Sacrificial Anode Replacement: How Often, and When It Is Not Worth Doing

What the anode does, and what happens without it

A domestic storage hot water tank is a mild steel pressure vessel with vitreous enamel fired onto the inside. That enamel is the corrosion barrier and it is never perfect: it carries microscopic porosity from manufacture and develops fine cracks every time the tank heats and cools. Wherever it is broken, hot oxygenated water meets bare steel, and steel in hot water rusts fast.

The anode answers that. It is a rod of magnesium alloy, or in some units aluminium and zinc, screwed into the top of the tank. Magnesium sits further along the galvanic series than steel, so in water carrying dissolved salts it gives up electrons preferentially. The rod corrodes. The exposed steel at every enamel flaw does not. That is why the part is called sacrificial.

The consequence of letting it run out is blunt. The tank becomes the most reactive metal left in the system, and the corrosion that was eating a replaceable rod starts on the pressure vessel. The wall thins from the inside where nobody can see it, and then the unit lets go, usually all at once.

How often it actually needs replacing

The rule for a glass-lined tank on Adelaide mains water is a first inspection at about 5 years, then every 2 to 3 years, replacing once more than half the rod is consumed. Rod condition decides it. The calendar only tells you when to look.

Read the warranty document that came with the unit before relying on any general rule. Storage cylinder warranties are commonly conditional on servicing and specify their own interval, and that document is what the manufacturer reads if the tank fails in year 6.

The factors that shorten the interval are the difference between a rod lasting 8 years and lasting 3.

• Tank age. The enamel lining picks up fine cracks with every heat and cool cycle, so an older cylinder exposes more bare steel for the rod to cover. The same rod, in the same water, is consumed faster in a 10 year old tank than in a 2 year old one, which is why the interval shortens as the unit ages rather than staying fixed.
• A domestic water softener. Softening swaps calcium for sodium, raising conductivity, and it can consume a magnesium rod in a fraction of the usual time.
• Thermostat setting. Corrosion rate roughly doubles for every 10 degree rise, so a tank at 70 degrees ages its rod far faster than one at 60.
• Heavy draw. More cold water in means more dissolved oxygen in, and oxygen feeds the reaction. A household of 5 consumes a rod faster than a household of 2 on the same unit.

What Adelaide water does to an anode

Adelaide is not served by a single water chemistry. SA Water supplies different parts of the metropolitan area from different sources, blending reservoir catchment water with River Murray water by zone and by season, so hardness, pH and alkalinity vary between supply zones and through the year within one. SA Water publishes a drinking water profile by suburb and postcode listing the figures for your address, and it is worth 2 minutes before assuming a general rule applies.

The 2 failure modes pull in opposite directions. Harder water builds scale, which insulates an electric element, drives running cost up and settles as sediment in the bottom of the tank. Softer, lower-alkalinity water scales less but is more aggressive to the anode and to any exposed steel at a flaw in the enamel. A unit sitting on the softer end of the Adelaide range can consume a rod noticeably faster than an identical unit a few supply zones away, which is exactly why rod condition decides the interval and a general figure does not.

Coastal suburbs cause a different confusion. At Glenelg, Hove and Brighton, salt air attacks the casing, the flue cowl and the external fittings, and a rusty jacket reads as a dying tank. External and internal corrosion are separate problems: a weathered unit can be sound inside.

Which systems have an anode and which have none

• Electric storage, glass lined. Has an anode. The classic case, and the one that fails when nobody checks.
• Gas storage, glass lined. Has an anode. Routinely forgotten, because gas storage attracts less servicing attention than electric.
• Solar storage tank. Has an anode and consumes it faster, because these tanks run well above a standard set point on a clear Adelaide summer day. Roof-mounted close-coupled tanks make the service awkward, which is why it gets skipped.
• Heat pump storage tank. Has an anode on most models. Buying an efficient system does not remove the maintenance item, though plenty of owners assume it does.
• Continuous flow gas, also called instantaneous. No tank, no anode, nothing to check.
• Stainless steel tanks. Most carry no sacrificial anode, because the stainless itself is the barrier. Check the manual rather than assuming.

Powered anodes are the exception. A titanium rod and a small mains supply impress a protective current onto the tank instead of consuming metal, so nothing needs replacing on a schedule. The catch: it works only while the power supply works, and the sole sign of failure is an indicator light going out. Check that light when you check the smoke alarms.

Checking and changing it, and where DIY stops

1. Isolate the energy. Electric units go off at the switchboard circuit, not at a wall switch. Gas units go to off on the control valve.
2. Close the cold water isolation valve, then open a hot tap to release pressure.
3. Drain enough water to bring the level below the anode port, running the hose to a legal discharge point.
4. Lift the cap or cut back the insulation over the anode port on top of the tank.
5. Break the rod free with a 27mm socket on a long bar, bracing the tank so it cannot rotate or rock on its stand.
6. Withdraw and inspect. Replace when more than half is consumed, when the steel core wire is exposed over a long section, or when the rod is smooth and coated.
7. Fit the correct replacement rod, sealed and torqued to the manufacturer specification.
8. Refill completely and purge air from a hot tap until it runs steady before restoring power or relighting gas.
9. Record the date on the unit and on the invoice, so the next interval starts from a known point.

There are 3 things that go wrong on this job, and between them they are the reason it is worth a licensed plumber rather than a Saturday.

• The rod seizes. After a decade of heat and scale it can take more torque than the tank fitting is built to survive, and twisting the port out turns a maintenance visit into a replacement.
• Headroom. A standard rod is close to full tank height and comes out vertically. In a narrow Goodwood or Unley side passage with a low eave over the unit, that clearance is not there. Flexible segmented anodes exist for exactly that, and knowing to bring one is the difference between a job done and a job abandoned.
• Warranty. Many manufacturers require servicing by a licensed plumber and a record of it, so a DIY change inside the warranty period is a fast way to lose cover on the tank you were protecting.

When a new anode is not worth fitting

A new rod is the right call on most tanks, and there are 4 situations where it is not.

• The tank is already leaking. A tank that weeps or drips from the body has corroded through, and no rod reverses that. At that point the unit needs replacing.
• The unit is past 12 years and out of warranty. Servicing the anode on a tank with perhaps 2 years left in it puts money in the wrong place. It belongs in the replacement.
• The system has no anode. A continuous flow gas unit has no tank and no rod, so there is nothing to inspect or replace.
• The existing rod is barely touched. A rod pulled at 30% consumed goes back in. The correct outcome of that inspection is a recorded date and a shorter next interval.

The rule of thumb is timing. A new rod earns its keep on a tank young enough to be worth protecting: on a 6 year old unit it buys years, and on a 13 year old unit it buys an argument about whose fault the leak was.

What a proper anode service includes

• The old rod laid out where you can see it, and a straight explanation of its condition.
• The correct replacement for that unit and that water: magnesium as standard, aluminium and zinc alloy where the water has been smelling of sulfur.
• A test of the temperature and pressure relief valve, the part that stops a tank becoming a hazard and the one that seizes.
• A check of the tempering valve, since it delivers 50 degree water while the tank holds 60.
• A sediment flush while the tank is already drained.
• The service date recorded on the unit and on the invoice.

Marc is a licensed plumber and gas fitter with 20 years of hands-on experience. He texts an arrival time before setting off rather than leaving you a 4 hour window, the price is discussed before work starts, and if the rod comes out in good condition you will be told so.

Questions we get asked

References

• SA Water: your drinking water profile, hardness and pH by suburb
• YourHome (Australian Government): hot water systems
• Energy Rating (Australian Government): water heating