What causes smelly hot water, and is it safe?

Stand near the water heater with the water off and sniff, then run the hot tap and sniff there. What you notice decides what happens next.

• You smell it in the air near a gas heater, stove or meter, with no water running. Treat it as natural gas. Southwest Gas says to exit the building right away, tell others to leave, and call 911 and Southwest Gas at 877-860-6020 from a safe place. It says not to touch switches, thermostats or appliance controls on the way out. Our page on what to do if your house smells like gas or rotten eggs walks through it.
• The smell only shows up when you run hot water. That points to the water heater, and the rest of this page applies.
• The smell is in cold water too. Then the source is the supply or the plumbing, not the tank.

### The hot and cold test

The Minnesota Department of Health gives a simple test. After you have been away for a few hours, smell the water from both the hot and cold faucets and note which ones have the rotten-egg odor. If cold water does not smell, it says the problem is likely in the water heater. Use a clear glass and smell it near the faucet, since the gas escapes as the water runs.

If both hot and cold smell, note that the Phoenix Water Services FAQ says taste, odor and hardness are not regulated under drinking water and health standards, and that taste and odor variations are often seasonal, stronger near the fall, because of increased algae in the canals that carry surface water to Phoenix's treatment plants. It also says the chlorine used in treatment can affect taste and odor. That kind of smell is different from rotten eggs, but it is a reason to call your water provider if the smell is on the cold side too. Our page on the musty or earthy taste in Phoenix water in the fall covers that seasonal case.

### Sulfate, the anode rod and hydrogen

Rheem's manual explains it directly. The tank has an anode rod that is slowly consumed to protect the glass-lined steel. Water sometimes contains a high sulfate or mineral content, and together with that protection process it can produce hydrogen sulfide, a rotten egg odor in the heated water. Rheem says chlorination of the water supply should minimize the problem. Bradford White says the magnesium anode rod generates enough hydrogen to cause odor problems, and that replacing it with a different anode may minimize the problem.

The Minnesota Department of Health says the same thing from the health side: a magnesium anode can supply electrons that help turn sulfate into hydrogen sulfide gas, and replacing it with one made of a different material, such as aluminum, can help.

### Sulfur bacteria

Bradford White says the smell needs three elements: sulfur, hydrogen and bacteria, and it says the bacteria can survive heating up to approximately 225 degrees F. The Minnesota Department of Health says a water heater can create a warm environment where sulfur bacteria live. Its advice for stubborn cases is to disinfect and flush the heater with a chlorine bleach solution.

Bradford White describes a chlorination procedure that involves draining the tank, swapping the anode, adding bleach and letting it sit for a minimum of three hours before a thorough flush. That is a plumber's job. It involves draining a hot tank, opening the anode port and handling bleach, and mistakes can shorten the tank's life or leave a chlorine smell in the water.

The Minnesota Department of Health says sulfur bacteria are not harmful, while hydrogen sulfide gas in the air can be harmful at high levels. In a household water heater the gas is released in small amounts at the tap, and the usual complaint is the smell, not illness. If anyone in the home gets headaches or nausea that clears when they leave, or the smell is strong in a small closed room, ventilate and have a professional look at it. For the health side in more depth, including the air levels OSHA lists and whether a powered anode rod helps, see whether a rotten egg smell in tap water is harmful.

A related caution is worth knowing if the heater sits idle. Rheem's manual says hydrogen gas can build up in a hot water system that has not been used for a long time, generally two weeks or more. It says hydrogen is extremely flammable, that opening the hot faucet at the kitchen sink for several minutes before using any electrical appliance connected to the hot water system helps dissipate it, and that you should not smoke or use an open flame near the faucet. If you hear air escaping through the pipe as the water begins to flow, that can mean hydrogen is present. This is a different issue from a rotten-egg smell, but the two often show up together after a vacation or a vacant house.

• Do not turn the heater up to kill bacteria. The Minnesota Department of Health lists raising the temperature to 160 degrees F for several hours as a treatment for sulfur bacteria. Rheem warns that water above 125 degrees F can cause severe burns or death from scalding, and children and older adults are at highest risk. That is a scald risk in your house, so leave it to a plumber with the right valves and procedure.
• Do not remove the anode rod and leave the port open. Rheem says not to remove the rod except for inspection or replacement, because operating without it will greatly shorten the life of the tank and will exclude warranty coverage. Bradford White says complete removal of the anode, magnesium or aluminum, will void the warranty and shorten tank life.
• Do not lower the temperature to fix the smell. The CDC's guidance for building water systems is to store hot water above 140 degrees F and keep circulating hot water at or above 120 degrees F, because Legionella grows in warm water between roughly 77 and 113 degrees F. The CDC says to install thermostatic mixing valves near fixtures to prevent scalding. A home tank at a lower setting is a trade-off that our page on what temperature to set a water heater covers.

A plumber checks whether the smell is really in the hot side only, looks at the age and type of the anode rod, and flushes the tank. From there the usual options, in order from least to most involved, are the ones the manufacturers and the Minnesota Department of Health list. Replace the anode with a different material. Disinfect the tank with chlorine. Treat the water supply. Or replace an old heater, since a tank that is past its life is a hard one to keep clean. Our page on whether to repair or replace a water heater helps with that call, and our page on how often to flush a water heater in Phoenix covers the flush.

If you have grit or discolored water along with the smell, read why there is sand or grit in your hot water. HQ Plumbing and Air is licensed and insured, takes emergency calls 24/7, and gives free estimates with an upfront price before work starts. See our water heater services.

In a hotel, apartment complex or care facility, water that sits in pipes is a health matter as well as a smell; see whether your building needs a Legionella water management plan.