This is my second WiFi thermostat. A year or so ago, I made one with a Particle Core, collected data and experimented with it. This one uses the ESP8266 and a Nextion display. Actually it’s 2 units, if you want to build the full project. One is wired and the other is wireless and can be placed anywhere. Both have temperature and humidity sensors, so either can be used to control the heating, cooling and humidity, or an average between the two, or a simple remote sender can be added to the mix. Lots of features, and settings, and it’s easy to modify most aspects of it, including the graphics using the Nextion editor.
Here’s the project: https://github.com/CuriousTech/ESP8266-HVAC