I read with interest your article on cylinders. We too have the same problem – a Peter Cocks mains pressure stainless cylinder installed 4 Feb 2011. Our elements blow on average once a year due to corrosion. I have replaced the elements so many time that I have had a plumber recently rotate the cylinder in our cupboard so the elements could be more easily accessed – I change my own elements and it has sped up the process significantly. But that cost around $200+ and each element costs around $70. I think out hot water supply costs more in elements than electricity! It was perhaps Murphy’s law, but the element would always blow when it was least convenient for me to repair so I have installed elements not suited to the application but that was all that was available at the local hardware on those weekends. Your letter has prompted me to look further into the problem.
Usually the metal sheath surrounding the element heating wire corrodes right through. The attached picture shows how the nickel coating has corroded but so has the copper element tube. The bottom of shiny new cylinder is now littered with metal debris and fragments of the ceramic insulation I can’t reach to remove. Sometimes the elements last only six months and the most recent failure was after only three months. We have two elements in our cylinder – the top element is not powered but still failed with corrosion albeit after a longer period of time.
As an Engineer, I have reviewed the Electrode Potential of the various metals involved. It is no surprise that the nickel and tin plated copper elements didn’t last as these metals corrode galvanically compared to the copper base metal of the element tubes. The copper corrodes galvanically relative to the stainless steel cylinder casing. I was recommended to use an incoloy element but it didn’t survive much over 12 months which was disappointing as that is normally a special corrosion resistant alloy. The latest attempt is another incoloy element with a grey protective coating, around $110 + GST.
To have galvanic corrosion take place, there needs to be the electrical circuit created by the metals and there needs to be an electrolyte (it performs the same function as the acid in older car batteries). Pure water is a poor electrolyte so there has to be a dissolved chemical in the water supply to enable the electrons to flow and make the corrosive electrical circuit. This may be the key issue for Cashmere as it doesn’t happen in other areas of the city. The manager at Peter Cocks was very familiar with the issue and I have attached a water quality analysis test he provided me.* Hopefully someone can comment further on this water quality. This test was dated 2012 post EQ. It would be interesting to have another current test for comparison.
An Engineering colleague experienced in hot water cylinders has suggested that the element electrical earth be checked for its real earth potential. If the earth functioned well, then it should dissipate this small galvanic potential and reduce the corrosion. There are cases where earth wires have been disconnected by the earthquakes or building repairs. The resistance between a house and the “earth” should be no more than 10 ohms, this can be measured by electricians.
Traditionally we have all relied upon the network of metal plumbing supply pipes buried in the ground to provide earth bonding and back up the earth stake for each property. Most modern water pipes being installed including those in the Council network are plastic so the traditional earthing capability of plumbing systems may no longer be as effective compared to the past. The hill suburbs have a problem where apparently many earth rods don’t reach into the ground far enough to get to wet ground because of rock. This isn’t the problem for us as I can reach ground water at 700 mm depth in our part of Bowenvale valley.
Our next step of remediation after Christmas will be to install a low cost sacrificial anode in our cylinder that corrodes before the element of the cylinder. We are fortunate to have spare connections on our cylinder but others may not be so lucky.
Another plumbing issue that I have stumbled over in Bowenvale is very high water pressure. Mains pressure is rated 450 kPa (65 psi). On the nights where the pumping stations are refilling the hill reservoirs (dry summer evenings) we can receive water at 950 kPa (140 psi). It was such a problem that I installed a pressure regulating valve to correct the problem. I couldn’t find maximum water pressure supply limit from the CCC, only a minimum.
Symptoms included hose fittings blowing apart, washing machine solenoid valves making a lot of noise but worst of all was the toilet cistern ball cock valves. Typically (late at night) as the ball cock was about to close, the flow was very low and the pressure very high. The valve would “scream” with a high pitched tone that would loudly echo around the whole house! Such high pressures are not covered by typical appliance warranties.
I hope this adds to the discussion. It would be good to have support from a larger group such as CRA to ask CCC what is in the Cashmere water supply.
* See Cashmere Water Analysis Report
Editor’s ref. http://www.argusheating.co.nz/metal-sheath-elements Argus Heating Ltd, 211 Maces Road, Bromley, Christchurch. See esp. http://www.argusheating.co.nz/heavy-duty-tank-heating-element-water-heaters