If there is any damage to the belt replace it with a new one.
Attic fan does not run on high speed.
High speed and low speed.
Little or no airflow.
If the attic fan isn t turning on it could be an electrical issue a problem with the thermostat or the motor of the fan that s not working.
Homes built after 2015 are required to have all ductwork sealed and tested for airtightness.
Parts can come loose or erode from high amounts of fan usage while turning the speed up to high can cause a persistent loud hum.
If you have leaky ductwork.
For this you can use a 9 v battery a dvm and a 1 megohm resistor from radio shack.
Check that the blade moves freely.
Your attic fan may not be turning on because of an electrical problem like a tripped circuit breaker a blown fuse or a bad connection.
You select which speed it runs at based on the switch position.
Unfortunately homes built before 2015 may have very leaky ducts.
Remove the outer casing of the attic fan and check to see if the belt is sagging cracked worn or broken.
When running at a high speed setting does the attic fan run hard or sluggishly.
Check motor current draw and compare with specs.
I ve never heard of one that starts on high and switches to low.
Some info on the motor is needed.
Attic fan not running at top speed.
Most of those attic fan motors have two motor windings.
Attic fan not turning on.
Then after checking the capacitor for a short or an open measure its value.
If it is rotating slowly or not producing a lot of air in the high speed setting you may have a problem with the exhaust or intake.