Walking into your garage or utility closet and hearing a distinct, continuous hissing noise near your water heater requires quick, careful evaluation. A hissing sound can stem from three distinct causes, ranging from completely harmless to life-threatening.
Cause 1: An Active Natural Gas Leak (DANGEROUS)
Pressurized natural gas escaping through a cracked flex line, loose flared fitting, or failing gas control valve produces a steady hiss. If you smell rotten eggs, do not flip any light switches. Evacuate immediately and call our 24/7 hotline.
Cause 2: Temperature & Pressure (T&P) Relief Valve Weeping
If your home's water pressure regulator has failed and pressure exceeds 150 PSI, or water temperature exceeds 210°F, the brass safety valve releases steam and hot water, creating a hissing sound.
Cause 3: Condensation Dripping on Burner (Normal)
When cold groundwater enters an empty water heater tank, moisture condenses on the exterior of the flue and drips onto the hot burner, creating a sizzling hiss. This stops once the tank warms up.