Abstract: The present invention discloses an energy conserving thermostat for controlling the supply of energy within a commercial or industrial building during unoccupied periods and for providing additional energy to the building when required. The present system includes: a night thermostat for establishing a low temperature necessary for the maintenance of the building during a non-occupancy period; a seven-day timer clock calibrated for every hour of each day, with manually adjustable pins on the clock settable for a time interval within each day during which a normal comfort condition is required; mechanical switch means responsive to such pins of the clock; electrical switch means including a relay energized by operation of the mechanical switch; and an override timer including a time meter and a manually operable switch to increase the time setting for any day wherein an unusual supply of energy is required within a zone of the building. The extra time of such setting is recorded on a time clock meter.