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  1. #1
    Join Date
    Nov 2002
    Location
    Redondo Beach, CA
    Posts
    68

    Hmm How is the Humidstat suppose to be used in the FL area?

    What does the humidstat do?
    We installed a new Trane with a digital Honeywell and separate humidstat.
    The installer told us to "turn off the humidstat" and use the thermastat to control the temp.

    What is it used for and how does it work?

    Thanks in advance for any advise.

    Rick

  2. #2
    Join Date
    May 2004
    Location
    Rapid City, SD
    Posts
    7,403
    Normally a humidistat is used to control a humidifier, but, since in your area you don't need humidity I'd say it's used to control the fan speed.

    If you slow the fan speed down when the A/C runs, it'll help pull moisture out of the air and make it much more comfortable. Once the humidity is gone, it'll ramp up to full speed.

    Of course, if you don't have a variable speed blower, the odds of this type of setup is pretty small.
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  3. #3
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    66,750
    If you have a VS blower, it slows the blower as amick said.
    If your intaller hooked it up.
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  4. #4
    Join Date
    Nov 2004
    Location
    SW FL
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    5,987
    Quote Originally Posted by richg44006 View Post
    What does the humidstat do?
    We installed a new Trane with a digital Honeywell and separate humidistat.
    The installer told us to "turn off the humidstat" and use the thermostat to control the temp.

    What is it used for and how does it work? Thanks in advance for any advise.
    Rick
    DEhumidistat will control the Relative Humidity.
    A dehumidistat is good for a extended leave/ vacation period.
    Dehumidistat should be set to ~ 56% with the thermostat set to about 84'F. A/C will likely only run < 3 hours per day in a relatively well sealed FL residence to maintain a proper environment where mold is not able to grow.

    There should not be a need to use a humidistat
    when a residence is occupied because a proper operating
    A/C system should maintain Relative Humidity at < 52%.
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  5. #5
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,061
    Ideally, the dehumidistat continues the operation of a/c below the temperature cutout of the t-stat. The hope is that while overcooling the home, enough moisture is removed to achieve the desired %RH. This was sop for the a/c industry for years. If a 2,500 sqft. home is 75^F, 60%RH and 50% RH is desired, 3 lbs. of water (3,150 btus latent) need to be removed from the air in the home. Most a/cs remove 3X more sensible heat than moisture. Cooling 9,450 btus sensible to remove the 3,150 btus of latent overcools the home dramatically before removing the moisture. If its warm enough outside to reheat the home, its ok. If not, coils freeze-up, customers are cold and clamy. A/c mfgs more recently limited over-cooling the home less than 4^F. Then wait for the home to warmup to the set point and try again. During cool wet weather the home may not warm-up. This works better than nothing. A whole house dehumidifier is a better solution. The a/c cools the home when needed. During low/no cooling loads the dehumidifier maintains 50%RH without overcooling. Supplemental dehumidification also allows not operating a/c when the home is unoccupied for extended periods of time. Another issue is retained moisture from the cooling coil keeping the ducts wet and growing mold. Ducting the dry air from the dehu throughout the home through the a/c dries out the ducts reducing the potiential for mold growth. Honeywell, Ultra-Aire, Aprilaire, and Santa Fe are some of the brands of whole house dehumidifiers. Dehu TB

  6. #6
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    A humidistat to slow the blower down, helps keep the humidity down during low load conditions.
    And can often bring the humidity down below 50&#37;.
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  7. #7
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
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    6,061
    Quote Originally Posted by beenthere View Post
    A humidistat to slow the blower down, helps keep the humidity down during low load conditions.
    And can often bring the humidity down below 50%.
    Slowing the fan speed only changes the sensible/latent ratio on most a/cs 10% unless the speed was not properly set for comfort operation. Typically 80/20 ratios for high eff. 75/25 ratios to max dehumidification. All stops pulled 65/35 is possible. The ducts/grills sweat(cold surface), distribution is poor(low air flow), and coils freeze easily as you over-cool. This is from the majors specs. House still get cold before the dry. It take a least a ton of constant cooling load to remove a couple lbs. of moisture an hour. We have been blessed with one of the driest springs I recall. My moisture removal is about 12 lbs/day to maintain <50%RH. Usually 2-3X more this time of year. No a/c load yet. More humidity in SE TX. Thank God. Dehu TB

  8. #8
    Join Date
    Dec 2006
    Location
    s.w FL
    Posts
    19
    He is referring to the dehumidistat. Normally we recommend when away from home for an extended period of time the homeowner set the thermostat to 82 and the dehumidistat to 60%-65%

  9. #9
    Join Date
    Apr 2005
    Location
    Tampa, Florida
    Posts
    1,634
    Normally, I see these installed in SW FL homes which are designed to be winter homes for retirees. They're almost always installed on single stage systems inbetween the Y terminal of the thermostat and air handler/condensing unit.

    The temperature/humidity specs Downeasthvac suggests are what I've normally seen recommended to people... BUT, in practice, I find that dehumidistats installed this way are minimally useful. Latent load is highest at night, sensible is highest during the day. For a system such as this, you need high latent + high sensible for the system to run. Have a hurricane/tropical storm absolutely saturating your home? Well, the system's not going to respond. In the meantime, the moisture level in your home is going to be well above "safe" levels. Not sure about typical FL tract housing, but at my homes, the inside temperature rarely gets up to 85F, 82F on a rainy day isn't common either. I find this sort of setup CAN work, but requires much lower setpoints to keep the inside of the home within reasonable humidity limits. Better is a variable-speed system which can properly handle humidity removal. Best is a stand-alone dehumidifier, which is a wise investment for someone who's away from their home for months at a time. Better performance, lower electric bills.

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