Porcelain is a ceramic material that expands when heated. Pouring boiling water (212°F / 100°C) into a cold toilet causes , which can instantly crack the bowl or the trapway below. If the crack is small, you might not notice it until water begins leaking through your ceiling.
When used correctly, hot water can dissolve and dislodge clogs caused by soap scum, grease, and soft organic matter. However, . Using water that is too hot can crack your porcelain bowl, turning a $20 DIY fix into a $500 bathroom renovation.
Here is the safe and correct way to use hot water to unclog a toilet. This is the most important rule. Do not pour boiling water directly into your toilet bowl.