That advisory was issued for parts of Norristown and Plymouth Township.
More than 1,000 customers are being asked to boil their water until 8 a.m. Friday as a precaution.
Pennsylvania American Water said the advisory was issued after a loss of positive water pressure in the distribution system due to an equipment failure.
The company said that when that happens, contaminants might be able to enter the system.
Any water that is used for drinking, making ice, preparing food, washing dishes, or tooth brushing should be boiled for at least one minute.
For more information, visit Plymouth Township Emergency Management's Facebook page.