Replacing a water heater anode: the cheapest part that saves your tank

Short answer: you should replace the anode in a hot water cylinder as soon as more than half of it has been eaten away; so have it checked every two to five years. The anode is a magnesium or titanium rod that sacrifices itself so that the steel tank does not rust, and it is the cheapest part of your cylinder. A new anode costs € 35 to € 97, a new tank twenty times as much. Replacing it takes half an hour to an hour: depressurise, partly drain, remove the old anode, fit the new one with a new seal.

Why there is a piece of metal hanging in your water cylinder

A cylinder is made of steel with an enamelled inner lining. That lining is never completely perfect: it contains microscopically small pores. At those spots the steel would come into contact with water and start to rust. The anode prevents this with a simple electrochemical principle: of two metals in the same water, the less noble one dissolves. Magnesium is less noble than steel, so the anode corrodes and the cylinder stays intact.

This also means the anode is, by definition, used up over time. It is not broken when it has been eaten away; it has done exactly what it is there for. The catch: once it is gone, the cylinder is next in line.

When should you check it?

The rule of thumb is every two to five years, and more often the more aggressive the water is or the higher the cylinder temperature is set. An anode check really belongs in the annual service of an indirect cylinder or a heat pump water heater; ask about it, because it is not done as standard everywhere.

Signs that it is getting too late: brownish water from the hot tap, a rotten-egg smell (a known consequence of a depleted magnesium anode in combination with certain types of water), or traces of rust at the connections. A system with active anode protection can also report this itself; A.O. Smith has a separate message for it, anode protection not working, where the appliance still works but the protection is off.

Three types of anode

De magnesium anode is the classic and most widely used type. It works without electricity and is slowly eaten away. The titanium anode or powered anode runs on a small DC voltage and does not wear out; you do not replace it, but its electronics can fail. Heat pump water heaters sometimes have a combination.

Part	Manufacturer number	Price
ATAG anode, 21.3 x 110 mm	65103768-01	€ 34.95
A.O. Smith magnesium anode, 584 mm	0183463023(S)	€ 49.95
A.O. Smith powered anode BFC	0197709(S)	€ 96.95
Stiebel Eltron titaniumanode, 420 x 3 mm	333297	€ 95.95

All variants can be found under anodes. Pay attention to the length, the diameter and the thread; those three determine whether it fits. For a cramped installation there is also a chain anode, made up of segments so you can insert it without needing a lot of space above the cylinder.

Replacing it: how does it work?

The cylinder is depressurised and partially drained, the old anode is unscrewed and the new one fitted, with a new seal. While at it, the engineer checks the limescale at the bottom and flushes the cylinder if necessary. Allow half an hour to an hour of work.

You can do it yourself on a free-standing electric cylinder with enough space above it: water off, power off, drain, swap the anode, refill and vent. With an indirect cylinder or a heat pump water heater there is more involved and an installer is the wiser choice.

The calculation

An anode costs a few tens of euros and half an hour of work. A new cylinder, depending on type and capacity, easily costs twenty times as much plus installation. Yet in practice the anode is rarely replaced on time, simply because nobody thinks about it as long as hot water comes out of the tap. Put it in your diary alongside the service of your heating system; it is the cheapest way there is to extend its life.

Frequently asked questions

How often should you replace the anode in a water cylinder?

Have the anode checked every two to five years, more often with aggressive water or a high cylinder temperature setting. If more than half of the rod has been eaten away or the core is visible, replace it. A titanium or powered anode does not wear out and does not need replacing.

How do I know if my anode is used up?

Only by taking it out and looking at it. If the core is visible or most of the rod has disappeared, it needs replacing. A visual check during servicing is the only reliable method.

Why does my hot water smell of rotten eggs?

This is usually a reaction between a magnesium anode, certain bacteria and sulphate in the water. A titanium or powered anode usually solves the problem. Do have the temperature setting checked first, though.

Does every water cylinder have an anode?

Almost every enamelled steel cylinder does. Stainless steel cylinders usually do not need one. Not sure which type you have? Check the rating plate or ask us, stating the make and model.

Will my warranty be void if I do not replace the anode?

Many manufacturers make periodic anode checks a condition of the warranty on the cylinder. So keep your service receipts; without proof of checks, a warranty claim for a leaking cylinder is difficult.