I have a Rinnai REU-55T water heater and need to know how to adjust the water temperature.

trouble shooting section will help https://sahotwater.com.au/specifications/Rinnai/Service%20manual/smV1616W_V2018W_V2020W.pdf

says on tag its lp

[PDF]Operation / Installation Manual - Rinnai

https://www.rinnai.us/.../V_Series_Indoor_25xx_U...

Rinnai America Corp.temperature (shown on the digital remote control) the gas burner will ... Without the connection of a remote controller(s), the Rinnai Water Heater is preset to.

PDF]Tankless Water Heater Service Manual - Rinnai

https://www.rinnai.us/.../100000098_Rev_B.pdf

Rinnai America Corp.Indicates condensing water heater models. ... Rinnai Water Heater Service Manual. 2 ..... Digital manometer or U tube type manometer with 14 inch water column (W.C.) scale, a hose and two .... will flash on the display of the remote controller.

http://www.rinnai.co.nz/tradesmart/product_108_rinnai_reu-55ft.html

Most tankless are preset to 120 degrees F.
30 degrees C is 86 degrees F.

If you want adjustable product, then tankless is not best choice.
Tankless are high tech gadgets with multiple interrelated parts controlled by sensors feeding information to control board. Everything internal and external has to be working correctly for tankless to work at all.
Most tankless brands do not suggest self-install, or self-adjustment, or any adjustment for reason given above.
You would be better off buying typical tank-type gas or electric and then thermostat adjustment is simple.
http://waterheatertimer.org/How-to-adjust-water-heater-temperature.html
http://waterheatertimer.org/pro-n-con.html
http://waterheatertimer.org/How-to-install-gas-water-heater.html

Add a comment for more free help.
Also take advantage of fixya expert assistance live.
For a price, expert works with you while you work on water heater or any do-it-yourself project.
Fixya is always less expensive than a service call.

The manual says:
http://www.coinwash.com/temp/rinnai.pdf

"For the name of the Authorized Distributor or Installer nearest you, please contact your local HVAC dealer or gas service technician, visit the Rinnai website (www.rinnai.us), call Rinnai at1-800-621-9419 or write to Rinnai, 103 International Drive, Peachtree City, Georgia 30269"

Manual says that the remote is for setting temperature.
When you turn on hot water spigot, tankless water heater will heat the water to the set point temperature.
If water heater is not set up to use remote, then tankless unit will heat to 140 degrees F.
"With the MCC-45-4US remote control installed the water temperature is adjustable from 96
to 180°F. The water temperature cannot be set to a temperature other than 140°F
without the use of the MCC-45-4US remote control unit. If you require a temperature
other than 140°F you must install the MCC-45-4US remote control unit."

"Error messages are displayed on the remote control units, simplifying service calls."

Page 9 of manual shows hot to set temperature using remote.
Water taps must be shut off to set temperature.
Circulating pump must be turned off.
"Simply press the H or C button until desired temperature is displayed"
"green in-use indicator will display"

If your unit is not working, the manual says to contact local plumber.
Fixya has specialists who will talk to you by phone about your problem for a fee.

This is an error code 12, not 12 degrees. You have a short on your low voltage side by the sounds of it.. First disconnect the controllers inside the heater. Turn of the power first, 2 screws top and bottom to take the frnot panel off. Follow the low voltage wires (not the power point ones) to the PCB on the ride side and disconnect them both. Panel back on, power on and try the heater again. If this fixes it you have a short circuit in the controller or the wires leading to it. If not the most likely short circuit is on the thermal fuse which loops around the heat exchanger. This will need an expert, Perth has plenty north and south of the river. Give Rinnai a ring first, they have some good staff in Perth who might help.

curios to find out if you had any help on this subjuct at all?