Can’t openhab be used as a smart thermostat ?
Why bind to external devices when openhab + good sensors + mobile app/webapp can be as good as a expensive smart thermostat.
Is there already an initiative to build something like that, with standard sensors that have bindings to openhab and control units ?
You are missing the actor in your listing ( openhab + good sensors + mobile app/webapp). To know the actual and desired temperature is half of it, you need to change the heating somehow.
An thermostat does that, if it is smart, you can control it from OH.
You´d have to care about control technology and PID filter elements as well. With some simple “if temp<settemp then switch=on” mechanism you might suffer over- or undershoot and oscilation. A PID-element is quite easy to implements but a bit harder to tune.
Just my 2c, regards, Michael
The temperature collection and system control are the easy bits.
But making the smarts are the hard bit. It took me about 3 weeks to get a replacement for my turn on at 5 off at 8 “program” that a phytical timer could do, but i was able to add if temp > x don’t turn on - rather than using a manual switch.
Let alone a nest like learning heating system. It’s a pity noone has opensourced a system like that, that could be run and interfaced with home automation software.
Yup, Openhab can certainly be a part of it.
Some have already done this (me included) and you can find their posts on this forum (search for “PID”; I have not yet written about how my system work.)
I doubt you will see a standard binding to manage that. The various heating and cooling systems are too different from region to region.
I have used Oregon Scientific thermometers (20€/each, 2x AA battery) with a RFXCOM as bridge (100€) and an OH binding for the RFXCOM.
I do the logic in NodeRED (there is a PID node 
).
For the actuators I use RPis controlling relays, but that isn’t optimal. If I had to do it again, I’d use arduino’s with an ethernet shield and MQTT.
There has been an effort from @george.erhan on a PID Binding: https://github.com/openhab/openhab2-addons/pull/1903
I have seen his binding in practice and it is quite impressive.