Rusty Water and Odor from a Water Heater: Causes and What to Do

Rusty water and a rotten-egg smell are two different symptoms with different causes. They are often lumped together and the whole water heater is replaced, although the source may not be in it at all. We will look at each separately and give a diagnostic sequence.

Where Rusty Water Comes From

Rusty or brown water is a suspension of iron oxides and hydroxides. Most often it comes from the mains and the riser, rather than forming in the water heater tank itself.

The mechanism is as follows. The inner walls of old steel pipelines are covered with deposits of corrosion products. As long as the flow is steady, these deposits stay in place. Any disturbance tears them off into the water. The disturbance may be repair work on the network, a pressure release, restart after a shutoff, the operation of a fire hydrant or abrupt opening of a gate valve.

This is called secondary contamination. Water at the intake may meet the standard, yet reach the consumer with iron and turbidity. The standard for iron in drinking water is 0.3 mg/L. The threshold of visible discoloration and staining of plumbing fixtures lies in about the same region.

The water heater itself produces rust in two cases. The first is damage to the enamel or glass-ceramic lining of the tank when the anode has been completely consumed. The second is corrosion of the steel flange and connections. In both cases the rust comes only from the hot side and is accompanied by sediment at the bottom of the tank.

Why It Smells of Rotten Eggs

The smell of rotten eggs is hydrogen sulfide. The odor perception threshold is low. According to the University of Georgia, a concentration above about 0.5 mg/L already produces a distinctly unpleasant smell.

In a household water heater, hydrogen sulfide forms for two related reasons.

The first is sulfate-reducing bacteria. They are harmless to humans, but they use sulfates dissolved in the water as a source of oxygen and release hydrogen sulfide. A warm tank with slow-moving water is a suitable environment for them. The Minnesota Department of Health puts it directly: a water heater creates ideal conditions for converting sulfate to hydrogen sulfide.

The second is a reaction at the magnesium anode rod. The magnesium rod is placed in the tank for cathodic protection of the steel. It corrodes in place of the tank and in doing so gives up electrons. These electrons take part in the reduction of sulfate to hydrogen sulfide. The Minnesota wording: the anode is usually made of magnesium metal, which can supply electrons that promote the conversion of sulfate to hydrogen sulfide. University of Georgia Extension describes the same thing as a direct chemical reaction of the magnesium rod with sulfate.

Manufacturers confirm the link. Bradford White lists three components required for the odor to appear: sulfur, hydrogen and bacteria. The magnesium anode protects the tank, but at the same time releases enough hydrogen to start the process.

The key sign: the smell is present only on hot water. If the cold water smells too, the source is in the mains or in the well, not in the water heater.

Diagnosis in One Evening

Go through the steps and write down the result.

• Run the cold water for 2–3 minutes. Assess the color and smell. If rust and odor are present on the cold side, the water heater is not the cause.
• Run the hot water. Compare it with the cold. A difference points to the water heater.
• Check the neighboring draw-off points and, if possible, the neighbors on the same riser. A shared problem means the network.
• Drain half a liter of water from the bottom connection or the safety valve of the water heater. Sediment and flakes at the bottom of the glass indicate the condition of the tank.
• Check when the water heater was last serviced. The magnesium anode rod is a consumable, and its condition decides everything.

What to Do About Rust

Start with mechanical protection at the inlet. A backwashable screen filter traps coarse scale. A 5–20 µm cartridge filter removes fine suspended solids. Both are needed in an apartment and in a house.

If the iron is dissolved rather than in suspension, mechanical filtration will not help. Dissolved ferrous iron passes through any screen filter and oxidizes only in the tank and on the plumbing fixtures. An iron removal stage with oxidation is needed here. Selection is based on the water analysis, not on appearance.

Check and service the water heater. Drain the tank, flush out the sediment, inspect the anode and the flange. If the anode is completely consumed and rust-colored spots are visible on the walls, the tank lining has already been compromised.

What to Do About the Odor

The sequence of actions is as follows.

Thermal treatment. Raise the temperature in the tank to 71 °C and hold it for several hours. This regime kills sulfur bacteria. Take into account the scalding risk; after the procedure return the working setpoint and make sure a thermostatic mixing valve is installed at the draw-off point.

Disinfection. Drain the tank, introduce a hypochlorite solution through the hot water outlet, hold for at least several hours, then flush until the smell of chlorine disappears. Bradford White recommends a hold time of at least three hours, preferably a day.

Anode replacement. The magnesium rod is replaced with an aluminum or zinc one. This reduces hydrogen release and breaks the reaction chain. The anode must not be removed entirely. The manufacturer states explicitly that removing the anode voids the warranty and shortens the service life of the tank. At an operating temperature above 65 °C, an aluminum rod is specifically recommended.

Eliminating stagnation. Design the system so that the water in the tank and in the recirculation circuit keeps moving. The British HSE, for legionella control, requires storing hot water at 60 °C or above, distributing it at 50 °C or above, and keeping cold water below 20 °C. These same regimes also suppress sulfur bacteria.

If, after all these measures, the smell returns within weeks, the cause is in the raw water. In that case a water treatment stage at the inlet is needed, not another flushing of the tank.

What this means for a project in Tashkent

• The water is hard and of low quality in approximately 85% of the city and in all other regions of the republic. Scale speeds up the destruction of the anode and creates stagnation zones under the layer of deposits, where bacteria multiply.
• Old central mains cause secondary contamination. Brown water after shutoffs and repairs on the network comes from the network, not from your water heater. A mechanical filter at the inlet removes most of the problem.
• Inspect the magnesium anode rod at least once a year, and more often on hard water. This is cheaper than replacing the tank.
• Before buying equipment, have the water analyzed at the address. Sulfates, iron and hardness determine whether you need water treatment ahead of the water heater.