We have an electric Reliance 606 640dors water heater. After moving in we discovered the hot water was set too hot so we reset the temperature to 120. After doing so, the temperature dropped but after getting a shower and washing dishes we ran out of hot water and the hot water heater stopped working. We replaced the upper heating element as directed by an online guide and had hot water again. Unfortunately after getting a shower and washing some dishes... we ran out of hot water and the hot water heater stopped working again.

Could anyone please help me troubleshoot my hot water heater?

I applaud you for attempting to repair your water heater; please make sure you are working safely!
Most dual element water heaters are wired for non - simultaneous operation. That means the upper tstat / element will heat the water until the upper tstat temperature is met, then the upper tstat / element will be de - energized, and the lower tstat / element will be powered; if the lower tstat or element are faulty, no water heating will take place... a more in depth explanation of electric water heater operation ( a
And maintenance ) can be found at " waterheaterresue.com " .
Also, please make sure to operate your water heater at a minimum setting of 140 degrees, lower temperature settings in tank type water heaters can provide the perfect environment for the very dangerous Legionella bacteria. A thermostatic tempering valve should be installed if there are concerns about the delivery of scalding hot water...

hot water heater stopped working again.
Please explain. Does that mean that you had not hot water for a period? Does it mean you had no hot water no matter how long you waited? Does it mean you had to reset the over temperature (red button)?

Mike mentioned turning the temperature up to 140F, and as much as he is correct (and appropriately mentioned the thermostatic mixing valve) it is not something we see done very often across the country.

Ideally, the water heater would be set to 140F and that 140F water would be individually piped to the dishwasher (DW manufacturer's like this) and a thermostatic mixing valve would be installed and reduce the water temperature to 120F to be delivered to the rest of the fixtures in the home.

Unfortunately, most plumbing codes do NOT require this to be done as presented. To this day, I am not exactly sure why that is??

Just FYI guys!

Mark