I think I have narrowed this down to a water heater issue, but here is the background...

Well water with slight iron content and low pH. Softener and ph filter installed. No issues for the first year of use. Then started getting sulfur smell from faucets/shower. I shocked the well and entire system (filters and water heater) with chlorine and the water was fine for 3 months. Now the smell is returning.

I have done more research and found that I do not smell it in the cold water.. even after running cold shower for a few minutes. Switch to hot and it is immediately noticeable. So, my assumption is that it is being caused by the water heater.

I have an electric water heater in series with a gas Polaris high efficiency stainless steel tank water heater. The electric is not energized, and is for future hookup to allow my geothermal to preheat the water... but for now, it is just a passive pass through for cold water. It DOES have an anode rod. The main gas Polaris does NOT have an anode rod since it it stainless steel tank.

So, does the sulfur reaction with the anode rod also take place without heat and could the electric tank be the cause of my sulfur smell? Or is heat required, and since there is no heat in the electric tank it is not the cause... and since there is no anode in the gas stainless steel tank that is not the cause... and I should keep on looking?

Any advice would be appreciated.