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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.…