The water is hot when only hot water is turned on . but when I try to add cold it goes cold to mix with cold goes cold

SOURCE: banging from boiler when hot water turned on.

Install a knocker in the line and bleed the air out

SOURCE: Only hot water in sink/basin when toilet is flushed

I had a customer a few weeks ago that had the same problem. When the water heater was installed they crossed the supply lines. They had the inlet as the outlet. Esentially, the water heater was plumbed in backwards. Also there could be a miss piping somewhere in the house but it would probably be near impossible to find it right now without tearing up foundations. Also check that the faucet is not plumbed backwards as well. Especially if this is a single handle faucet that you are referring to.

SOURCE: Bosch AquaStar 125b ng suddenly goes cold during shower

Does it happen to any other fixture? If not, it's most likely your shower valve and not heater.

SOURCE: 125FX (with FXhood) turns off while in use

may be overheat stat or fan check little hose at fan area for pinhole best of luck

SOURCE: I have a problem with my Rinnai hot water heater

We had the same problem - the hot water would only be hot for 3 -5 minutes and shut off, then we had to turn the water off, wait a few minutes, and do it again. VERY FRUSTRATING.

Solution: The Rinnai serviceman showed me the heat-exchanger blower/fan: It was clogged with dust and moths.

Another clue: Our Rinnai was extremely noisy when the heat came on. After the fan was cleaned and replaced, it is nearly silent.

If you are reasonably mechanically inclined:
TURN THE BREAKER / POWER OFF to the unit.

The blower-fan is located in the lower central area of the unit (at least on our unit). If you draw imaginary north-south, east-west lines through the center of the unit, the fan is in the lower-left quadrant. There is a spring clamp securing the black rubber accordian-like tube to the fan. There are two screws that secure the fan to the unit. there are two plugs that connect to the fan, as well as a third air-flow sensor that should be removed. then you can look at the fan unit, as well as the grill on the underside of the heat-exchanger that should be cleaned.

My thought was to go out and get a simple bug-screen, and place it over the air-input outlet vent going outside, but the repair guy said that although it may sound good, if that screen ever gets clogged, or reduces airflow to the Rinnai, then the unit will work harder, or even shut off, giving the same problem.