Programmable electronic thermostat with means for enabling economical recovery

Claim 1

In a programmable elctronic thermostat for controlling operation of multistage heating apparatus, the thermostat including a microcomputer which provides means for storing a time-temperature program comprising a plurality of chronologically arranged set times and associated set point temperatures; means for looking ahead a predetermined number of time intervals in said time-temperature program for a next one of said set times; means, effective when said means for looking ahead determines that said next one of said set times is programmed to occur within said predetermined number of time intervals, for comparing a next one of said set point temperatures with a present one of said set point temperatures; means, responsive to said comparison indicating that said next one of said set point temperatures is greater than said present one of said set point temperatures, for calculating the value of temperature difference between said next one of said set point temperatures and a buffered temperature, said buffered temperature being a composite temperature value including a component based on sensed temperature and a component based on a characteristic related to demand for operation of the multistage heating apparatus; means for comparing said calculated value of temperature difference and the number of time intervals, of said predetermined number of time intervals, remaining until said occurrence of said next one of said set times; means for initiating recovery to said next one of said set point temperatures at a time when said calculated value of temperature difference is greater than or equal to said remaining number of time intervals, said means for initiating recovery including means for changing the set point temperature from said present one of said set point temperatures to said next one of said set point temperatures, locking out all stages except a first stage of said multistage heating apparatus, and immediately unlocking one or more of said locked stages as determined by said characteristic related to demand; means for maintaining recovery with said first stage and only the stage or stages of said one or more of said locked stages which are unlocked at said time when recovery is initiated; and means for calculating the value of temperature difference between said next one of said set point temperatures and said buffered temperature at predetermined times prior to said occurrence of said next one of said set times and for selectively unlocking said locked stages at said predetermined times if said calculated temperature difference at said predetermined times is sufficiently great.