Example methods, apparatus and articles of manufacture to detect impurities in flow-through water heaters are disclosed. An example apparatus includes a flow-through water heater having a conduit to conduct a fluid flowing therethrough, a heating element extending around at least a portion of the conduit, and first and second temperature measuring elements to determine respective first and second temperatures at different locations on the conduit; and a computing unit to at least detect an impurity deposit in the conduit based on a difference between the first and second temperatures.