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WaterHeaterCalc

Maintenance calculator

Anode Rod Life Calculator

Quick answer

Inspect a magnesium anode rod after about four years in ordinary water and after about two years in softened water, because softened water is aggressive to magnesium specifically. The rod is consumed on purpose: replacing it is what keeps the steel tank from rusting through.

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Result

Inspect after 4 yr Convention
Rod material Magnesium Convention
Then check every 2 yr Convention

Replace the rod when more than about six inches of bare steel core wire is showing. A flexible or segmented rod is the only kind that fits where there is less than four feet of clearance above the tank, which is most closets.

The formula

Inspection interval = base interval for the material, reduced for hardness above 10 and 20 grains per gallon, reduced by two years for softened water on a magnesium rod, and reduced for a setpoint at or above 140 F.

THIS IS A CONVENTION. The anode is consumed at a rate set by water chemistry, temperature and how much water moves through the tank, none of which a calculator can know. The only honest output is an interval at which to LOOK at it. Pull the rod and replace it when more than about six inches of bare core wire is showing.

The figures above are computed with these exact inputs: Water hardness 10 gpg, Rod material magnesium, Water is softened no, Tank setpoint 120 F.

What this calculator assumes

Some of these are published test procedures or code tables and some are planning conventions. They are not the same kind of number, so each one is labeled.

AssumptionKindWhere it comes from
Anode inspection intervalConventionAnode inspection interval. The rod is consumed at a rate that depends on water chemistry a calculator cannot know

Gear that matches this result

Every card below is in the page source. The calculator re-ranks them against your numbers and marks anything that falls short of what you just calculated.

Frequently asked questions

What does an anode rod actually do?
It corrodes instead of the tank. A water heater is a steel vessel with a thin glass lining that always has small imperfections, and the anode rod is a more reactive metal that gives up its electrons preferentially at those spots. While the rod has material left, the steel does not rust. When the rod is gone, the tank starts corroding, and that is what eventually puts a hole in it.
How often should I replace the anode rod?
Look at it after about four years in ordinary water and every two years after that, replacing it when more than roughly six inches of bare core wire shows. In softened water a magnesium rod can be consumed in two years, so inspect sooner. A thirty dollar rod fitted twice is far cheaper than a tank, which is the entire argument for doing it.
Why does my hot water smell like rotten eggs?
A magnesium anode rod in water containing sulphate reducing bacteria produces hydrogen sulphide, which is the smell. Switching to an aluminium zinc rod usually solves it, because the zinc suppresses those bacteria. A powered impressed current anode removes the cause entirely, since it is not consumed and does not feed the reaction at all.
Is a powered anode rod worth it?
In hard or softened water, often yes. An impressed current anode is not consumed, so it is fitted once rather than every few years, and it eliminates the sulphur smell outright. It costs several times what a sacrificial rod does and needs a power outlet nearby, so it makes most sense on a tank you intend to keep for a long time.

Researched guidance, not professional advice. These results are planning figures from published test procedures, code tables and manufacturer specifications, and they do not replace a sizing done by the person who will sign the permit. A water heater is a pressure vessel: never cap, plug or remove the temperature and pressure relief valve, and replace one that will not open when the test lever is lifted. Gas line, flue and vent work belongs to a licensed plumber or gas fitter, because improper venting produces carbon monoxide. Water at 140 F scalds a child in seconds. Turn off the breaker before touching an element, and never energise an element that is not submerged.