I recently performed a home inspection on a home that has been vacant for some time now. Shortly after turning on the hot water faucets the odor became offensive. Rotten eggs and other descriptions would fit this well. This particular home is supplied with municipal water. This smells like a fairly common problem and is called hydrogen sulfide. If the smell is only from the hot water faucet the problem is likely to be in the water heater. How is this hydrogen sulfide gas produced in a water heater? A water heater can provide an ideal breeding ground for the conversion of sulfate to hydrogen sulfide gas. The water heater can produce hydrogen sulfide gas in two ways – creating a warm environment where sulfur bacteria can live, and sustaining a reaction between sulfate in the water and the water heater anode. A water heater usually contains a metal rod (sometimes plastic) called an “anode,” which is installed to reduce corrosion of the water heater tank. The anode is usually made of magnesium metal, which can supply electrons that aid in the conversion of sulfate to hydrogen sulfide gas. The anode is 1/2 to 3/4 inches in diameter and 30 to 40 inches long depending on the heater design. Is there a cure? 1) Disinfect and flush the water heater with a chlorine bleach solution and sometimes hydrogen peroxide. This can kill sulfur bacteria, if done properly. If all bacteria are not destroyed the problem may return within a few weeks. 2) Increase the water heater temperature to 160 degrees for several hours. This will destroy the sulfur bacteria. Flushing to remove the dead bacteria after treatment might control the odor problem. CAUTION: Increasing the water heater temperature can be dangerous. Before proceeding, consult with the manufacturer or dealer regarding an operable pressure relief valve, and for other recommendations. 3) Replace or remove the magnesium anode. Many water heaters have a magnesium anode tube, which is attached to a plug located on top of the water heater. It can be removed. Removal of the anode, however, may significantly decrease the life of the water heater. You should consult with a water heater dealer to determine if a replacement anode made of a different material, such as aluminum or zinc, can be installed.