If you have extraction running when the lights are on you probably dont need the fans but always ensure the plants have access to fresh air. My fan is timed to my lights, when they come on so dose the fan but my extractor fan runs 24 hours a day so fresh air is being pulled through my grow constantly. If the fan is on when the lights are off and it is a cold day the fan could be making it worse, i know if i blow my fan on my plants on a cold day with the lights off it just makes the plant colder. Mind you my fan is more to keep the heat from building upand stop the tops of the plants burning under the lights rather than to circulate air as my extractor is powerfull to do this on its own. Peace