Thermostat for control of an add-on heat pump system

Claim 1

A thermostat to control the heating of a space with said thermostat adapted to be connected to control a furnace and a heat pump wherein the output heat from said furnace passes through a portion of said heat pump, including: two-stage thermostat means with a first stage including single pole switch means having a temperature control range smaller than a temperature control range of a second stage of said thermostat means; both of said stages being responsive to a temperature within said space to be heated; said switch means having contacts which are open circuited upon a temperature rise and closed circuited upon a temperature drop within said first stage temperature control range; said second stage having single pole, double throw switch means with said second stage switch means having contacts operated between open and closed positions on temperature changes within said second stage temperature control range; internal thermostat circuit means connecting said first stage switch means to said second stage switch means to allow said thermostat to complete a first electric circuit through said two switch means upon said first stage sensing a temperature drop to a limit of said first stage temperature range; said second stage switch means opening said first electric circuit and completing a second electric circuit upon a sufficient temperature drop; and terminal means connected to said two thermostat circuits with said terminal means adapted to connect said thermostat to said heat pump and said furnace; said heat pump and said furnace being controlled so that they can not operate at the same time.