In addition to the boil water notice issued by the City of Houston, Harris County has advised utility districts and wholesale water providers from across the County to perform testing of water to ensure the safety of the water supply. Water testing conducted by these entities - which operate independently from the City of Houston and Harris County - takes approximately 24 hours.
Local municipal utility districts and water wholesale providers are required to inform their customers if their water does not meet safety standards established by the Texas Commission on Environmental Quality. They are also required to perform water quality testing, which can take 24 hours or more. Residents should expect to be informed by their water utility provider if they need to boil water before they use it.
These graphics from Harris County Public Health provide easy to understand directions on how to boil water.