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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