Metal pipes expand when heated. Pouring rapidly expands the metal. If there is any existing rust or corrosion (and in old metal pipes, there always is), that rapid expansion can crack the pipe or dislodge rust flakes, causing a sudden clog.
Use a plunger or a snake. Never hot water. boiling water down drain
GENERALLY SAFE. These materials handle high temperatures well. However, even in metal systems, the rubber or plastic seals at connections can still be damaged by extreme heat. Metal pipes expand when heated
Most homes built after the 1970s use plastic pipes for drains—specifically PVC (polyvinyl chloride) or ABS (acrylonitrile butadiene styrene). These materials are lightweight, cheap, and resistant to corrosion. They are not resistant to thermal shock. Use a plunger or a snake
If you have a garbage disposal, you have an extra layer of risk. Disposals are often housed in plastic casings and use rubber seals to prevent leaks. Repeatedly hitting these components with boiling water can cause them to melt, crack, or fail prematurely. Does Boiling Water Clear Clogs?
Pouring boiling water down a drain is a common household habit, but plumbing experts generally advise against it due to the risk of significant damage to modern piping systems.
Modern residential plumbing often utilizes various types of plastic, which are highly sensitive to thermal stress. PVC (Polyvinyl Chloride):