A method and apparatus for conserving energy in buildings consisting of an indoor temperature sensor, an outdoor temperature sensor, a programmable electronic thermostat, and a bi-directional power ventilator. The programmable electronic thermostat contains software instructions to switch on or switch off the power ventilator during pre- determined time schedules in response to changes in the indoor and the outdoor temperatures. During a summer day-time schedule, the thermostat switches on the power ventilator in the normal flow mode to exhaust the building of hot accumulated indoor air if the indoor temperature is greater than a selected set-point temperature and if the indoor temperature is also greater than the outdoor temperature by a predetermined ratio. During a summer night-time schedule, the thermostat switches on the power ventilator in the reverse flow mode to force cooler outdoor air into the warm building if the indoor temperature is greater than a selected set- point temperature and if the indoor temperature is also greater than the outdoor temperature by a predetermined ratio. During a winter day-time schedule, the thermostat switches on the power ventilator in the reverse flow mode to force warmer outdoor air into the cold building if the indoor temperature is less than a selected set-point temperature and if the indoor temperature is also less than the outdoor temperature by a predetermined ratio. The use of cooler summer night-time air to pre-cool the building during warm summer days reduces the air- conditioning energy requirements of the building during the summer. The use of warmer winter day-time air to warm the cold building during cold winter days reduces the space heating energy requirements of the building during the winter.
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