Question about thermostat circuit
NTC on lower side of bridge to Vin-
Cool causes R to increase and Voltage rise across thermistor causes output low and NON Off with relay off.
Excess heat drops Rth value and also Vin- drops voltage 


when Vin- &lt; Vin+= V+/2 the ref. point, it causes Op Amp out high and NPN ON and Relay ON.  




  
  THIS MUST BE FOR A/C mode or COOL setting.
  



HEAT setting reverses the output logic.
Notice the feedback resistor ratio to Vin+ causes hysteresis of 11k/3M3 which  translates into the allowed backlash and temperature error. 
It also affects the Relay cycle rate so that it is not too fast or too slow.  Using a more precise Vcc accuracy and a trimmer, you can reduce this ratio further from 3' C to 0.5'C in terms of the voltage sensitivity per degree.  
Often this is done with a precision 2.5Vref IC with 0.1% to 0.5% stability in modern inexpensive digital Thermostats.  
However the accuracy might still be 3'C depending on other tolerances so one adjusts to how it feels, but a 3 'C hysteresis would be excessive unless sensed closer to the A/C source.


Cool.