Gas Fitter Practice Test 2026 - Free Gas Fitter Practice Questions and Study Guide

The intermittent hissing sounds from a water heater are likely caused by condensation in the flue passage, particularly when water is drawn from the tank. When the water heater operates, especially during heating cycles, cold water enters the tank, causing a temperature drop within the system. This drastic change in temperature can lead to condensation forming in the flue passage as warm gases rise and come into contact with cooler surfaces.

As water vapor in the exhaust gases cools down, it may condense and create an audible sound as the water droplets collide with the flue's surfaces or as they move through the flue. This phenomenon is common in flue systems where temperature fluctuations are significant, especially in climates with varying humidity and temperatures. The sound will likely be intermittent, correlating with the heating cycles of the water heater, which explains its variability.

Understanding the impact of drawing water from the tank on condensation helps to identify the source of the noise and suggests that the water heater is functioning as designed, with flue efficiency being a contributing factor.