So you are looking for a smart thermostat based on an open source platform? The HestiaPi Touch is worth a look. It comes integrated with a Raspberry Pi Zero W, 3.5″ touch display, and sensors to measure temperature. humidity, and atomic pressure.
The device ships with openHAB 2 pre-installed. Mobile apps for Android, iOS, and Windows will be available. The HestiaPi Touch supports most HVAC systems with five wires.