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Sacrifice Your Sacrificial Anode


By Leo Simpson

Click for larger image
This sacrificial anode is about five years old. Even though it’s not too far gone, for the purpose of this article we decided to have it replaced. Note that there is insufficient headroom above the tank to enable it to be fully removed. You would need a segmented anode for this job (the inset photo shows a non-segmented type – see overleaf).

In these days of “carbon pollution panic” and dire predictions of climate change havoc, electric off-peak hot-water systems are supposedly regarded as wasteful and to be avoided.

So much so that they are prohibited in new homes and there is a possibility that they will be banned from sale in the future for replacement of existing systems.

Either way, it is in your interest to keep your existing off-peak storage hot-water system going as long as possible. Replacing it will be costly and if replaced with a gas instantaneous or storage system, it is likely to be more expensive to run.

There are two ways to ensure long life in any mains-pressure storage hot-water system. First, keep the thermostat to as low a setting as is practical.

Normally, it should be set to no more than 60°C. Any higher setting causes increased thermal cycling stress in the tank itself, not to mention the  increased risk of scalding to infants and elderly people.

Second, make sure that the sacrificial anode is working, ie, being sacrificed to protect the tank. That’s if it is indeed still there – there’s a distinct possibility that it will have been worn away, either mostly or even completely.

What’s a sacrificial anode?

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A somewhat stylised diagram of a storage (mains pressure) hot water heater showing the sacrifical anode. It doesn’t show the heating coils, inlets/outlets or control gear.

By way of explanation, all storage hot-water systems, be they electric, gas-fired, solar or heat pump, use a steel tank which is lined with a vitreous coating. As time goes on, that vitreous coating is subjected to a lot of stresses and inevitably, very fine cracks develop and allow the hot water under pressure to come into contact with the tank and then corrosion starts.

Or at least it would, if the tank was not fitted with a sacrificial anode. As its name suggest, it is “sacrificed” and it corrodes before the tank does. The anode is usually made of magnesium, a metal which is more “active” (with a more negative electrochemical potential) than the steel of the tank.

All of which is good but if you leave the sacrificial anode for too long, it will be sacrificed too much and then the tank’s life is quite limited. Unfortunately, by the time you notice that the tank is leaking, it is too late to do anything about it and it must be replaced. That’s expensive.

If you live in Sydney or other Australian city or town with a “soft” water supply you can normally expect to get about ten years or less from a hot-water tank. Or at least, that’s what most people get because they don’t know about the sacrificial anode and its function.

Incidentally, sacrificial anodes are also found on ships, larger boats and even outboard motors, for exactly the same reason – they prevent the hull or motor being eaten away by electrolysis. But we digress.

In most hot water systems the sacrificial anode is in the form of a long (magnesium) rod which hangs down inside the tank and is suspended from the top plate. It is quite easy to inspect and replace and we will go through the steps in a moment. You can do it yourself with the only tool required being a socket spanner or if you are not confident about meddling with your tank, a plumber can do it.

At this stage we should state that most plumbers seem quite ignorant of the facts that first, hot-water tanks do have sacrificial anodes and second, that they should be inspected or replaced at specified intervals. And some plumbers take the attitude that if the tank is more than a few years old, it should not be disturbed in any way.

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An example of a sacrificial anode that really has worn right away. The central iron core is now clearly visible which means that the anode is now playing no part in protecting the iron tank. It would be very suprising if this hot water system was not badly leaking.

That’s silly. First, it’s an easy service for a plumber and second, they only have to read the manufacturers’ info to find out the details.

To illustrate, some 13 years ago a company specialising in servicing hot water systems contacted me and suggested that the sacrificial anode in our hot-water tank should be inspected and replaced. By that time, the tank was about seven years old. Inspection revealed that the anode was very heavily corroded but still intact and it was strongly recommended that it should be immediately replaced. I agreed and it was only about ten minutes work.

Fast forward some seven or eight years to late 2007 and one day I noticed that the outside of the tank seemed quite warm. In fact the top of the tank was more than warm; it was hot. I duly removed the plastic inspection disc at the top of the tank, only to discover that the insulation was quite wet.

Hmm. I had left it too long to replace the anode.

About a month or so later, the tank was clearly leaking and subsequently I had it replaced with a virtually identical 315-litre model.

As a matter of interest, the sacrificial anode had completely gone and its mating thread in the tank had heavily corroded, leading to the leak. Still, the good thing was that I had achieved about 15 years from the tank; quite a bit longer than the average of 10 years which is typical of a mains pressure off-peak hot-water system in Sydney.

But how much more life would I have obtained if the anode had been replaced in reasonable time?

Fast forward again, to July 2012, and we were about to have some major home renovations done and as part of the deal, the hot water tank was to be moved from inside the laundry to a store-room nearby. By this time the tank was only five years old but I decided to obtain a new sacrificial anode and have it replaced at the same time.

Now here is the tricky bit. Many storage hot-water systems are in rooms where there is limited headroom above the tank. A 315-litre tank is about 1.6 metres high (depending on whether it has been mounted on a plastic or concrete pad) while the anode itself is 1.4 metres or thereabouts.

You need more than one metre of head-room above the tank if you are to remove it without bending it. And even if you do manage to bend the old anode sufficiently to remove it, how can you manage to get the new one in?

Fortunately there is a solution: flexible segmented anodes. The segments are about 300mm long and so the anode can be bent (carefully!) to insert it into tanks with limited headroom.

In my case the limited headroom did not matter because the anode replacement could be done when the tank was being moved by the plumber. But for the purpose of this article, I purchased a segmented anode.

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