Don't ask me how I know. Some people will say that there has to be a sealed pipe, otherwise there's room to expand up the pipe. Balderdash, I say. And what's the big deal taking 15 minutes and two gallons of RV antifreeze and going through the house? Also, don't forget the toilet tanks. There is about a pint of water in there that won't drain, and it needs some antifreeze in there. And the bow of snake sprayers in the kitchen. Again, don't ask me how I know.