Let me start by saying this isn’t your average DIY thermostat. The Honeywell THX9421R5021WW is a beast—meant for pros, installed by pros, and honestly, it shows. After struggling with a 24-year-old furnace and three clueless HVAC companies, this was the only thermostat that could handle our humidifier (a rare feature in 'smart' thermostats).

The touchscreen is crisp, and programming feels intuitive—once you get past the initial 'wait, why isn’t this connecting?' panic. Pro tip: You’ll need an extra EIM module (sold separately) to unlock features like humidity control. Oh, and forget Amazon’s 30-day return window; Honeywell’s warranty activation requires an HVAC pro to register it. Learned that the hard way.

I added wireless outdoor sensors, and seeing real-time temp/humidity on the display is satisfying. But beware: Wi-Fi requires yet another add-on (of course). It works flawlessly... when it works. Some units arrive DOA or fail within months—hence the mixed reviews. If you’re not a contractor, Honeywell’s customer service might ghost you.

Bottom line? If your system needs pro-level control (and you have a trusted HVAC tech on speed dial), this is gold. For everyone else? Maybe stick with Nest.