You’ve probably heard the advice: after a long vacation, or when an empty rental has sat idle for a while, run every tap for a few minutes before you drink, shower, or brush your teeth. The logic feels airtight. Water that sits still goes “stale,” bacteria multiply, so you rinse the old water out and swap in fresh. Simple.

Except the science turns out to be messier than that. A handful of recent field studies — several prompted by the enormous natural experiment of COVID-19 lockdowns, when whole office towers sat unused for weeks — found that flushing sometimes increases the very bacteria you’re trying to get rid of, at least for a while. Whether flushing helps, hurts, or does nothing depends on your pipes, your water heater, and even whether your plumbing is copper or plastic.