When buildings have had low or no water use—for example, during COVID-19 pandemic closures—it is important to know how to restart building plumbing systems safely. Normally, water is actively flowing in buildings: it is pulled in from the water main via the service line, then flows to, and exits from, taps, appliances and other fittings. Before sending water out to the public water mains, the public water system (water utility) undertakes a variety of treatments to ensure tha