The Rheem water heaters say

The Rheem Performance Platinum 50 Gal. Electric medium water heater provides an ample supply of hot water for households with 3 to 5 people. This unit comes with two 5500-Watt stainless steel heating elements and an automatic thermostat which keeps the water at the desired temperature. Plus One Water Management control includes multiple user modes and water sensor with audible alarm. A 240-Volt electrical connection is required for installation. A patented magnesium anode rod design incorporates a special resistor that protects the tanks from rust, providing longer tank life. Factory installed temperature and pressure relief valve and 3/4 in. water connections are included with the water heater for easy installation. This model has a 12 year limited tank and parts warranty plus a 3 year in home labor warranty. This Rheem Performance Platinum model is Wi-Fi compatible allowing for remote monitoring of the water heater's operation (Wi-Fi Module REWRA631EWH sold separately).

Ok, so that's how a sacrificial anode works. What the hell is a "special resistor" and how does that provide longer tank life?

I have written Rheem now on this a couple of times, with no response. They market this all kinds of ways calling it a special resistor,

Here on their site for Professional Ultra Low NOx Powered Damper,

Longer Life: Patented magnesium anode rod design incorporates a special resistor which protects the tank from rust.

In their spec sheet,

Longer Life: Patented magnesium anode rod with resistor protects the tank from rust

Doing a Google patent search showed CA 2569779 C: Improved resistored anode construction.

But, I have no idea if it works or if it's nonsense, and how effective it is.

  • Did you ask them?
    – user22865
    Dec 21 '15 at 8:08
  • 4
    This doesn't strike me as on-topic. The question at hand is "What does Rheem mean by 'special resistor'?" Once that question is answered, then it may be on-topic to ask if a device that meets that definition can successfully protect from corrosion. But without that definition, the question is otherwise unanswerable, as far as I can tell.
    – Flimzy
    Dec 21 '15 at 9:54
  • The only way to find out what Rheem mean is to ask them. Possibly asking them for the patent number would tell you. Dec 22 '15 at 14:29
  • @Flimzy updated. Mar 8 '16 at 20:54
  • @DJClayworth I tried, no response. Mar 8 '16 at 20:54

The "special resistor" doesn't protect the tank from rust, the sacrificial anode does. The special resistor - or any resistor for that matter - makes the anode last longer, which has been recognized for a long time.

Rather than interpreting the sentence in the advertisement as:

A patented magnesium anode rod design incorporates a special resistor that protects the tanks from rust, providing longer tank life.

The sentence in the advertisement should be interpreted as:

A patented magnesium anode rod design incorporates a special resistor that protects the tanks from rust, providing longer tank life.

From the patent cited in the question:

In order to control the rate of consumption of a sacrificial anode, various anode constructions have been previously proposed in which a resistor is incorporated in the anode, and electrically connected between the anode and its protected tank, to automatically regulate the electrical current generated by the anode during its operation and thereby increase the service life of the anode.

The "special" part about the resistor is the design which is intended to be mass-produced and less-subject to failure during manufacturing and use. The key description from the patent is as follows:

A hollow metal cap receives the end wall section of the sleeve member, and the assembly further includes an electrical resistor having a body portion supported on the end wall section of the sleeve member and a lead wire conductively anchored, illustratively by soldering or spot welding, to the core portion and having a curved stress relieving portion, representatively having a generally U-shaped configuration, received in the through-opening. The receipt of the stress relieving portion of the lead wire in the sleeve end wall through-opening inhibits this portion of the lead wire from being deflected outwardly away from the sleeve end wall during the soldering or spot welding process, or thereafter, and contacting and shorting out against the metal cap when the sleeve is subsequently inserted into the metal cap.

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