duct tape or foil tape; you want mastic on there. This alone will improve your new system's performance...could even improve your existing system's performance provided it still runs.
    • 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.

  3. #3
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,614
    Just remember that even a fancy A/C can only dehumidify if it is cooling so when it satisfies, no more dehumidification. So in milder weather, especially 60s & 70s out, little dehumidification will be taking place, even on 2 stage equipment. Remember most are 75% give or take capacity on low so not that much less. Also some have some poor latent ratios compared to their single stage counterparts.

    There is the Lennox setup with reheat coil that will dehumidify without freezing you out. Or the whole house dehums.

  4. #4
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,646
    Quote Originally Posted by muggy View Post
    Hello all.

    I live in Houston, and am looking to replace both upstairs and downstairs systems in my 16yr old house. I tend to be very humidity-sensitive... 50% RH at 73F the other night, and I was still sweating in bed, under very light covers and a ceiling fan on high.

    I'm in the process of getting proposals from several local contractors who I think are pretty knowledgeable (and we ARE doing a Manual J), but I'm getting different opinions about humidity control.

    One school of thought seems to be to go for top-of-the-line AC equipment (say Trane XL20i & XC80, or Carrier Infinity) to get maximum humidity control from the HVAC. The other approach is to go with single-stage cooling + VS furnace (Trane XL15i & XV80) plus a whole-house dehumidifier such as the Ultra-Aire XT150H or equivalent Honeywell DH150.

    The cost to me is virtually the same either way (dehumidifier option maybe 5% more expensive). Going in, I was really expecting to go the whole-house dehumidifier route, but one of the contractors I'm talking with is pretty insistent that I'll be happy with the XL20i, and argues that it'll be cheaper to operate, more reliable in the long run (less stuff to break down), and quieter.

    So what do you guys think?? If the cost is ~equal, go for maximum humidity control via the HVAC system, or go with the VS furnace + whole house dehu?

    Should I be worried about noise with an attic-installed XT150H?

    Much thanks!
    Thank you for the chance for us posters to express our strong held opinions about the strengths of your choices.
    I feel that maintaining low humidity (<50%RH) during high cooling loads can be easily done by properly setup high efficiency a/c. Certianly, the fresh air infiltration/ventilation must minimized to avoid overwheleming the latent capacity of the a/c. On the other hand, you need enough fresh air in a home to change the air every 5-6 hours when the home is occupied. Purging indoor pollutants and renewing oxygen are critical issues for health and comfort. Manual J does a good job of matching the a/c's capacity to the sensible/latent loads during high cooling load conditions. Typical estimates at design load is 80% sensible/20%latent. Typical design cooling load may 60,000 btus total with 48,000 btus of sensible/12,000 btus latent. That is 12 lbs. of moisture removal per hour. That will keep your home <50%RH by removing the 8lbs. of moisture from +100 cfm of fresh infiltration/ventilating and 4-6 occupants.

    The fun starts as the sensible sensible load declines while latent loads usually remain constant. This is over-night or on a rainy day scenerio. Over-nite the temperature drops to 80^F while the outdoor dew point drop very little. The occupants are in the home generating moisture load and hopefully you are getting 100 cfm of fresh air infiltration/ventilation. The a/c is cycling on/off. During the off cycle, the a/c removes no moisture and some of the moisture that condensed on the cooling coil during the "on" re-evaporates back into the home. At this partial load condition, the sensible load is a fraction of "full" load while the latent maybe as much as during the peak of the day. The will become more humid as the sensible load declines. Being able to lengthen the cooling cycle helps some but the sensible cooling loads decline to the point where even over-cooling has long off cycles. Off time allows the large amount of moisture on the coil to evaporate back into the home.
    During no cooling loads with outdoor dew points +58^F, moisture levels in the are uncontrollable. This gives the whole house dehumidifier the definite advantage of being able to remove the 2-6 lbs. per hour of from fresh air and occupants needed to provide <50%RH during regardless of the cooling load or the weather conditions. Having a whole house dehumidifier also provides <50%RH without operating the a/c when the home is not occupied for extends periods. This save a tremendous amount of electricity when compared to maintaining <50% by over-cooling. The UA 150H removes 7.8 lbs. of moisture per KW compared to a much lesser lbs. per KW with a/c.
    The other issue is intial installation and long term maintaince. Set-up and maintaince on the complex multispeed a/c requires more expertise that the simplier high SEER a/c and wh dehu.

    What ever you do, get your expectations in writing from whoever. I suggest that you also incorporate some fresh air ventilation in either system you select. During calm weather, even air leaky homes do not get enough fresh air to purge indoor pollutants and renew oxygen. Thanks for the question.
    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"

  5. #5
    Join Date
    Apr 2002
    Location
    Houston
    Posts
    25
    Thanks everyone.

    Shophound - For what it's worth, I do plan to try and reduce the amount of damp outside air we're bringing in... duct sealing in the attic, supply ceiling penetrations, and can lights.

    Bald and TB - I appreciate the "caution" about shoulder seasons (AC can't dehumidify when there's no call for cooling) and about humidity creeping up over night for the same reason. This was really the reason for me originally leaning towards a whole-house dehumidifier. I've been surprised that all of my contractors so far are telling me basically: "buy the 20i or the Infinity, and you won't need the dehumidifier".

    My inclination was, if the 16i plus DH150 costs the same as the 20i/Infinity, then I think I prefer the former... knowing that I can control humidity 24/7/365, and get fresh air to. The contractors disagree, and two of them have significant experience with Aprilaire and UA dehus (one of the reasons I sought them out).

    So are they right, and I'm too hung up on getting the extra box?

    And (again), should I be concerned about noise with a DH150 installed in the attic?

    Oh, and TB, I appreciate all of the strongly held (expert) opinions I can get!

  6. #6
    Join Date
    Jun 2004
    Location
    4H: Hot, Humid Houston H.O.
    Posts
    3,304
    I live in Houston too, and am a homeowner who pursues low humidity. If you think it might be informative to chat a bit, my email address is in my profile.

    Best of luck -- Pstu

  7. #7
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,646
    Yes, isolate the UA 150 from the ceiling of the home. Locate the unit over none sensitive areas of the home, not the master bedroom. Hanging from the trusses is good with well insulated flex duct.
    If you want <50%RH during all seasons, get the UA xt150H. Everyday, we add dehus to the multispeed a/c installs make the homes comfortable. Areas like SO FL install dehus with all a/c installs in premium housing.
    If the contractor feels strongly, have him assure you in writing that you will have <50%RH during low/no cooling load conditions during occupancy and minimal fresh air ventilation. Get the 10 yr. service warranty. Keep us posted on how you go and the results.
    The amount of fresh air entering your home can be determined by checking the CO2 levels in the home. These meters cost less than $200.
    Most a/c contractors have a problem admitting that an a/c with all the bells and whistles is unable to maintain <50%RH during all weather conditions. nothing like a <50%RH without over-cooling for real comfort. Also a air change every 5-6 hours keeps the health and fresh.
    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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