Installed an ARISTON point of use hot water heater. Water is warm when faucet is first turned on turn just barely warm after a minute. Is there a way to adjust this heater, I misplaced the manual that came with it or didn't get one.

I am assuming that you are referring to a calcium or rust build up in the water heater.
If you are having this issue you will need to turn off the water to the heater. You will need a garden hose, access to a outside water soucre, cold water is fine.
Shut off the gas or power to the hot water heater before starting.
Now you will need to place the garden house on the drain fitting on the water heater, placing the end of the hose outside and lower than the drain valve. Open the valve and drain the heater, you will need to open a hot water faucet to help drain the heater. I don't suggest opening the TPR ( Temp Pressures relief )valve due to the fact that they sometimes leak or don't reseal after opening them.
Hopefully the drain valve is not clogged, you should hear and see water exiting the garden hose.
If you are not getting any water from the drain valve you are going to have a challenging task.
If no water is coming out, my recommendation is to get the required tools to remove the hot water line from the top or side or the heater, also a few towels to soak up water A bucket is helpfull also.
remove the hot water line, then you will have to use a small hose to siphon the water from the tank. Once the tank is empty you can then remove the drain valve and replace it.
once you have replaced the drain valve you should ensure that it's closed and use a Gallon of CLR. Poor this into the water heater through the hot water outlet where you removed the pipe from. Now make sure the faucets are closed and replace the hot water line. You can now turn the water back on to the water heater.
After a few minutes open a hot water faucet to let out any air until water flows out, than close the faucet.
Let the water heater set for a few hours, then drain the tank!
when the tank drains, close the drain valve again and refill the water heater. Once again open a a hot water faucet and let the air out till the water heater fills. Again let sit for a hour.
At this point open a hot water faucet at the farthest point from the heater and let the water run for 5 min. , than close it and move to the next closest faucet and repeat until all faucets have been opened and run for 5 minutes.
If you still have a slow flow in a faucet, you may need to repeat the CLR step.
I also suggest that you remove all faucet screens and clean them.
If you still have a clog than you are going to have to remove or replace a piece of water line in the house. The blockage may be hard to find but if you have access to most of the pipe or it's visable you may be able to hear the blockage with the water on. Otherwise this will result in using some water line taps and a pressure guage to find the blockage.

Luke warm water is the result of top element bad and the bottom is the only one heater. The best solution I can reccomend is to change both out due to the fact the bottom element may go bad and since you have to change one you might as well change both.

Turn power and water off, drain heater down and once water is drained out change them out. Once new ones installed turn water on to fill heater and open faucets and get air out on the hot side of each faucet once water is flowing out of each faucet you are now ready to turn power back on.

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.

Tankless water heaters normally take a little longer than a regular heater to provide hot water because they have to start and get warmed up. I'm guessing the heater uses a pilot light and that the short amount of hot water when you turn on the tap is hot because when the water is sitting still in the heater the heat from the pilot light is enough to get it hot. The tub faucet will get hot a lot faster than the shower because it lets a lot more water through and empties out the pipes faster than the shower. You could probably speed things up by letting the tub faucet run on completely hot for like 10-15 seconds and then turning on the shower.

that's a faucet problem, nothing to do with the tank
either replace the faucet washer (The one that's noisy)
or get a diffeent style faucet

WE THOUGHT WE NEEDED ANOTHER THERMOSTAR, BUT WHEN WE WERE LOOKING FOR THE PART THE EMPLOYEE ASKED WHAT WAS THE TROUBLE. WE EXPLAINED, HE SAID YOU DEFINATLY DO NOT HAVE A PROBLEM WITH YOUR TANK AND ASKED IF WE HAD A SINGLE HANDLED FAUCET WITHIN THE HOUSE. NO WE DONT. HE SAID IT SOUNDS LIKE WATER IS MIXING WITH THE COLD WATER THATS THE REASON FOR THE WARM--COLD--WARM WATER. COME TO FIND OUT SINCE WE ARE REMODLING THE BATHROOM, WE HAD BOTH THE HOT AND COLD WATER TURNED ON IN THE SHOWER--BUT WERE PLUGGED OFF AT THE SHOWER HEAD AND THE FAUCET UNTIL THE TILE IS INSTALLED. SOOO ALL MY HUSBAND HAD TO DO WAS TURN OFF THE WATER IN THE SHOWER. NOW WE HAVE PLENTY OF HOT WATER, NO WARM COLD. YEA!!!

Absolutely no hot water indicates the top element or thermostat on the heater. Check to make sure that you have 240 volts on the top two terminals of the top thermostat. If you do then check the top element for voltage. It should also read 240 volts between the two screws. If you have 240 volts there shut off the power, drain the heater to a point below the top element and change the top element. When the element is installed turn your WATER back on and let the air bleed out of several hot faucets in the house. When the faucets stop spitting air you can turn them off. Check for leaks around the element. If you have no leaks turn the power back on the the heater and replace the covers.

That should get you 'back in hot water' in two hours.

Mike

Something is wrong with the power . You check the connecter and you will proabbly find only side is making.

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?