I suppose it's where in the country the boat is but for me here in the tropics mold and mildew are concerns. The fan runs at low speed on both units 24-7 to circulate air to control the biologics. The AC is also on 24-7 and when not using the boat set at 78. The materials used inside the boat especially the old Vitracore laminates are not very tolerant to wide swings in environmental conditions and have been known to delaminate and crack. I leave the fan running to also circulate air over the temperature probe so the unit operates to maintain average temperature throughout the boat.