Why Does My Hot Water Smell of Rotting Eggs? Sydney Homes Causes and Fixes.

you switch on the hot tap and the kitchen stinks of sulphur. The icy water is alright. Just the hot. Here’s what’s happening inside your tank and how to fix it.

Hot water smells like rotten eggs; this is a gas of hydrogen sulphide. It is a chemical change between the rod of sacrificial anode you have in your tank and the naturally growing sulphate-reducing bacteria in the water. The bacteria are not harmful: they are present in most water supplies at low concentrations, yet when they grow in a warm, low-oxygen atmosphere in a hot water tank, they generate hydrogen sulphide as a byproduct.

The odour is more frequent with magnesium anode rods than with aluminium ones as magnesium is a more vigorous reactor with the water and produces more hydrogen during the process-on which the bacteria thrive. It also tends to occur more in tanks that are not used frequently (holiday properties, periods of idle residence) where the water will be lying stagnant and the bacteria will be left multiplying.

Three Common Causes

1. The anode rod is reacting with the bacteria in the water

This is by far the most common cause. Hydrogen is formed inside the tank by the corrosion of the magnesium anode rod that is inside the tank. The Sulphate-reducing bacteria present in that water use that hydrogen and form hydrogen sulphide gas as a by-product. It is a natural process not dangerous but unpleasant.

The remedy: Use an aluminium-aluminium anode or a zinc-aluminium anode instead of magnesium. These materials create less hydrogen, leaving the bacteria without food. The smell generally fades in several days after the swap. The replacement also recovers corrosion protection on the tank.

2. The tank hasn’t been used or flushed in a long time

When the tank has had the same water a long time–sitting idle on a holiday, or vacant, or having had very little use–the bacteria have had time to multiply in the warm, stagnant water. Hydrogen sulphide concentration accumulates to the extent that it is felt at the tap.

The remedy: Wash out the tank. Allow the hot taps to run a few minutes to flush out the stale water. When the odor continues even after a complete flush, then the anode rod is probably the continuing root cause and ought to be evaluated.

3. The thermostat is set too low

Legionella bacteria can be killed by hot water, which is why even hot water tanks have a temperature of 60 C, and prevents the development of other microorganisms, such as the sulphate-reducing bacteria responsible for the smell. When the thermostat has been adjusted to 50OC or below (then maybe to save on electricity), the bacteria proliferates and the odour becomes worse.

The fix: Check the thermostat setting and restore it to 60°C. This is the lowest safe hot water level of storage that you can have at temperatures lower than that; you run the risk of bacterial growth. The tempering valve at the outlet controls the temperature of the water at the tap to a safe level, therefore the water will show 50 o C at the tap, yet it is maintained at 60 o C in the tank.

When the Smell Means Something More Serious

There are rare situations whereby the rotten-egg smell is a sign of something that is out of the anode rod. When it smells so strongly, when it occurs with a hot and a cold tap, when there is discoloration, the problem could be in the water supply itself, a contamination of the mains, or a cross-connection. When the odor is due to the hot tap and to specific fixtures, it is practically always the tank. Call Sydney Water should it be of all taps.

The rotten-egg smell is unattractive and often easy to remedy. The most frequent solution is an anode change where the magnesium is replaced with aluminium and makes your tank last longer. If you’ve been ignoring the smell because you thought it was “just the water”—it’s not. Your tank is saying that you need to tend to the anode.