That rotten egg smell from your tap is hydrogen sulfide gas. It’s not dangerous at the concentrations typically found in a home water supply, but it’s unpleasant and it does have a cause you can track down.
The most common source is your water heater, not the water itself. Second most common is well water with naturally occurring sulfur bacteria. Municipal water can occasionally be the culprit too, but it’s the least likely if you’re in a city.
The Water Heater Anode Rod (Most Likely Cause for Hot Water)
If the smell only comes from hot water, the anode rod in your water heater is almost certainly the problem. Anode rods are magnesium or aluminum rods that sit inside the tank and corrode sacrificially to protect the steel. That’s the job they’re designed to do.
When a magnesium anode rod reacts with water that has a high sulfate content or certain sulfate-reducing bacteria, it produces hydrogen sulfide. The hot environment inside the tank accelerates it.
Rheem, Bradford White, and A.O. Smith all ship most tank water heaters with magnesium anode rods from the factory. Switching to an aluminum/zinc alloy anode rod often eliminates the smell entirely. Some plumbers also recommend flushing the tank and disinfecting it with a diluted bleach solution before installing the new rod, because the bacteria can persist in the sediment at the bottom.
This is a real fix, but it’s also a job that involves working with a pressurized hot water tank, and the rod can be extremely difficult to remove if it’s been in place for several years. A licensed plumber can handle the inspection and replacement.
Well Water Sulfur Bacteria
If the smell comes from both hot and cold water and you’re on a well, sulfur-reducing bacteria living in the well or in your plumbing are the more likely cause. These bacteria are naturally present in some groundwater and feed on sulfur compounds, releasing hydrogen sulfide as a byproduct.
A few indicators that point to the well rather than the water heater:
- Cold water smells just as bad as hot
- The smell has gotten worse over time or appeared after heavy rain (which can push surface bacteria deeper into the well)
- Your water has a slightly yellow or brown tint
A water test from a certified lab will tell you what you’re dealing with. Testing for total coliform, sulfate-reducing bacteria, and hydrogen sulfide covers the most common issues. Your county health department can usually point you to approved testing labs.
If bacteria are confirmed, shock chlorination is the standard first treatment. It requires calculating chlorine concentration for your specific well depth and casing diameter, distributing it correctly through the system, and flushing it out afterward. Get a licensed water well contractor to handle it. The chlorine dose has to be right for your well’s dimensions, and if the treatment is done wrong it either fails or creates a bigger problem to clean up.
Municipal Water That Smells Sulfurous
If you’re on city water and both hot and cold water smell, call your water utility first. They’re required to provide annual water quality reports and can tell you if there’s a known issue. Sometimes this is seasonal, especially in late summer when certain algae blooms affect source water.
If only the hot water smells and you’re on municipal supply, the anode rod explanation above still applies. That’s still the most likely cause.
How a Technician Diagnoses It
A plumber or water treatment specialist will typically work through this in sequence:
- Smell test at the tap, then at a hose bib (outdoor spigot) that bypasses the water heater. If the outdoor tap smells fine, the heater is the source.
- Check the anode rod condition, water heater age, and whether it’s ever been serviced.
- If well water, review the last water test or pull a new sample.
- In some cases, a water softener or existing filtration system can harbor bacteria and become a secondary source. That gets checked too.
Most experienced plumbers have seen this specific problem many times.
What You Can Safely Do Yourself
Run cold water from an outdoor tap or a tap that’s nowhere near the water heater for a few minutes. If it smells fine, you’ve narrowed the source to the heater.
You can also check your water heater’s age. Most tank water heaters have a manufacture date coded into the serial number, though the format varies by brand. If it’s more than 8 to 10 years old and has never had the anode rod replaced, that’s a strong indicator.
Beyond that, leave the actual repairs to a licensed plumber. Pressurized water heaters, well disinfection, and water treatment equipment all involve risks that aren’t worth taking on without the right training and tools.
When to Call a Licensed Plumber
Call a licensed plumber if:
- You’ve confirmed the smell comes from the water heater and you want the anode rod inspected or replaced
- Your water test comes back positive for sulfur bacteria
- The smell is strong, has appeared suddenly, or is getting worse
- You’re on a well and haven’t had the water tested in more than a year
In California, plumbing work requires a C-36 license. You can verify any plumber’s license at cslb.ca.gov before they start work. It takes about thirty seconds and tells you if the license is active and whether there are any disciplinary actions on record.
This guide is for information only. We don’t perform plumbing work or provide quotes. If you need service, hire a licensed plumber in your area and verify their credentials before they start.