No hot water comes into the house

most electric hot water systems use a heat transfer process where the element is in one tank and the house water is in another
the house water is what is being drawn off when you turn on a tap
there is a regulator valve at the top of the tank with a small lever that you open up to let water in and air out of the system
another point is that there may be a hot/cold mixer on the hot water outlet which limits the temperature of hot water to prevent scalding injuries in the bath and shower or sink
If you have one fitted they are known to malfunction and cut the hot water temperature down as, they mix too much cold water into the hot water pipe.
have a plumber check it out

The cold water enters the top, but the water is not released into tank at the top. The cold water goes through a "dip tube" This takes the cold water to the bottom of the tank. If you dip tube fails, or breaks, your faucet temperature will slowly drop as you use hot water (vs giving you 60% of tank at full temperature before temperature starts dropping) In normal operation, the cold water is released near bottom of tank, the hot water rises to top, and comes out of the top of the tank.

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I do not know how old this unit is or what model but many water heaters are not sophisticated enough to measure the temperature, but heat based on water flow and temperature rise. To start, tankless water heaters turn off every time you turn the hot water off. Lets use an example. You use the facilities, wash your hands then turned off the water and take a shower. You will bring the already heated water (from washing your hands) from the pipe up to the shower. Then the cold water that comes into the pipe from the reactivation of the water heater comes up the hot water pipe with warm then hot water just behind.

If this is not your problem, keep reading. Every unit I have been called to see were installed wrong. Check your installation manual to see if the problem with the system started with the contractor.

First, check the temperature of the water from the heater. If it is too hot and you are mixing too much cold water in, the unit will turn off because there is not enough water flow to keep it running.

Second, turn off the valve at the water heater and turn on the hot water at a faucet. If the water does not stop running, there is a "plumbing cross over" usually caused by a failing pressure balance valve in a shower.

Third, if the unit has not been serviced, go to bosch-climate.us to find a tankless water heating professional in your area. Totally Tankless LLC

Limit message is when the output water set temperature can not be reached, this is normally when the incoming hot water to the shower unit is not sufficiently hot. Service code 20 is something else inside the unit that is faulty, it could be a number of faults. Try reset Power down unit and re-start, it will either say Wait or Priming, after that it should restart, hopefully it will work. If not, you could have a internal fault with: The mixer valve/motor, the flow control valve or motor, the flow sensor. If you have hot water from other taps, then the mixer valve/motor is the most likely fault. If you only have cold water only from the shower head with limit message it could be the Hot solenoid controlled valve is faulty. Good luck as there are no spares available!

Sounds like a plumbing problem.

Since no tap gets hot water, chances are good the problem lies in the hot water pipes between the water heater to the first Tee.
Look for sediment lodged in water heater hot water pipe.
Or sediment lodged in elbow or Tee.

Other problems can be in play.
Faulty check valve or mixing valve if these are installed.
Faulty recirculation system if this is installed.
Also crossover at single-handled faucet where the cartridge is bad and lets the cold water flow into hot water pipe whenever another faucet is turned on. If you are getting cold water out the hot tap, and you know the hot water heater has hot water, then crossover is suspect > in addition to sediment blockage at hot water heater.

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Fixya is always less expensive than a service call.

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.

If this water is coming from the pressure/temperature relief valve on your tank type water heater, and it is piped from inside to outside of the house, you could try pulling up on the lever atop the pressure/temperature relief valve and releasing it. See if it sets properly. It may have to be replaced if I understand your problem accurately.
Gary

Believe it or not one of the sink or shower fixtures is by-passing when closed.Gravity circulation in standby mode(not being used) is the cause.Trace the (warm )cold water pipe to the fixture after no one uses the water for a while.Good luck.

I found a way to get warm water in the shower, but I'm not sure it qualifies as a solution. After trying everything else I thought that maybe when I was feathering the faucet in the shower toward the cold side so I would get warm water that at a certain point it blocked the flow on the hot water side enough so that the sensor in the hot water heater sensed there was no demand in the line and it shut the water heater off. To test this, I went to the kitchen and turned the kitchen sink faucet all the way to the hot side and then turned it on so that I got a small flow of water. I waited for the water to get hot to be sure the hot water heater was working. Once it was hot, I left it running to keep demand in the line and then I went to the shower and turned the faucet on. When the water got hot I started feathering the control towards the cold side and soon I had warm water. So I think I'm right in that the sensor in the hot water heater was shutting the water heater off when the demand in the shower was lowered to a certain point when I was trying to adjust the water temp in the shower.

So now I can get warm water, but it means I have to leave the hot water running at another fixture in the house to create a false demand in the system so the hot water heater doesn't shut down. Is there a way to adjust the senor in the hot water heater so that it will stay on when I'm using the shower only, so I don't have to waste water by running another faucet when I want to take a shower?

Some circulation pumps have a 3 phase mixing valve (cold,warm,hot) to regulate the water pressure in the building; no way to know unless you trace the plumbing. Basically, the hot water temperature could be lost because the circ-pump is cooling it with the cold water supplied to it. This is used to balance the pressure (aka Pressure Balancing Circ-Pump), so heater is really working overtime to heat the water. If the pump has two in and one out that's more than likely your problem; but you should consult a "old-school" plumber in your area.