I found a few discontinued discussions about this so I'm starting a new one. Many talked about the inaccuracy of the humidity reading on the Nest Learning Thermostats. I noticed the same thing, about 10% more on the Nest than my other measuring units. I did a few changes in testing and thermostat install and it seems to have improved. I post this because I may have figured a couple of things out to improve your testing and install that may make the Nest and your other measuring tools become closer together in RH%. For reference I have the Learning-Gen 3 and I have 4 ThermoPro TP50 indoor hygrometer/thermometer units around the house. Before I explain, here's the setup: My Nest is in an 8 foot hallway that has an opening to the great room, a new insulated fire door to the garage, and a door to the MBR which is usually open and was for this period. The Nest is also right above the only return vent the house has (1962 old-school Florida home). And, the wall the Nest is mounted has the garage on the other side of it, right where the HVAC air handler is - which is why the air return vent is just below the Nest. That 2x4 wall was also recently insulated with batt insulation. And note that it is August in SW Florida so hot and humid. The first thing I did was move one of my ThermoPro's to a small wall mounted cabinet across from the Nest. It isn't really accurate to leave it in another room for comparison. It moved the RH%, closer to about 5-6%. Then I thought about where and how the Nest was mounted. It's on a wall that backs up to the garage, and I recalled that there's about a 1" hole in the drywall for the wires from the air handler. So I removed the Nest and mounting plate and did some work. There were multiple screw holes in the drywall from previous thermostats in addition to the 1" hole for the wires. I assumed there was some, although small amount, of heat and humidity coming through those holes. So I caulked all of those screw holes shut. Then I took some fiberglass insulation from a scrap of batt insulation I had and shoved them in the 1" wire hole. I made sure the last one would stay put, partially inserted through the hole, when I slid the wire/Nest base back to the wall before screwing it all back in. And I taped the ThermoPro right above the Nest. Now the Nest reads 3-4% higher than the ThermoPro hygrometer. Note that this last measurement is at night, and the initial readings were in the afternoon. I'll watch it to see how it does throughout the day as well as between vs during the HVAC run periods. But again, my purpose here is to provide ideas on making sure your test RH% units are setup as close to possible as your Nest, and attempting to insulate the Nest as much as possible from outside (including garage) impacts.
