Don't just let your fans whirr on full power 24 / 7. You could be needlessly drying the air in your indoor garden and causing your plants to over-transpire. The key is to connect your fans to a fan-speed controller!
Growers use extraction fans to remove warm, spent air from their indoor gardens and replace it with cool, fresh air. Extraction fans are an essential piece of kit for keeping the air in your indoor garden suitable for plant growth in terms of temperature, relative humidity and carbon dixoide levels.
Some indoor gardeners miskenly believe "more is better" when it comes to ventilation. But the real trick when it comes to maintaining an optimum environment for your plants is to use a fan speed controller.
At their simplest, fan speed contollers switch your extraction fans ON when their thermostat senses that temperatures in your indoor garden have risen above a pre-determined maximum—e.g. 80 °F—and they switch your extraction fans OFF when temperatures fall below a minimum—e.g. 65 °F.
Some more advanced fan speed controller allow the grower the option of setting the minimum fan speed (aka idle speed) and also the ability to set minimum shut off temperatures for nighttime as well as daytime. Be sure to check that your chosen fan speed controller is rated to control the size fan you have. Most hobbyists look for a controller that can handle fans powered up to 750 Watts.
Other fan speed controllers offer even more features. Look out for the "fire-up" feature that automatically switches your fan to 100% when it comes on and then slowly reduces the power to the desired setting. This feature can dramatically increase the life of your fans by controlling the electrical load.