Beyond the pipes, consider your own skin. When you pour a gallon of boiling water into a sink, it hits the cold metal drain cover and the standing water in the trap. This creates an immediate burst of superheated steam.

If you aren't supposed to use boiling water, what should you use? Here are five professional-grade solutions.

Enzyme drain cleaners (Green Gobbler, Bio-Clean) use bacteria to eat organic waste. They require cold or lukewarm water to survive. Boiling water kills the enzymes instantly. For a fresh-smelling, slow drain, use an enzyme treatment overnight once a week with cold water.