A method of heating or cooling a room of a building includes providing supplemental warmer or cooler air to the room with an inline duct supplemental heating and cooling device mounted along an air duct that supplies the room with air from a main furnace or cooling unit associated with the building. A thermostat is used to set a desired temperature for the room, and a controller of the supplemental heating and cooling device is in communication with the thermostat, and activates an electric heater and/or fan as appropriate to warm or cool the room as needed. When the device is used to cool the room, the fan may be operated to draw cooled air through the duct from the main cooling unit and/or from other areas in the building that are linked via ductwork.