I have a $120 30 gallon electric water heater that I rewired so that both 4500 watt elements are on at the same time - it is used for radiant heat and the house hot water together. This unit flows into a 50 gallon non working $2500 gas Polaris water heater, functioning as a storage tank. I went nuts for a year trying different elements of every flavor and type, all of them burning out in @ 2 or 3 weeks. In my case it was not voltage but rather hard water. I could hear the 'bubbling' or boiling sound on the elements, and you could chip the calcium off the failed elements. And yes, I tried the so called low watt per inch double fold back elements and they failed just as soon with added fun of having to rip them out with huge vice grips since they turned into a unfolded mess.
Normal voltage in feed. Finally, I put in 2 of the cheapest high watt density straight elements. Then I turned the temperature down to about 125' . Boiling sound went away, and the unit simply stays on more often, probably constantly in winter when the circulating pump is on often or always pushing through 900' of 1/2 " pex in slab. I have not changed these $9 elements for 4+ years now. Surprised the hell out of me. When I finally take them out, I am sure the insulating layer of calcium that burned out the other elements has not been deposited on the elements. Either the flow of cooler water across the elements is the fix OR the increased watts per inch of elements causes the calcium layer to pop off due to element expansion, rather than gather like on the low watt density ones - totally contrary to conventional wisdom. A higher voltage element, like 250 volts might have done it also, but I saw none available.
FWIW - If I needed a 90 gallon water heater, I would buy 3+ $150 units and inline them. Likely many hundreds of $ cheaper than a 80 or 90 gallon heater with the benefit of redundency, 6 elements instead of 2 monsters, and the guarantee of some hot water no matter what happens. Also 3 anodes instead of one.
I was changing these so often I started to do "wet" changes. No draining - just fast work in and out of the element, would not do it any other way again.
