J n o d tankless Electric hot water heater model #XFJ120FDCH water will not heat is there a reset button or something or what to do?

Question edited for clarity. Looks like you encountered autocorrect

The manual is here. It doesn't mention a reset button.

Rheem RETEX-27 instruction manual and user guide

https://mans.io > Rheem > Tankless Electric Water Heaters

Rheem RETEX-27 Performance 27 kw Self-Modulating 5.27 GPM Tankless Electric Water Heater download pdf instruction manual and user guide.

But this says it does, or may have?

Where Is the Reset Button on a Rheem Hot Water Heater? On Rheem water heaters, the reset button is usually a red button located near the thermostat, at the middle of the limit switch. To access it, you will need to remove the metal panel on the unit. The red button is behind the insulation.

How to Reset Rheem Tankless Water Heater - Quick Fix

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 ...

Reset Button.....be nice if you gave us a brand/model to go with.
If you mean the small GFCI inside the heater, you'll need to get a Tech Service Call and call Mfrs Tech Support as something is short to ground. This can't be done remotely as you have to track currents and voltages and resistances with a multimeter.
Tankless usually draw less than 2A even when fully on so doubtful you're overloading a general house circuit.
Might even be the small interior GFCI that's bad. That's why you need Tech Support.

Do you have 3-phase water heater? Is this balanced or unbalanced 3-phase unit. Add comment and describe product in detail, including brand since i have some service resources for both types.
Or do you have ordinary residential split-phase 240Volt Bosch tankless computer that heats water, and has 2 or 3 separate elements, and a separate circuit breaker supplying power to each element? And testing across both hot leads coming from each breaker yields 238Volt?
Copy following link for Bosch tankless manuals, with brief troubleshoot section that does not provide detailed service procedures for repairing tankless computer:
http://waterheatertimer.org/How-to-wire-Tankless-electric-water-heater.html#manual
If this is new water heater, then call installer. If you installed yourself, then remove tankless and return to seller for refund.
Tankless computers require certain amount of water flow to activate computer sensors. Turn on Hot tap and listen to pipe to hear if water if flowing through hot pipe. Check for crossover:
http://waterheatertimer.org/Crossover.html
Clean water filter usually located just above where cold water connects to unit.
Delime tankless ... and use water softener. Tankless cannot be exposed to more than 11 grains hardness in water, or parts will fail within 2-3 years. Simply deliming tankless heat exchanger each year will not maintain full efficiency... although for typical family of 4, tankless comper water heater costs $100 more to operate than tank-type heater according to manufacturer's promo literature and independent studies.
Call local technician who specializes in fixing tankless computers. For plumbing problems call local plumber. These are two different things.
If tankless computer is older, then cut losses, and buy tank-type electric water heater at Lowes for $285 and make quick changeover and have hot water same day. Sell old tankless on ebay and cheat next guy, or sell at scrapyard for $3 per 100.
Gene
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If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

How to wire tankless electric water heater. Reading required.
http://waterheatertimer.org/How-to-wire-Tankless-electric-water-heater.html
Add a comment and describe more detail, including product and number of terminals, including information shown on product label.

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

Tank-type Electric water heater?
Press reset button.
http://waterheatertimer.org/How-to-troubleshoot-electric-problems-with-water-heater.html

Add comment and describe water heater, and more detail about problem

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

10-22-12
Copy following links to help identify product and download possible troubleshoot resources:

Tank-type gas
http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html

Tank-type electric:
http://waterheatertimer.org/How-to-troubleshoot-electric-problems-with-water-heater.html

Rheem tankless:
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html

Other types of water heaters include boiler, tankless electric, indirect, solar-assisted, solar, hybrid electric, heat pump, wood-burning, gas hybrid, 3-phase commercial electric.

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

If you have tankless electric, it can simply be insulated... but tankless electric are seldom mounted outdoor.
Service bulletin says: "the only way of assuring protection is to completely remove the heater during those times the area may experience freezing temperatures.

There are tankless with built-in heater for exterior installation.
Check your manual, or add a comment with specific model number and maybe I can find the information.

Here's why they install tankless outdoors: Because tankless burn more fuel per gallon of hot water. This requires larger gas line and larger vent pipe than typical tank-type gas water heater that is usually installed indoors.
They install tankless outdoors, so there is no need for expensive vent pipe, and because there is ample incoming air to support combustion.
So if tankless does not have factory-installed internal heater, then the unit can freeze easily when the combustion draws in cold air.
Of course you can put heat-tapes on the pipes but the heat exchanger inside will quickly freeze during periods when hot water is not being used, and combustion is not taking place.

You can move the tankless installation inside and use correctly-sized vent and intake pipes directly through the wall. This will generally protect tanklesss from freezing if room is heated adequately.

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

how is heating element ? Wasit on with no water running thru it. Does it have a reset?

You need to get a real tankless water heater, I had one of this brand and what a waste of money, when you call the company they do not speak very good English and if you talk to Richard whom is supposedly the VP he is as dumb as a mule.