Abstract: A method of controlling heating and cooling sources utilizes a computer or an electronic thermostat to control a plurality of heating or cooling resources for regulating indoor temperature within a narrow temperature range. The method maintains a temperature within the control range by activating a required number of heating or cooling sources and proportionally controlling one of the sources within the control range. An additional heat source is activated each time the indoor temperature falls below a preset "add heat" temperature, and a heat source is deactivated each time the indoor temperature passes a preset "delete Heat" temperature. Similarly, when the method is operating in the cooling mode, an additional cooling source is activated when the indoor temperature passes an "add cooling" temperature, and an additional cooling source is deactivated each time the indoor temperature passes a "delete cooling" temperature.