Quote Originally Posted by dmenning View Post
The anode rod is aluminum. I think it's hydrogen, because it's only in the hot water.
Interesting --------- typically there is a chemical reaction with magnesium anode rods when there are high sulphates in the water causing hydrogen-sulfate gases ---- generally you would have a rotten egg type of smell.

Since this suddenly appeared with the new water heater try removing the anode rod and see it the problem disappears------- if it does then contact the manufacturer as to their recommended way of correcting this.