We've always left the gray open and the black closed. I wash too many clothes and we take too many showers to leave the gray closed. Been doing it for ten years with no problems. The only time we've had problem is when we left the gray closed and I did a load of clothes! Fills up the gray tank FAST! Then all the drains start gurgling, lol. However, I do close the gray and fill it up with water when we get ready to dump the black tank, so it can be flushed.H2H1 - 8/23/2010 3:46 PM just a little advice here,,, never open or leave open the gray tank open until you are ready to emphty both. The Black is open first and let drain, then open the GRAY to wash out the Black TANK waste from the hose. By doing this it helps stop the build up inside the tanks. I know I did it the way you are saying and it created a big problem until I was advised the correct way to do it.