ATMOX is of course a manufacturer of crawlspace fans designed to ventilate and exhaust air from the crawlspace. There are other fans on the market. Most other fans on the market are designed as exhaust-only fans using open vents as intake points for air. ATMOX believes that air flow should be balanced and even throughout the crawlspace and generally recommends using both exhaust fans and intake fans to reduce too much negative pressure on the crawlspace. While you do not want crawlspace air being pushed into the home, you also do not want to be pulling too much air from the house down into the crawlspace either as it would not be energy efficient to do so.
Ventilation is a good thing for a vented crawlspace but now the question becomes what is triggering those fans to come on and off or are they running continuously. Most other fan manufacturers will incorporate the use of a humidistat at the fan location to trigger the fan to operate. While it sounds logical enough to run the fan when high humidity is triggered, it can actually have the opposite effect of that desired as the fan is now only running when the humidity is high. The humidity measurement is only taken right at the fan location meaning it may be more of a measure of the outside air than the air in most locations in the crawlspace. The fan is now going to run most often when the humidity is high, probably also when the humidity is high outside. So, in effect, you are now bringing hot humid air from the outside into the crawlspace at the worst possible time as the crawlspace air generally cooler from the air conditioning ducts and will not be able to hold that moisture being brought in. As we have stated before, you cannot put a wetter air mass from the outside into a crawlspace and expect conditions to improve.
Some fans may be running continuously. Whether or not this is beneficial or detrimental to the crawlspace will greatly depend on the geographic location and climate of the house. Most crawlspaces do not need or benefit from continuously running fans unless there is another factor at play unrelated to moisture control.
Fans can be triggered by other means as well such as a timer. While this is not our ideal recommendation, it can work in certain climates to improve ventilation and keep air fresh in the crawlspace. Once again, it will depend on the climate. If the home is located where the air generally cools off in the evening, running fans at night can generally benefit the crawlspace as you are making the assumption that more nights will be drier than wetter as compared to conditions in the crawlspace. Fans on a timer have one drawback in that without any temperature reading, they will continue to run in extreme cold temperatures and could contribute to freezing pipes if not checked. Learn more about running ATMOX Fans on a Timer.