 So if the drain line has a P-trap, kind of the same kind of that sort of U-shaped pipe that's underneath the bathroom sink, then that pipe will stay filled with water and will not allow any gases, any air to back up through it and get into the furnace. That's not built into the furnace already? Not always. I mean, it depends on the workmanship of the installer. But no, you would see it on the outside. If you don't see a P-trap, it doesn't have one.