Have 9yo electric Rheem 81V52D hotwater heater (HWH) with thermostats set to 120F (assume upper and lower should be set the same), but water is too hot. About 2-3 months ago started noticing that I didn't have to turn the single-point shower value to mix much hot water. Same with other faucets in house. Also electric bill has climbed from around $300/mo (avg summer) to now over $500/mo. Given that nothing else much has changed as far as usage in the all-electric house (stove, oven, HVAC, lights, dryer, etc), I suspect one or both thermostats are faulty. How do I confirm that my problem is one of the thermostats or something else on the HWH? Can't find manuals to troubleshoot circuits and have no idea how to test HWH. I am very handy DIY and am electronics tech. Any help would be appreciated. Thanks.

You could test the temp of the water with a thermometer and if its above 120 and either the top or bottom thermostat is still sending power to the heating elements you know you have a bad stat. But, thermostats are extremely cheap, so I suggest just replacing both themostats and know you have the problem fixed. Trouble shooting electric water heater thermostats can be very time consuming as you need to wait for water to heat, then run water and wait for water to cool, then repeat the process over and over again just to make sure the stat is working correctly. I say save yourself the heartache and just replace both stats. Good luck and please let us know how things go.

I replaced both thermostats and the water is now around 120F. One of them must have been running away, but not enough to trip the relief valve. Thanks for all the help.

Thanks for the update. If you have more plumbing problems you know who to call. US!
Good luck, Toim

Glad you got it fixed.