An attic fan vents the attic of hot air through a set of shutters, or louvers. The shutters automatically open when the fan comes on to allow the flowing air to escape to the outside. If the shutters do not open, back pressure will form in the attic, forcing superheated air into the home living space, making it uncomfortable and causing the air-conditioning system to run constantly in an attempt to cool things down.
Check the fan and make sure it is functioning properly. Check the breaker to the fan and make sure it is flipped to the "on" position. If the fan is getting power, determine whether the fan motor is working properly or not. A fan motor that is burning out won't spin the fan blades fast enough to open the shutters. If the fan motor is not working properly, it must be replaced.
Adjust the Shutters
Shutters are spring-loaded to automatically close when the fan turns off. Make sure the shutters have a spring and it is connected properly. If the spring is there but the tension is insufficient, move the spring up a notch to increase the tension. This enables the shutters to close when the fan turns off.
Make sure the shutters open and close easily. If the shutter is jammed one way or the other, free any obstructions you find. Painted-over shutters do not open and close easily. Lubricate the axles or the pivots with a common spray-type lubricant until the shutters move freely. In all cases, as soon as the fan begins to blow, the shutters must easily open.
The fan draws air from the home interior to clear the attic of hot air. If the home is closed up tight, airflow through the fan blades might be insufficient to open the shutters. When this occurs, the fan cannot move air successfully through the shutters and blow it outside. An open window or two may be needed in the home to equalize the pressure in the attic and allow the fan to adequately circulate the air and vent it outside.