Abstract: A cap covers an open end of a drain pan fitting of an appliance drain pan. The cap comprises a cylindrical body and a non-planar water-soluble membrane. The cylindrical body has a first end with a first end opening, a second end with a second end opening, and a longitudinal through-hole extending between the first end opening and the second end opening. The non-planar water-soluble membrane is positioned to at least partially obstruct the through-hole when the membrane is intact such that air flow through the through-hole is inhibited in the absence of water in the drain pan. The membrane is adapted to dissolve when exposed to water in the drain pan such that water in the drain pan is not inhibited from exiting the drain pan through the through-hole.