Gas Fitter Practice Test 2026 – The All-in-One Guide to Mastering Your Certification!

The most likely cause of an intermittent hissing sound from a water heater is condensation in the flue from drawing hot water. When hot water is drawn from the heater, it can lower the temperature in the flue momentarily as the hot exhaust gases are vented out. This temperature drop can lead to condensation forming within the flue system as the steam from the hot water interacts with the cooler surfaces of the flue. The hissing sound is often the result of this condensation evaporating or interacting with surfaces, creating an intermittent noise that correlates with the operation of the water heater.

In typical operating conditions, the flue should remain hot enough to minimize condensation, but any fluctuation, such as increased demand for hot water, may result in temporary cooling that interrupts normal flow and creates the hissing noise. Understanding this process is important for diagnosing potential issues in the water heater's exhaust system and ensuring it functions efficiently and safely.