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WANT TO $AVE REAL DOLLAR$ IN THE FUTURE?
Sacrifice you
Sacrificial A
Do you have a mains-pressure storage hot water system? Is
it five years old or older? Then you probably should make
arrangements to have its sacrificial anode replaced, to
ensure at least another five or six years life before it fails.
Replacing the anode is straightforward, cheap and will save
you lots of money.
I
n 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 mainspressure 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?
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
14 Silicon Chip
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”
ACCESS COVER SEAL
SACRIFICIAL
ANODE
THREADED
TOP
SACRIFICIAL
ANODE
CUT-AWAY OF
PRESSURE
TANK
OUTER
TANK
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.
siliconchip.com.au
By LEO SIMPSON
ur
Anode!
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 nonsegmented type – see overleaf).
(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
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a few years old, it should not be disturbed in any way.
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
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.
November 2012 15
Here’s a “segmented” sacrificial anode, designed
to be inserted into the tank where there is
insufficient headroom to fit the “straight” variety.
Each of these segments can bend with respect to
one another and re-straightened as it is inserted
into the tank. It’s important to note that the anode
must not touch the walls of the tank inside or it
may actually contribute to rapid corrosion, not
protect from it.
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.
Where to buy a sacrificial anode
You need to contact a plumbing supplier with the make
and model of your hot-water tank, and if possible, the
recommended anode type, which should be in owner’s
handbook. If not, contact the tank manufacturer.
You’re most unlikely to find a sacrificial anode at your
local hardware store, even if it does have a good range of
plumbing fittings – it is most unlikely to stock sacrificial
anodes.
In fact, you may well find that the plumbing specialist
store will have to order one in for you. However, this is
16 Silicon Chip
not foolproof: they need to know what you want!
After talking about sacrificial anodes, another of our
staff members was prompted to change his. So he went
into a local plumber’s “wholesaler” (a major store with
large amounts of stock) and asked for a sacrificial anode to
suit a Siebel Eltron WA300 Heat Pump hot water system.
The instruction manual for this heater says to check the
sacrificial anode after 12 months – he’s had it for nearly
three years. And heat pumps cost far more to replace than
conventional hot water systems!
The (usually quite knowledgable) girl behind the counter
looked at him as if he was talking Swahili and turned to
the other salesman to see if he knew what was being asked
for. Blank looks abound! So they asked a plumber who was
also being served at the time and he got this quizzical look
on his face while shaking his head.
The girl then said “I’ll have to ask Bob when he comes
back from lunch and ring you back”. Bob, as it turned out,
was the manager of the place who had been a plumber
most of his working life.
Apparently he asked “why would anyone want one of
those?” but at least knew what a sacrificial anode was! The
end result was that they couldn’t locate one from any of
their normal suppliers.
Our guy then rang the importers and was told that it
was a bog-standard 1.2m “black top” anode and “anyone
could sell you one of those”. He told the bloke on the
phone his tale of woe at the wholesalers, who responded
with comments along the lines of what we said earlier in
this article. “Plumbers don’t replace them,” he said, “but
that could be because it’s much more lucrative to replace
a whole hot water system when it starts to leak.”
If that’s not an indictment of an industry, then what is?
But he was able to steer our guy in the right direction
– “call Reece Plumbing Supplies,” he said. They should
be able to help you.
He was right – they could and did (although they had to
order one in, as even then it was not a normal stock line).
And if you don’t want to DIY?
If you do not want to do the job yourself, contact a local
company who specialise in hot-water service, or even the
manufacturer of your unit. Most have service departments.
But be prepared for a long silence when you tell them
what you want!
DANGER: Hydrogen build-up in hot water tanks
When changing a sacrificial anode, or even when using a pressurised hot water system after a period of non-use (eg, returning
from holidays) be careful of hydrogen build-up in the tank.
Hydrogen, a colourless and odourless (but highly explosive)
gas, is released from the anode as part of the chemical reaction
which allows it to protect the tank. In normal use it is eliminated
from the tank by the flow of water (you may experience a sputter
at times when you turn the hot water tap on).
Therefore this is not normally a problem when changing sacrificial anodes because you remove the pressure (and hydrogen
gas) by turning on a hot tap and activating the pressure release
valve but you should avoid smoking and also remove any other
ignition sources before starting work.
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The steps you need to replace your anode
[a] Check the specifications label on your tank. You need to know the anode type required (eg,
black or blue; dependent on water quality) and the brand, model and size of tank. If the anode
type is not listed, your supplier will need the other information to get the right one.
[b] Having obtained the required sacrificial anode, check that
its length is about right for the tank; it must not be too long.
If it touches the sides or bottom inside the tank, it can cause
rapid corrosion – exactly what you don’t want!
[c] Turn off the electricity supply at the switchboard by
removing the hot-water fuse or switching off the relevant
circuit breaker or main switch (or both).
[d] Turn off the water supply with the stop-cock on the cold
water inlet.
[e] Release the pressure inside the tank by lifting the lever
on the pressure relief valve near the top of the tank. Lift it
gently and release it gently; do not let it snap back into place
because that might damage the valve seat.
[f] Open one or more of the hot water taps in your home to
bleed off a little water. Not much should come out.
[g] Remove the plastic inspection cover in centre of the top plate
of the tank. You should see the black nut (typically 11/16 inch).
[h] Use the correct socket to loosen the nut. Do it slowly; just
in case you did not do the preceding steps! No water should
leak out.
[i] Before removing the old anode, check that its threaded
section at the top is identical to the replacement anode. If it is
not identical, you will need to re-install the old anode and get
the correct replacement.
[j] OK. So is the old anode heavily corroded? If not, you may
want to leave it for another year. Before screwing it back into
place, remove the old Teflon tape from the threads and run a few
turns of new tape around it to ensure a good seal. Do it up tight but
not overly so. Remember that you will want to remove it in the future.
If the anode is heavily corroded, it should be replaced and if there
is limited head-room, you will need to carefully bend it as you lift it
out. Do not apply any leverage to the threaded section in the tank
as you do this. Mind you, if the anode is heavily corroded there may
not be much of it left so removing it should be easy.
[k] Now you need to insert the new anode. If you have a straight
anode and no head-room limitation, you can just lower it into the
tank. Note that some anodes will be supplied with Teflon tape already
wound on to the threaded section. If this is not present, you must
wind several turns of Teflon tape around thread, in the same directions as the thread. Almost invariably, that is clockwise.
If you have restricted head-room above the tank,
you will need a flexible (ie, segmented) anode. It must
be inserted into the tank so that when it is fully in, it
is not touching the inside (of the tank). You do this by
straightening each pair of segments as they are lowered into the tank. When you are sure
that nothing is touching inside, tighten (but do not over-tighten) the nut.
Then reverse the other steps of the removal process. That is, turn on the water supply to
the tank, let the water run briefly from one or more the taps inside you house and then check
that no water is seeping from around the top of the new anode. When everything is hunkydory, snap the plastic inspection cover back into place and restore the electricity supply. SC
siliconchip.com.au
November 2012 17
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