Skip to content Many HVAC systems in North America operate off 24V systems, which can be readily upgraded with off-the-shelf  smart thermostats quite easily. However, there are many people living in buildings with 120-volt fan coil units who aren’t so lucky. [mackswan] is one such individual, who set about building a smart thermostat to work in these situations. The build is based around an ESP32 running ESPHome firmware. It rocks a 2.42″ OLED screen with automatic brightness adjustment for showing temperature and control parameters. There’s a rotary encoder on the front with an integrated button for control, with [mackswan] building the physical device to look as clean and neat as possible. The device uses a relay to switch the fan coil system on and off to heat or cool as needed, with an SHTC3 temperature and humidity sensor used to monitor current conditions in the home.…