Inspect the magnesium anode every two years and replace it once about 60% of its mass is consumed or its diameter falls below 10–15 mm. In average mains water that means a new anode every 3–5 years; soft, conductive or heavily used water can consume one in 1–2 years. Most enamelled-tank warranties are conditional on documented anode checks.
Why does an enamelled tank need a magnesium anode?
Because enamel is never perfect. Firing the glass coating leaves microscopic pores where water can reach steel, and the magnesium anode protects those spots electrochemically: as the less noble metal it corrodes sacrificially, so the tank wall cannot. The protection lasts exactly as long as the anode has mass left to give. Once it is consumed, the pores in the enamel coating stand undefended and pitting corrosion works through the wall from the inside — unnoticed until the tank leaks, and by then irreversible.
How often should the anode be inspected?
Every two years is the standard interval, and it is written into most manufacturers' warranty conditions for enamelled cylinders — keep dated photos or the installer's invoice as proof, because a skipped inspection is the most common reason a corrosion claim fails. Make the first check at the two-year mark, then let the measured wear set the rhythm: an anode half gone after two years in your water deserves annual checks, while one barely touched can stay on the two-year cycle. Solar cylinders running hot summers and large families with high turnover sit at the fast end of the range.
When is the anode actually due for replacement?
Replace at roughly 60% consumption — in practice, when the diameter has fallen from the original 26–33 mm to about 10–15 mm, when the steel core wire shows through, or when the surface is deeply pitted and crumbling. Electrical testing does the same job without opening the tank: an anode tester measuring the protective current between anode and vessel flags a dead anode in seconds, and permanently installed inspection anodes make the check a routine service item rather than a half-day drain-down.
What makes an anode wear faster?
Water chemistry sets the pace. Soft, low-mineral water is the most aggressive because it is the least buffered; high conductivity accelerates the electrochemical reaction; storage above 60 °C and heavy daily turnover both feed the process; and a water softener upstream shortens anode life noticeably.
| Water type | Typical magnesium anode life |
|---|---|
| Hard, mineral-rich mains water | 4–6 years |
| Average municipal supply | 3–5 years |
| Softened or naturally soft water | 1–3 years |
| Well water or variable quality | Check yearly |
Treat the table as a starting point and the two-year inspection as the truth — no rule of thumb survives contact with a water softener or a new municipal supply blend.
Should you switch to a powered anode?
An impressed-current titanium anode replaces sacrificial protection with a small controlled current: it never wears out, needs no scheduled replacement, and stops the rotten-egg smell that magnesium can produce with sulphate-reducing bacteria in soft or well water. The trade-offs are a higher upfront price and a permanent mains connection that must never be switched off. For hard-to-access tanks, aggressive water or smelly hot water it is the clean fix — but never simply remove a magnesium anode without fitting replacement protection, and resolve any bacteria issue with a proper anti-legionella heating cycle rather than by lowering the storage temperature.
Frequently asked questions
Can I check the anode without draining the tank?
Partially. An anode tester measures the protective current from outside, and tanks fitted with an inspection anode allow checks with minimal water loss. A conventional screw-in anode still needs the pressure released and a partial drain to withdraw and measure.
What happens if the anode is never replaced?
Once it is consumed, the enamel's microscopic pores expose bare steel and pitting begins. The tank typically perforates within a few years — damage that is irreversible and, because the inspection was skipped, usually outside the warranty.
Why does my hot water smell like rotten eggs?
Sulphate-reducing bacteria reacting with the magnesium anode in soft or well water produce hydrogen sulphide. The fix is a powered titanium anode plus a thermal disinfection cycle — not removing the anode, which would sacrifice the tank's corrosion protection.
Do solar water heaters have anodes too?
Yes — enamelled solar cylinders, including thermosiphon tanks, carry the same magnesium anode and follow the same two-year rule. Higher summer storage temperatures tend to put them at the faster end of anode consumption.