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by • July 31, 2012 • HVAC & Dehumidification, UncategorizedComments (16)45770

Humidity Controlling Thermostats

  I’m going to be straightforward with you, I don’t like Thermostats that control humidity.  (Unless they are connected to a stand-alone dehumidifier)  Yes, these Thermostats will add a some level of humidity control to your house, but it will be at the expense of odd temperature management.   

  If you set your standard thermostat to 76 degrees your AC will run so that it maintains 76 degrees.  However, the thermostat doesn’t know the % Relative Humidity in your house.  With very tight homes you might have a house at 76 degrees inside, but if it’s 82 degrees outside & 80% Relative Humidity outside this well built home won’t have to work very hard to maintain 76 degrees (meaning the AC won’t come on alot) unless it’s in the upper 80’s or 90’s outside.  Remember that an AC unit controls temperature by running a long time so the coil gets cold and the air running past the cold coil will condense and thus dehumidify the house.  The AC unit needs to run for at least 10 minutes before it’ll control humidity by air condensing on the cold coil.  On top of this we also generate humidity in our houses by living there; cooking, showering, breathing, all these add moisture to the air inside.  There are lots of times of the year the AC simply won’t run long enough to bring down the humidity inside.

  Here’s how a standard HVAC system with a Humidity Controlling Thermostat works; the humidity controlling thermostat will tell the AC that even though the inside air is 76 degrees the humidity is too high (say 65% RH), this control module will kick on the AC to control that humidity but the furnace will need to run for 10-15 minutes in order to do any moisture (humidity) removal which ends up taking the house to a lower temperature.  This is what I don’t like about the humidity control on a standard HVAC system.  You’ll walk by the thermostat and it’ll show the set point as 76 degrees but the AC is still going down to 72 degrees in order to bring down the humidity.  It’s hard for people to understand it who are living in the house.  It’ll make the house too cold just to bring the humidity down.

  This is why I like the stand alone dehumidifier.  It has it’s own controller that says keep the house at 50% RH.  It’s independent of the the AC unit.  The Dehumidifier doesn’t add cool air to bring down the humidity like the AC does.  It’s more of a low/slow/steady approach to controlling humidity compared to the AC unit which is like a V8 engine that only runs full ON or full OFF.  The other benefit is that the AC with it’s large engine uses alot of electricity where as the Dehumidifier runs at a very low 7-8 Amps.  The result is a comfortable house year round that uses less power. 

  If you are considering a Humidity Controlling Thermostat, my advice is don’t.  Instead add some ability to control humidity apart from you HVAC system.  HVAC+D as I recently heard it.  Here’s a link to what I’ve done in my personal house until I can get an more advanced UltraAire unit installed to control humidity. 
-Matt Risinger

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  • You can also try some other humidity controller from here.

  • Tim

    ” It’ll make the house too cold just to bring the humidity down.”

    You are not setting it up correctly. The way it is supposed to function is to allow the house to warm up as the humidity drops. My Honeywell can drop the temp by as much as 3 degrees to bring down humidity. In the summer here in Alabama when the ac is running all day, the house is most comfortable @ 72 and the humidity will be somewhere between 55-59%.

    So I set the thermostat to 75 and the humidity control to 45%. What this does is keep the house at 72 if the humidity is above 53%. But in the spring and fall when we have warm but dryer weather, it will allow the house to warm up with the lower humidity. The house will be 73 when the inside humidity goes down to 53%, 74 @ 48%, 75 @ 43%. This keeps the house more comfortable as it maintains the same humidex – http://www.ccohs.ca/oshanswers/phys_agents/humidex.html, without me having to mess with the settings.

    It’s not intuitive to set up right, and the instruction book doesn’t explain anything, but it actually works well.

    • Sabrina Persaud

      Which Honeywell model are you using? I need to replace my thermostat and I’m having a hard time finding one that allows you to control The humidifier. Thanks.

      • Tim

        I’m using the TH8321U1006. I don’t have a humidifier, it just helps control your ac to help regulate the temp based on the humidity level in your house. The place it falls short on doing this is when the outside temp is around 72 and it’s raining. Then you would have to set it up to make it cold in your house to bring down humidity, which would be uncomfortable, this is where a separate dehumidifier would be handy, or the ability to run the ac and heat at the same time.

        • Sabrina Persaud

          Thanks. I’m really liking for a thermostat that can control my humidifier as well since it’s attached to my heating system.

    • HonkyTonkHero

      Tim…brilliant explanation! Yes this is a bit hard to conceptualize, but the more I read your writeup and thought about it the more it made sense. I just configured my thermostat the same way. Looking forward to the results. Thank you so much for taking time to write this up. Huge help.

      For those following along….another example. Our comfort zone is around 77 with 55-59% rH. So my set point is 80 and rH set to 45%.. On heavy humidity days we will be nice and cool at 77% and on low humidity days we will be just fine around 80.

      • Tim

        No problem, glad it helped and I hope it works well for you!

    • ROSALIND

      Tim, my house is about 3 years old. I came home today and the humidity was 77 and my temperature was 70. It was muggy in my house. I changed my fan to circulate instead of “ON or “AUTOMATIC”. My humidity is actually dropping. Now my temperature in the house toggles between 70 to 72 and my humidity is down to 66%. It feels cooler now. Your posting was very helpful, it does make sense.

      • Susan Staley

        My Thermostat doesn’t have “Circulate” as part of the equation. It has “On” and “Automatic” only. help……

  • KevinYoung11

    Humidity Adjusting essential a kit which container maintain humidity least possible 75% & temperature 25-30 degree c in the couch area, for well-being purpose. http://www.portableac.com/industrial-rentals/dehumidifiers/

  • Dennis Baver

    Matt, A dehumidifier is an energy pig because it is essentially an air conditioner with reheat. If yo are going to go that route buy an air conditioner with a gas reheat coil. It will make your AC run like a dehumidifier when cooling is not necessary.

  • Susan Staley

    This really isn’t helping me. I have a Thermostat for the Air Conditioner and apparently, a separate Humidity “thermostat”. I live in Florida and it’s “Hell Hot” here. I can’t get my AC to cool the house down – it feels like a sauna with breezes of cool air now and again. I like the temp to be at 76. How do I set this stupid, stupid RH thermostat so my home is cool and DRY?!?! HELP!!!!

    • This is a bigger problem than your thermostat. You need an expert local HVAC person to evaluate OR a Home Performance Contractor. Look for a person who is trained by the BPI Building Performance Institute. They have a link on their website for local pros. Google them. Best to you, Matt

  • Paul B

    My Cor thermostat is set to 20% humidity but the reading is showing 68%. With this set point, the valve should not be turning on with heat, but it turns on every time the furnace runs. We can’t see through our windows! Something must be wrong here..If the actual humidity is higher than the set point, shouldn’t the valve stay closed until it drops to the set point?

  • Pratik patil

    I want to maintain humidity 75 %and temp low er than 30 c. What I should do it????

  • bgdude

    Dude, you’re totally wrong. De-humidification control on the thermostat sends a signal to HVAC control board to keep running the fan on so that air moves through the whole-house dehumidifier. Once humidity level is reached, the dehumidification cycle is interrupted until reached back again. This is independent of both heating and cooling cycles.