If you ain't got a fan controller - get one. I had some troubles with low temperatures and too dry air in vegetative period which all went away with a simple controller. There is a lot to choose from and i just happen to have the one which reduces fan speed in the wrong way - but it works.
My extraction fan and the fans beneath the lights are hooked up to a temperature/humidity controlled sensor.
If temps drop below 78 the fans will shut off automatically. If humidity rises above 60% or the heat rises the fans will kick back on.
(both the temperature setting and the humidity setting can be independently adjusted.)