The answer to 'water heater foul smell' is that a foul odor coming from your water heater is often caused by a buildup of bacteria or minerals. This can lead to a rotten egg or sulfur-like smell in the hot water. Some common causes include an anode rod issue, sediment buildup, or bacterial growth in the tank.
If you notice a foul smell, first try flushing the water heater to remove any sediment buildup. You can also replace the anode rod, which helps prevent corrosion. However, if the smell persists, it may be a sign of more serious issues that require a professional plumber to diagnose and repair.