Water heater keeps cutting off and flashing code 12, will work great for hours then do this, i have checked electrical and ground all ok.

Code 12 is a flame failure.
First step in diagnosing the root of the issue is to check for any separation, obstruction, or failure in the vent system.
Second step is to use the isolator kit under the heater to flush the heat exchanger with vinegar or rectorseal calci-free.
These two things are a great starting point to working on any tankless water heater. Flame failure can be as simple as the heat exchanger being clogged not allowing the sensors to work properly or the exhaust being mixed with the air intake. If the problem persists I would suggest having a Rinnai Certified Tech look at the heater.

Both gas and electric wtr htrs require electricity to work. Check your breaker or fuses to see if that may be the issue.

Cause: Negative pressure on vent system (pressure difference)
Solution: Check vent system. Vent system and air intake system must be installed according installation instructions. IF vent system is OK: Replace fan

1) Test elements, and if elements check out, then replace both thermostats.
http://waterheatertimer.org/How-to-troubleshoot-electric-problems-with-water-heater.html#new
http://waterheatertimer.org/How-to-troubleshoot-electric-problems-with-water-heater.html#warranty

2) Add a comment with result.

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

large water heater, turn p the themo stat to a higher setting ( watch for burns with young childern and older folks)

or the best install a tempering valve. this valve mixs hot water with cold water before it goes into the building.

Again if you have young ones or old people they can burn faster than any other group.the best is a larger heater for the DIY person.

Thanks Tom

The red reset button or ECO or energy cut off turns off power to electric water heater.
The ECO responds to temperatures in excess of 150 degrees F.

Preliminary work:
You can use thermometer to check water temperature immediately after ECO trips.
Compare water temperature against the thermostat setting.

There are 4 things that can cause ECO to trip:
1) Bad thermostat is sticking and letting element continue heating water, or ECO is reading incorrect temperature.
2) Factory installed insulation and cover missing from tank, and thermostat is exposed to cooler air and misreads tank temperature.
3) Element shorted to ground in center of element. This completes 120V circuit to ground which will cause element to continuously heat the water.
4) Loose wire near ECO causing high-heat electrical event.

Solution:
Check for missing insulation.
Check screw terminals for signs of high heat and burning >> tighten screws.
Replace both thermostats using detailed instructions given on link below.
http://waterheatertimer.org/How-to-replace-thermostat-on-electric-water-heater.html

If the problem continues after replacing thermostats, more troubleshooting is needed:
http://waterheatertimer.org/How-to-troubleshoot-electric-problems-with-water-heater.html

For more self-help information about your electric water heater:
http://waterheatertimer.org/Do-it-yourself-water-heater-timer.html

For more help, add a comment and I will respond

The reset button or ECO (emergency cut-off) is a safety device.
The ECO trips and cuts off power to entire water heater when it detects temperatures over 150 degrees.
This prevents overheating and eventual explosion of water heater.

The ECO will trip for 4 possible reasons:
1) Bad thermostat. Water temperature inside tank exceeds 150 degrees F because thermostat is sticking in ON position, and thermostat does not turn off element correctly.
2) Loose wire inside or near upper thermostat that causes ECO to sense higher temperature
3) Bad element shorted to ground near center of element, and short does not cause breaker to trip
4) Factory-installed insulation and cover are removed from water heater >>> cooler air causes thermostat to read wrong temperature, and thermostat keeps heating water inside tank

Steps you can take:
First thing is to check if factory-insulation-and-cover are present.
Next look for signs of burning at or near screw terminals. Tighten loose wire if found.
Especially check the lower element screw terminals.
Take a photograph of the parts so you can see the parts larger on your computer, and compare parts more easily when buying new parts.
Next most likely suspect are the thermostats.
For around $22, you can buy generic upper-and-lower thermostats at Lowes or hardware store > then replace the thermostat(s) and see if problem persists.
http://waterheatertimer.org/How-to-replace-thermostat-on-electric-water-heater.html

If problem continues after steps above
You will need to test water heater parts to track down the exact problem:
You need a inexpensive multimeter from Radio Shack, and the steps are simple, and you will know everything about an electric water heater afterward.
http://waterheatertimer.org/How-to-troubleshoot-electric-water-heater.html

Answer back if you encounter something not covered above, and I will help.

Maybe you want to think about getting a tankless heater.The gad unit works real good.But the electric water heater i want to think you should check the thermostate.Maybe it is struck and not cutting off every time.

12 - Flame Failure or Poor Ground

1. Immediate code 12 is usually a defective gas valve, power supply problem, or defective fan motor wiring harness.

2. If code 12 occurs after a few seconds it indicates that proper operation was established, but then stopped unexpectedly.

3. Verify that fuel is being supplied to the unit.

4. Check for proper electrical ground. Unit must use a grounded power supply.

5. Check for flue blockage on inlet or exhaust.

http://www.foreverhotwater.com/service-codes.php