Is thermostat replacable on 81v40 - Rheem 40 GAL ELEC TALL Water Heater 6YR SC 82VH402

Double check the thermostat connection and setting; if it persists, I would add a pipe hugging thermostat connected to a shunt trip for the heater elements.

There is no reliable test for electric water heater thermostats
Replace thermostats
http://waterheatertimer.org/How-to-replace-thermostat-on-electric-water-heater.html

Gene

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Bad thermostat?
http://waterheatertimer.org/How-to-replace-thermostat-on-gas-water-heater.html
http://waterheatertimer.org/How-to-replace-thermostat-on-electric-water-heater.html

Incoming water too cold
http://waterheatertimer.org/Tempering-tank.html

Bad faucet cartridge:
http://waterheatertimer.org/Crossover.html

Gene
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If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

1) Rheem 81V40 electric water heater ... manufactured pre 2009 ... posted serial number does not match manufacturer
http://waterheatertimer.org/pdf/Find-Age-of-water-heater.pdf
http://waterheatertimer.org/pdf/Rheem-Ruud-2009-water-heater-parts-by-model-number.pdf page 5

2) Possible date of manufacture 11-1994

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Replace thermostats.
One of the thermostats is sticking causing overheating that trips ECO reset located on upper thermostat. There is no good way to test thermostats since testing means turning the temperature setting which temporarily unsticks bad thermostat so it looks like it works ok, so replace both thermostats.
http://waterheatertimer.org/How-to-replace-thermostat-on-electric-water-heater.html

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Try replacing the thermostat.
Cut back the burned wire, install new thermostat.
http://waterheatertimer.org/How-to-replace-thermostat-on-electric-water-heater.html
If problem persists, do full troubleshoot.
http://waterheatertimer.org/How-to-troubleshoot-electric-problems-with-water-heater.html

Open following link to see typical electric wiring for water heater thermostat.
http://waterheatertimer.org/How-to-wire-water-heater-thermostats.html

Always take photo of thermostat wiring before replacing thermostat.
http://waterheatertimer.org/How-to-replace-thermostat-on-electric-water-heater.html

I would replace both thermostats.
http://waterheatertimer.org/How-to-replace-thermostat-on-electric-water-heater.html

There are two other possible suspects:
1) Factory-installed insulation and covers have been taken off tank so thermostats exposed to cool air and misread temperatures and cause overheating

2) One of the elements is shorted in middle of element and still heating after thermostat turns off, plus upper thermostat ECO reset is not tripping because it is defective.

If replacing thermostats doesn't fix problem, then test elements, and/or do complete troubleshoot:
http://waterheatertimer.org/How-to-test-water-heater-element.html
http://waterheatertimer.org/How-to-troubleshoot-electric-problems-with-water-heater.html

Add a comment for more free help.
Also take advantage of fixya phone service.
For a price, expert speaks with you over phone while you work on water heater or any do-it-yourself project.
Fixya is always less expensive than a service call.

If water heater thermostat is bad, buy a generic thermostat at hardware store and install it according to instructions shown on link below:
http://waterheatertimer.org/How-to-replace-thermostat-on-electric-water-heater.html
I usually replace both upper and lower thermostats for about $28.
Lowes sells thermostats and elements.
If your water heater is a single-element/ single-thermostat, you can buy any thermostat with red reset button.
Technically a single-element thermostat has 1 less screw, but either thermostat will work.
If your water heater is 120V or 240V, the same thermostat will work of either voltage.
This is true because the thermostat is a simple bi-metal heat-reading device that does not operate on electric current.

That being said, you might want to test parts before replacing anything, in case problem is element.
To do comprehensive electric water heater troubleshoot:
http://waterheatertimer.org/How-to-troubleshoot-electric-problems-with-water-heater.html

Image below shows generic thermostat with reset button. Each of these will work for single-element or dual-element tanks with either 120V or 240V. Wires connect just as they did on old thermostat. Tighten wires very tight. Press in reset button to make sure ECO is set. Put insulation and cover back over thermostat when finished.

I would try it again and if water temp is still to high then I would adjust thermostat down to say 120 degrees and see what that does.If problem continues I would replace thermostat. Thank you