An automatic system for controlling the environmental conditions in a building divided into a plurality of zones includes a control station that sends control signals over the power lines of the building to control power consuming appliances in the various zones. The control station includes a programmable computer that is operably coupled to various displays and control switches on a control panel. In one mode of operation the switches and displays are utilized to program a schedule into the computer for controlling the various power consuming appliances such as heating and airconditioning units, lights, machinery and the like in order to provide a desired environment within the various zones of the building. In another mode of operation the displays on the control panel provide operator information as to the programmed schedule for the zones whereas the switches on the control panel enable the schedule stored within the programmed computer to be selectively overridden to accommodate changing conditions, such as holidays and the like, where newly desired environmental conditions in the various zones of the building differ from the schedule stored within the programmed computer. The system has the capability of automatically advancing the scheduled start up time of heating units dependent upon the difference between the temperature outside the building and a reference temperature in order to insure that environmental conditions in the various zones of the building are at a desired level at the time of occupancy.