Power out for 48 hours, tankless water heater located outside of house, temperature around 20 degrees, no hot water coming out of tap, where do i begin?

possible bad power transformer or unit is overheating

-Sounds like every thing out side are frozen.
-You can not do any thing at this time. Just wait until the temperature rise and find out the water is running or not.

Good luck

Mai

You might have crossover problem caused by bad faucet cartridge.
http://waterheatertimer.org/Crossover.html

The green standby light should come on when 240Volt power is applied. Page 6 manual.
Both red lights should come on when tankless is activated. Page 6 manual

RTE 13 requires 60 amp breaker and 6 gauge wire
http://waterheatertimer.org/How-to-wire-Tankless-electric-water-heater.html
http://waterheatertimer.org/pdf/Rheem-TanklessElectricUC_AP15447.pdf

Factory temperature setting is usually default 120 degrees.
Adjust temps page 11 manual
Thermostat calibration is always approximate.
120 degrees is not hot enough to burn the skin.
Exceptionally cold incoming water temperature can cause tankless not to heat enough hot water: install tempering tank to preheat water to room temperature.
Tankless cannot be exposed to more than 11 grains hardness in water. Delime tankless yearly, clean water filter monthly, do not install low-flow restrictions or conserve water since the flow rate must remain high for tankless to activate.
Rheem does not post service manuals for their tankless water heaters.
Call Rheem 800 432-8373

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

you need to test the elements and stat for heatert

Before calling service technician, open following link for Rheem troubleshooting resources.
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html

List of causes MIGHT include one of following:
Bad household circuit breaker that powers electronics, or loose wire inside outlet where tankless plugs in.
Tankless fuses and control board.
Water flow rate too low. Minimum .8 gallon required (depending on model)
Tankless inlet filter clogged.
Household aerators clogged at faucets.
Crossover at single-handled faucet
If installed, there can be crossover at mixing valve.
Faulty household check valve, including check valve on re-circulation system if that system is installed.
Restriction in household water line, such as clog at elbow.
Unequal pressure in household hot and cold lines, caused by restriction in hot line. When single-handled faucet turns on, the cold water pressure is higher so hot does not flow and tankless will not turn on.
Loose wire connection inside tankless.
Unit temperature set too high.
Unit temperature set too low.
Water temperature is very hot coming out of tap, so more cold is added, but that reduces amount of hot water flow, so tankless turns off because flow rate is too low.
On some tankless, if water is preheated by solar or other indirect source, the burner will not activate.
High mineral content in water > delime tankless yearly
Misaligned temperature sensor
Low gas pressure. Wrong type of gas. Undersized gas line.
Too many faucets on at same time.
Faulty tankless water valve. Water valve stuck.
Fluctuating household water pressure, for example MUD and PUD district water pressure problems or water well pressure.
Faulty household pressure reducing/regulating valve if installed.
Faulty household pressure tank, if installed -or- pressure tank needs to be installed.

Here is the A7 code for this heater:

Outlet temperature sensor fault

(Temperature below freezing or

above 210°F).

1. Check red wire connections at hot water temperature sensor.

Clean terminals with an eraser. If badly corroded, replace sensor

and wire harness. See Fig. 65, page 48.

2. Sensor may trip if water temperature drops below freezing to

protect heater from freezing conditions. Any damage due to

freezing conditions is not covered under warranty.

3. Clean inlet filter screen and clear any debris out of shower

heads, faucet aerators and whole house filters.

4. In areas where water has a high mineral content, periodic
descaling may be necessary.

I would be very surprised if your water temp is below freezing, even in Alaska, check it with a thermometer. Well water is usually around 45 degrees F.
Let me know,
Gary

1. COLD MIX, HEATER DEACTIVATES
   If inlet water temperature is over 70°F due to geographic location, water maybe very hot out of the tap. This requires a lot of cold water to be added with it in order to attain a useable hot water temperature. The addition of too much cold water will overpower hot water demand from the tankless water heater. This slows the flow within the tankless water heater, decreasing it below activation point, which shuts off the heater. The end result is nothing but cold water coming out of the outlet. See: TWH E2-04 Overcoming fluctuating temperatures [PDF].
2. FLUCTUATING WATER PRESSURE
   If the water pressure in the home is erratic and the water flow is not consistent while a tap is opened, then the temperature of hot water will fluctuate. The minimum water pressure for the home should be 30psi or greater. For people on well systems the minimum pressure range is 30-50psi. The use of a pressure reducing/regulating valve is an effective way to maintain constant water pressure to the water heater. Watts brand 25AUB- ¾" or N35B-¾" pressure reducing/regulating valves or equivalent is suggested.
3. CHANGING FLOW RATE
   Major changes in flow rate can adversely affect the output water temperature of the heater. Increases from one major fixture running to multiple fixtures running at one time can cause the temperature to fluctuate. Stay within the heater's specifications. See: TWH E2-03 Outlet temperature vs. flow rate [PDF].

From the Bosch website.

Good Luck,
Gary

Rheem makes a number of tankless models
http://www.rheem.com/products/tankless_water_heaters/

They make a tankless 95 outdoor series
http://www.rheem.com/product.aspx?id=EBA80FAA-3C0F-411D-AF83-82061C28ED1B

The 95 outdoor series spec sheet says .4 GPM minimum activation and .26 GPM flow to keep burner activated. The colder the incoming water, fewer users can take a shower at once.
http://globalimageserver.com/fetchDocument.aspx?id=ea5f0816-dfd4-4bcf-a605-79945619f5bb
The .4 GPM and .26GPM numbers are fairly typical for tankless units.
These numbers are good when the unit is new.
After a few years the number may be different because the tankless is a high-tech machine with sensors that are susceptible to particulate in the water, and susceptible to perfect-functioning of other parts in the machine.
The outdoor series spec sheet says the tankless is good to minus 30 F.
But remember that's when the unit is new.
Outdoor tankless water heaters have an electric heater that keeps water pipes inside the unit from freezing, as long as electricity is ON.
If electricity goes off, the house stays warm inside for several hours.
But a steel box located outside would get cold immediately without electric heat.
The electric heater adds standby cost to operational expense.
The tankless has a heater, but you still have to protect the incoming and outgoing pipes.
If your incoming water is very cold, the tankless heater may not be able raise the temperature fast enough to give two showers -or- even 1 shower depending on age and efficiency and capacity.
You can add a tempering tank located in naturally warm area that passively pre-heats incoming water before it enters the tankless burner.
It might be best to locate tankless inside the house >> and use direct vent through the wall for both incoming air supply and outgoing vent gas.
If incoming air is taken directly from the living space, then tankless will draw air into the house through cracks and around doors.
A lack of sufficient air, or poor air quality can cause the burner to shut down.

When buying a tankless, read the spec sheets and really ask questions about everything you don't understand.

check to see if you have a blocked line on the hot water side

Its A Possibility The Left Over Water In The Hot Is Frozen. If Your Line Is Exposed Outside Somewhere It Can Easily Freeze Up. Turn On The Hot Water Everywhere In The House And If Accessible Check The Lines To See Where The Warmth Ends. You Can Leave One Off Over Night In Hopes That It Will Eventually Defrost.