12 would need to operate at its maximum capacity in order to maintain an actual indoor temperature at or near a given desired target indoor temperature thermostat 10 can also determine whether unit 12 needs servicing by comparing the calculated tolerable outdoor temperature limit to a predetermined specified outdoor temperature limit in some embodiments thermostat 10 can also determine the minimum or maximum achievable indoor temperature for a given outdoor temperature to do all this in the example of an on off control scheme thermostat 10 follows the control algorithm outlined in fig2 in control block 28 thermostat 10 reads the outdoor temperature through feedback 20 provided by sensor 22 in block 30 thermostat 10 reads the actual indoor temperature as sensed by temperature sensor 18 in block 32 a user provides thermostat 10 with a desired target indoor temperature block 34 comprises a conventional on off control scheme that cycles unit 12 on and off to maintain the indoor temperature at or near the desired target indoor temperature generally the greater the difference between the outdoor temperature and the target indoor temperature the closer unit 12 must operate at its maximum capacity of 100 numeral 35 in fig3 for an on off control scheme the capacity of unit 12 is in terms of duty cycle i e the percentage of time that unit 12 is running on time on.