Fluctuating water temp. It is hot when I start the shower,

It *may* not be a problem with the water heater itself necessarily. When you say "keep adjusting the water temperature when you're taking a shower" I presume you're talking about turning the shower hot/cold knob(s) and not the temp thermostat on the water heater itself.
If so, when it starts to run colder, turn the knob all the way to full hot. Is the water then very hot? If so, the water heater is working fine & doing its job, and the problem is likely to be in the shower faucet mixing valve itself. Sometimes a faulty one will expand the inner seals/cartridge as hot water flows thru it, and starts to (wrongly) restrict the hot water flow, thus causing the constant need to "turn the dial up a notch".

The Rinnai will shut off it can't maintain the temperature you have set to within +/- 3 degs so I'd guess that might be the problem. The cause can be a few options. Check water flow first; any filters you have in tapware, main water flow etc. get cleaned. The Rinnai has a water line filter in the incoming supply, middle pipe under the heater just after the isolating valve. Gas supply could be the other option mainly being a possible intermittent fault in the gas regulator. Hard to check this one short of just changing it. Final option is some sort of electronic malfunction, pretty rare. I'd start with the water flow issue as you can do this yourself. Checking the gas will need a good technician with digital gas pressure guage to check incoming and working gas pressures; it might just need to be recalibrated. Just hasd a thought about the heat exchanger being scaled up if you are in an arera where water scale is a problem but this would normally take far less than 8 years to be a problem. Sorry can't be of more help; hard when it might be a few different causes.

Ok i intall tankless water heaters and sell them and repair them . The trouble your are hving is more than likely ,the mixing valve on your shower has a ajustment on it to prevent you from burning your self . that is the number one reason in showers when water never get to temp . What it is doing is addng cold water to your shower for you bummer . Take handle off and adjust water temp to your hottest setting . Some times you have to remove the low flow washer in shower head to get the right flow . also every time you start up your hot water it shouldbe set all the way ot other wise withe the battery 2 D models the unit will not sence flow and shut off the unit ,with some units it is a fine balancing act to get them adjusted right. If your buying a tankless water heater always buy a blower type unit that has a computer that runs it . You will be happy . If the Battery operated unit is for a cottage or a smallcabin then your just going to have to fix the pumbing to make it work right many of the safety features of the newer showers are not made to work well with tankless water heaters .

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

these units need lots of flow to start and run. Set the temp on the unit lower and the people in the shower will have to use more hot and less cold which will make the unit more stable.

Look for a water temp control on unit. Otherwise a lower temp can be attained by having a higher water flow to minimize temperature buildup in heating coil.

Problem new or old? How long you have unit? Sounds like not enough flow to keep burner on. Tankless need at least .6 gpm to keep hot water flowing or else the burner fires then shuts fires then shuts and you get a sandwich effect...hot cold hot cold. If you have new diverter valve in shower -vs- old two handle type try turning hot water ALL the way ON and see if hot stays hot. If it does stay hot then slowly adjust to cool and see if you maintain a good MIX for proper temp. If it goes cold again try this......LOWER control pad setting to a comfortable 105 degrees (ladies) 100 degrees(men) and turn shower valve all the way to HOT. See if your temp remains consistant now and if it does then problem solved. Tankless mindset...set temp on control pad and stop trying to MIX at shower valve. Thats how these things work. Thats why I install a control pad in each bath and one in the kitchen for adjusting ash temp during dishwasher on time. ditto for wash machine. You should never need to turn on cold water if your unit is set to proper temp at controller. Hope this helps

This is a great heater. It is however crucial that it be installed to every detail of the manual. If there is trouble send pictures to Bosch tankless you can call 1-800-283-3787 ask for Bosch Tech support or go to boschhotwater.com click find a contractor and call the closest Bosch Service Provider most will travel.

Hot Water Hot to Cold And back. I had that problem for 2 years. I had a very large shower head from the time I installed the tank-less hot water heater and the water temp would change from one extreme to another. Over time my shower head had stopped working as well as it did so I bought a smaller shower head and installed it and amazingly my water temperature quit changing temperature. I now turn my water to what temp I want and it stays there. It's been working correctly for 6 months now ... until yesterday and it quit working completely which is the reason I'm on here. lol

Sounds like there is not enough flow in order to keep the burner in the heater firing continuously