[timun]

Originally Posted by Geraldsh

Using an aluminum rod in an aluminum heater negates the purpose of the rod. In an aluminum heater you definitely need the magnesium rod. Aluminum rods is for steel or iron heater.

An aluminum heater? WhoTF builds an aluminum heater? Granted I don't work on residential stuff, but I've never seen an aluminum tank...

Originally Posted by Wormius

Maybe it isn’t a massive difference, but it sounds like the magnesium ones last longer and do a better job protecting the water heater.

Magnesium rods will tend to "do better" in the sense that Mg it's more electronegative than aluminum.

That said, because it is more electronegative and more readily corrodes, it will also react more readily with minerals in your water supply. What can happen with Mg anode rods in hard water—like Calgary's—is that the Mg rod reacts so readily with mineral ions in the water that it becomes covered in scale and stops effectively protecting the tank long before the supply of Mg is actually depleted. I've seen functionally useless Mg anode rods pulled out of tanks covered in white crust.

As such, more often than not, in my (admittedly institutional/commercial/light industrial) experience, anode rods sold in the Calgary market are de facto aluminum.

I should preface that before I posted, I didn’t know you could use an aluminum one where the magnesium rods were recommended by the water heater manufacturer (Rheem).

I'm surprised Rheem has any particular material recommendation at all.