Domestic Hot Water (DHW) and Legionnaire’s Disease

A homeowner forum post on Green Building Advisor points to a real concern for anyone with a hot water system: Legionnaire's disease, a lung infection caused by legionella bacteria that can grow in household plumbing. The bacteria thrive in certain conditions inside water heaters and pipes, and one factor called out is "dead legs" — sections of plumbing that sit unused for long stretches, like a line running to a guest bathroom that rarely gets turned on.

The post references the Centers for Disease Control and Prevention's guidance on controlling legionella in building water systems, which lays out the conditions that let the bacteria grow and design choices that help prevent it. This matters as more homes move toward heat pump water heaters and other efficiency upgrades, since some of these systems run at lower storage temperatures than traditional tank heaters, and lower temperatures can make bacteria growth more likely if water sits stagnant.

There's no specific product recall or new rule here, just a reminder that hot water systems need occasional use throughout the house, not just in frequently used bathrooms and kitchens. If you have a guest bathroom, basement sink, or any other fixture that goes weeks without running hot water, it's worth knowing that stagnant lines are one of the conditions the CDC flags as worth paying attention to when thinking about home water safety.