When to Change the Anode
Anodes are manufactured by extruding the magnesium alloy onto a central core of 3mm diameter mild steel. Where an anode is active and working effectively, it will appear sharply pitted and the "valleys" of this pitting will be grey in colour the same as the original magnesium. As the anode corrodes, it will wear down to the core and then progressively expose the core, usually from the top down. Ideally, the anode should be replaced before it has worn down to the core in order that there is sufficient surface area of magnesium to provide adequate corrosion protection.
As many as 50% of anodes become inactive, or passivated, and in these cases, the anode is not sharply pitted as above, but appears smoother and will be partially or fully coated with calcium carbonate, usually brown in colour or sometimes white. These anodes can be quite thick and appear to have sufficient magnesium left, but because it is heavily coated it is providing little or no effective protection to the tank and such anodes should be replaced.
Our experience tells us that, the longer and/or the thicker the anode, the longer its life. Nevertheless, all heater manufacturers are very definite in recommending that anodes should be replaced in order to ensure maximum effective life of their heaters.
Suggested Replacement Times of Hot Water System Anodes
Original Anode* 2nd Anode*
Dux 50 & 80 litre electric 5 years 2-3 years
Rheem & Vulcan 50 & 80 litre electric 6 years 3-4 years
Rheem & Vulcan 125 and 160 litre electric 7 years 3-4 years
Rheem & Vulcan 250 litre electric 8 years 4-5 years
Dux 250 litre electric 6 years 3-4 years
Rheem & Vulcan 315 & 400 litre electric 8 years 4-5 years
Dux 315 & 400 litre electric 7 years 3-5 years
Rheem 90 litre gas 7 years 4-5 years
Rheem & Vulcan 120, 135 & 145 litre gas 8 years 4-5 years
Rheem & Vulcan 170 & 185 litre gas 8 years 4-5 years
Rheem 200, 260 & 290 litre twin anode gas 9 years 5-6 years
SOLARHART 300 litre 10 years 7-8 years
OPTIMA 135 & 170 litre twin anode gas 10 years 6-8 years
OPTIMA 200 & 260 litre twin anode gas 10 years 6-8 years
OPTIMA 250, 315 & 400 litre electric 10 years 6-8 years
*These suggested replacement times of the original anode must be used only as a guide.
The life of an anode is dependent upon a number of variables:
1. Water Quality
Natural waters contain a wide variety of dissolved salts, all of which act as carriers of electric current. The higher the salt content of the water, the lower its electrical resistance and the greater the corrosion potential. Artificially softened waters are exceedingly corrosive, because the process merely substitutes a sodium ion for the magnesium and calcium ions and this takes away all of the scale forming ability, but does nothing to reduce electrical conductivity.
2. Size of Household
Hot water storage tanks are under constant pressure, so each time a hot water tap is turned on or off, the tank expands and contracts. Washing machine solenoid valves and ceramic mixer taps turn on and off more sharply than screw-type tap mechanisms which results in fast pressure changes inside the tank. These changes in pressure cause the glass lining of the tank to crack thereby exposing the steel of the tank to the effects of corrosion.
3. Method of Heating
Anodes in gas storage heaters usually last longer than those in electric tanks because gas heaters are typically operated at a lower temperature than electric heaters and having no copper element in the water means that there is less corrosion of the anode resulting from electrolysis.
4. Temperature Setting
Hot water is more corrosive than cold water, so the anode is consumed more quickly when the thermostat is set higher than necessary. It obviously consumes more heating energy as well. This point partially explains why anodes tend to wear more quickly at the top than elsewhere.
When is it too Late to Replace the Anode?
Most heaters should last around 12 to 15 years. There are always exceptions and we have seen heaters 20 or more years of age which are still providing good, clean hot water. On the other hand, we have also seen a few heaters which have rusted through as early as five years of age. Obviously both are extreme examples. Experience has shown us that, once rust gets started, it takes about three years for it to eat right through the tank wall. Therefore, the average heater only starts to rust at the 10 to 12 year mark.
For this reason, we believe 12 years should be the limit for replacing anodes in standard model heaters. Again, exceptions can be made when the heater looks in good condition and where the anode is still working. (That is when grey metal is clearly visible, indicating that anodic reaction is still taking place.) If water quality is tested and there is no discolouration, we see no reason why the anode should not be replaced. This is particularly applicable to Rheem Optima models which have a heavy duty anode in electric heaters and twin anodes in gas models as well as having a heavier coating of vitreous enamel. However, take special care with the twin element electric tanks as we have seen some of these models fail within the ten year guarantee period.