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Not worth the extra money

One difference between the Premium thermostat enhanced thermostat is the air quality monitor. The thermostat is fine. The air quality monitor is mostly useless. TL;DR: save your money, don't buy the "premium thermostat" for the air quality sensor. It has two problems: 1) It measures only volatile ...organic compounds. If you're astute, you'll see that ecobee infers CO2 concentration based on an average correlation from human breath or something. This is a fancy way of saying they take one number, and then it multiplies and divides and adds and subtracts to get another number. (It may be a nonlinear transformation, but it's still just a deterministic transformation.) Imagine if Ecobee marketed separate measurements of Fahrenheit and Celcius using "the average correlation between the two measures on the surface of the earth." Even if literally true, it's a fancy way of saying that they derive two numbers from one number. (My day job is a statistician. Most of statistics is to take many numbers and reduce it to fewer numbers. This is doing the opposite.) 2) The sensor is a relative sensor. Ecobee admits to this. This in itself is not bad--even your alcohol thermometer that you measure fevers is a relative sensor--the key difference is that your alcohol thermometer is calibrated. The Ecobee air quality sensor is both not calibrated and worst of all, the calibration changes, so it's impossible to infer when your house is relatively polluted. What this means in practice is that when I open the window, the sensor shows "Clean" and when I close it, it shows "Poor." During humid east coast summers when opening a window would let in a lot of humidity, "Poor" changes into "Fair" into "Clean" over the course of several weeks. If you open a window for a few hours and then close it, it immediately goes back to "Poor." Ecobee technical support suggested that I expose the sensor to indoor air pollutants. I've done this with all the detergents I could possibly find in my house. Nothing changes. "Poor" air quality stays "Poor."