I made a first write-up about some findings on the NTC and the W1209 sensor input. The datasheet referenced by @jeff1937 doesn't match, and it would be nice to learn more about the particular NTC used for Chinese thermostats!

Edit: I updated the Wiki page above: Chinese NTC sensors sometimes have the tag MFA52AT. I found a datasheet that that describes a typical W1209 sensor pretty well (error within 3%).

Grades of 1%, 2%, and 5% of such sensors are common which means that you one should be careful when commissioning a thermostat for applications where keeping a precise temperature is necessary (e.g. chicken incubator, sous-vide cooking).