There is not actually a top tank, just a drip tray with little drain holes that become easily clogged. After removing the rear cover of the unit and cleaning thoroughly, you can use a can of air (like the ones to dust pc's) wth the little straw attachment to try to clear te clogged drain holes in the tray below the evaporator. If your unit froze up condensate may have overflowed into the fan area. Let the unit defrost, and run it in fan mode for awhile to dry this moisture out.