Abstract: An external air assisted building forced air heating system equipped with or without air conditioning that uses ducted external air to cool or heat a building instead of using other energy resources (like electricity and gas) when the temperature of the external air is less than the desired cooling temperature or greater than the desired heating temperature. If the heating system is not equipped with air conditioning, the building is only cooled when the outside air temperature is less than the desired inside temperature. A controller (normally referred to as a Programmable Thermostat) monitors the indoor and outdoor air temperature in order to control the building's heating and air conditioning system and the external air duct system.