I'm not sure which category to put this one in, but here it goes. I am installing a pan for a washing machine (drain pan in case of leaks) in a closet that sits above my attached garage. The attached garage has 5/8" sheetrock on the ceilings as per code (the walls are concrete) but there is some plumbing protruding through the sheetrock that is sealed at the joints with the fireblocking caulk. The inspector said this is fine.

I wanted to just drain the washer pan directly into the attached garage, without linking it to the existing plumbing. This is for both space (height clearance) and observation reasons (you would easily see even a slow leak draining as it drips into the attached garage). However, this would leave a 1" opening (hole) in the middle of the PVC from the attached garage to the living space above, much more than the exposed pluming that is already sealed (and which doesn't penetrate the subfloor above like the drain pan drain would.

Does anyone know if I can just drain it this way or if there is some type of flapper valve or something I can put on the PVC to close it off when there is no water sitting in the pipe? Any other suggestions are welcome.