How so? The evaporator coil causes moisture to condense on the coil so it can drain away. How does that add moisture to the house? I think the OP's issue is quite simple. Not easily solved but the mechanics of it are quite simple. It reminds me of an unrelated issue involving the same principles where convection currents form in a loop within a hollow exterior wall. It causes convective heat losses through the wall. Such currents in the OPs issue would help equalize the differences in temps between basement and upstairs. Same concept but beneficial in this case. Marc You answered your own question Marc. the reason is because moisture condenses on the coil. It does not drain away immediately. The coil will stay wet and if you operate the unit with the fan in on position during the cooling months that moisture gets dumped into the home. From the Florida Power and Light website: Another reason to keep it on ?auto? Setting your A/C fan to auto also helps provide better dehumidification. Have you noticed how moisture from the air condenses on the outside of a cold drink on a humid day? Your A/C unit captures moisture the same way, helping your home feel more comfortable. When the fan cycles off using the auto mode, moisture has a chance to drip from the cold cooling coils into the condensation pan and then drain outside. However, when the fan runs all the time in the ?on? setting, less moisture has a chance to drip and drain outside. Instead, some gets blown back into the air again.