a wall fan includes a control circuit both controlling the wall fan and a temperature control unit comprising a heater , a cooler , or a heating , ventilating , and air conditioning system . the control system receives a desired temperature and compares the desired temperature to an outdoor temperature . when the outdoor temperature indicates that drawing outside air into indoor space will drive the indoor air temperature towards the desired temperature , the control system activates the wall fan , and de - energizes the heater , cooler , or hvac system . when the outdoor temperature indicates that drawing outside air into indoor space will not drive the indoor air temperature towards the desired temperature , the control system de - energizes the wall fan , and energizes the heater , cooler , or hvac system .