A standard thermostat control unit, such as used in residential heating systems, is modified to include a manually actuated, electro-mechanical timer that includes a switch connected across the unit' s thermostatically operated contacts, such that actuation of the timer produces a temporary, self-expiring, operating mode in which the normal thermostatic regulation of the furnace is overridden and the furnace is held "on" for the brief duration of the timer interval. The modified unit may be used in lieu of the standard unit as an energy and cost saving device in places such as in a vacant house, waiting to be sold or rented, with the unit preset at a minimum, thermostatically regulated temperature level. To briefly increase the temperature in the house to a comfort level, for such temporary occupancy as a realtor showing the house to prospective purchasers, the timer is manually actuated to commence a short timing interval, such as five minutes, during which the furnace is turned on and maintained so for the timing interval, and at the expiration thereof the unit automatically reverts to the energy saving, thermostatically regulated mode at the lower temperature.