I automated my air conditioning today!
The entire project obviously took way more than a day (it’s a learning process) but I deployed it today.
@JGKK how do you like that screen cutout. I filed it very, very carefully while thinking of your fan controller. 
It’s a Wemos D1 ESP8266 with an IR emitter (transistor powered), 200x200 pixel SPI display, and a 433 MHz remote receiver, running purpose-built software and talks to openHAB as an MQTT/Homie 3.0.1 device.
The display shows the setpoint. Yes, the setpoint currently is really 28 degrees celsius (82 F). Living in a tropical climate it makes no sense for the indoor temperature to differ too much from outdoor. I’ve found myself chasing the outdoor temperature 4-5 times a day, which meant walking to the bedroom (central air, controls the whole house) and using an infrared remote.
No more! It’s now automatic, based on an offset from the outdoor temperature, with formulas/rules that I am sure I will be tweaking for a while.
Next step is to take the astro binding local sun elevation/azimuth into account so that I can make it ignore the exterior temperature sensor when it gets hit by the sun for about an hour around 5:30 PM.
Those 200x200 SPI displays are painfully slow, it takes almost a whole second to update the entire screen, and there doesn’t seem to be any fill commands and no way to optimize things, you really have to write pixel values… ugh. But, for this purpose it works. At least I don’t have to rewrite the screen to control the brightness, I connected the backlight control pin directly to a PWM pin. 
The display shows the current setpoint, nothing more.
I have a separate temperature formula for sleep mode, and that means I can now finally tie things like exterior lights to sleep mode as well. Before, when “sleep mode” was just a toggle switch in openHAB, of course I never remembered… but there’s no way I’m forgetting to change the air conditioning temperature. Seeing my new A/C controller on the wall (instead of the original remote) will certainly be enough of a reminder to press the sleep mode button! which is an aeotec wallmote next to the bed.
The 433 MHz remote receiver does not have a purpose yet but I’m sure I will find a reason to put up a 433 MHz wall remote for something else now that I have a receiver in the bedroom.