I have a problem with my Rinnai hot water heater giving me intermitant hot and cold. The water gets hot and I get in the shower. A few minutes later the hot water stops and I have to shut of the hot water...
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.