CSLB C-36 Plumbing ContractorService, Repair, and MaintenanceMedium
During a routine inspection of a commercial water heater, you discover a strong 'rotten egg' smell emanating from the hot water. The cold water supply does not have this odor. What is the most probable cause?
- AA backflow prevention device failure allowing sewer gas into the hot water lines.
- BA natural gas leak near the water heater unit.
- CAn accumulation of anode rod degradation byproducts and sulfate-reducing bacteria in the tank.
- DExcessive chlorine levels in the municipal water supply reacting with tank materials.
Show answer & explanationAnswer & explanation
Correct answer: C. An accumulation of anode rod degradation byproducts and sulfate-reducing bacteria in the tank.
A 'rotten egg' smell (hydrogen sulfide) specifically in the hot water, but not the cold, is a classic symptom of sulfate-reducing bacteria reacting with the magnesium or aluminum anode rod in the water heater tank. These bacteria thrive in warm, dark, anaerobic conditions and produce hydrogen sulfide gas as a byproduct.
Why the other options are wrong
- A. A backflow prevention device failure would primarily cause contamination of the potable water system with non-potable water or sewer gases, but the odor would likely be more general or different, and not consistently localized to only hot water in this specific manner.
- B. A natural gas leak would produce a distinct sulfur-like odor, but it would be present in the air around the unit, not specifically in the hot water, and is a different chemical compound.
- D. While chlorine can react with plumbing, it typically doesn't produce a strong 'rotten egg' smell, and if it did, it would likely be present in both hot and cold water if it's a municipal issue.
Water Heater 'Rotten Egg' Smell
Odor of hydrogen sulfide gas in hot water, caused by sulfate-reducing bacteria interacting with the anode rod.
- Specific to hot water supply.
- Caused by anaerobic bacteria in the water heater tank.
- Anode rod material (magnesium, aluminum) contributes to the reaction.
- Can be remedied by flushing, chlorination, or replacing the anode rod with zinc.
Memory trick: Anode's decay, bacteria's way, makes hot water go astray.