What fuel source is used by Guardian water heaters from Rheem?

Guardian is the name of Rheem's fuel vapor ignition resistant technology, so it only applies to their gas models. Thus, there are no Rheem Guardian models, only gas models with Rheem's FVIR technology.

FVIR technology is designed to prevent gas vapor that exits the unit from igniting. These systems also detect the buildup of gas vapor outside the combustion chamber and shut off gas flow to prevent buildups and unwanted ignition. Many manufacturers, including Rheem, produce gas and electric models that use similar tanks. Because of this, those looking for an electric version of a gas Rheem water heater have options to choose from.