Left red button has to be constantly reset. Also, water gets extremely hot

PDF]Tankless Electric (RTE) Troubleshooting - Rheem

www.rheem.com/docs/FetchDocument.aspx?ID=87d15e35-5adf-4e9f...
Tankless Electric (RTE) Troubleshooting. Technical Competence .... For four chamber units (RTE18 & 27) go to paragraph 4T. 9. Conduct OHMS resistance ...

Rheem RTE 27 Series ' Use and Care Manual

https://www.manualowl.com/m/Rheem/RTE-27-Series/Manual/296033
Rheem RTE 27 Series ' Use and Care Manual - Page 1. Use & Care Manual Electric Tankless With Installation Instructions for the Installer Water Heaters The ...

Replace upper and lower thermostats.
If problem is not solved, replace both elements.
http://waterheatertimer.org/How-to-replace-thermostat-on-electric-water-heater.html
http://waterheatertimer.org/How-to-replace-water-heater-element.html

Gene

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

Your Titan water heater most likely have a bad SCR. Check the link below:
Titan water heater doesn't stop heating after I close the hot water

ECO reset button trips for several reasons.

1) Easiest thing without testing, is replace both thermostats.
http://waterheatertimer.org/How-to-replace-thermostat-on-electric-water-heater.html

2) If problem persists, then test elements.
http://waterheatertimer.org/How-to-test-water-heater-element.html

3) If problem continues, do full troubleshoot
http://waterheatertimer.org/How-to-troubleshoot-electric-problems-with-water-heater.html

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Hi,
Yes, it is a thermostat problem and most any local hardware or big box store will have the repair parts...

Electric Water Heater not Working
heatman101

Have you reset thermostat on the heater, it will be the top thermostat right above the element. It will have a red reset button to press to reset. They will trip causing the heater not to work till it's reset or replaced due to thermostat gone bad. Chances are you may only have to reset it.

Please go to link below with to get some very helpful information on these topic's. If you replaced everything but the thermostat or reset it that's all left to hold out power to water heater. The thermostat is a safety device to protect the home owner. You may have had a element short out to trip it.
It will take 30 to 45 minutes to start to produce hot water after power comes back on.

http://www.water-heater-repair-guide.com/electricwaterheatertroubleshooting.html

Even though the water is very hot do you run out quickly? If so the problem is the 'dip tube' if not it is most likely the upper thermostat.

probably has lime and mineral deposits on thermostat and is not getting a correct reading turn off water supply and electrical - drain tank remove screws to heater elements and if build up try cleaning or if to bad replace same with the stat

water temp setting should be appx 120, any higher heat exchanger trips on overload and you must reset each time..