MovingOnanswered grow question 5 years ago I bought a fan that only turns on when the temperature gets too hot, or when the humidity gets too high. It's the "AC Infinity: Cloudline T6", you can also get a 4" version. Or you can use a thermostat with a solenoid that is hooked up to your current fan. The times between when the fan turns on, the humidity and CO2 values are much more stable and less water and CO2 is wasted.