What is the likely cause of an intermittent hissing sound from a water heater?

The intermittent hissing sound from a water heater is likely caused by condensation in the flue due to the process of drawing hot water. When water is heated, it generates steam and changes temperature rapidly as it moves through the system. If the flue assembly is not properly insulated, or if there is a temperature differential when the heater is firing up and then shutting down, it can lead to condensation forming inside the flue. As this condensation drips and interacts with heated surfaces, it can create an intermittent hissing or popping sound.

This sound is often a normal occurrence in some systems and indicates that there is moisture present, which is a result of the flue being cooler than the steam generated, leading to condensation. Recognizing this can help in understanding the functioning of the water heater and addressing any potential issues.