Here is more than you ever wanted to know about fixing my tankless water heater.
A few years ago I bought and installed a Poloma model RMTG-53DVP tankless water heater in my home. This exact same model is also sold under the brand names of Rheem, Ruud and Richmond. After operating for about a year it shutdown and started flashing error codes C7, 13 on the remote control. If I unplugged it for a while it would work again but would only run for about 3 minutes before shutting down again. Some digging around on the WEB indicated this was a oxygen sensor fault. In other words the oxygen sensor was telling the controller there was low oxygen. Never having worked on a tankless water heater before I called a plumber that advertized fixing tankless water heaters. He came out and we had a nice conversation and basically recommended I fix it myself. For this I paid for a service call. Apparently he was not familiar with that brand of heater and did not want to figure it out.
Now a tankless is way more complicated than a conventional water heater with a microcontroller and lots of sensors and valves.
Here is my heater opened up. At the top is the copper coil assembly where the water gets heated. The plastic with the black lines on that is an electric heater that keeps the water from freezing if the temperature gets too low. Below that is where the burner assembly goes. The burner assembly and manifold is removed in this picture. Below the burner assembly is the controller, valve and fan. There is a whole bunch of screws that have to be removed to get thing apart.
The oxygen sensor work by monitoring a flame burning in a special chamber. There is a precision orifice that controls the amount of gas burning and a thermocouple to monitor the flame temperature. The controller waits for 3 minutes after the heater starts to check the flame temperature so that everything is stabilized. If the temperature is too low the controller shuts down the heater and sends an error code to the remote control.
This is the burner assembly. On the left is a round brass hole. That is where the sensor flame burns.
So what could cause the problem? Obviously there is plenty of oxygen and nothing blocking the air flow into the heater. The thermocouple could be bad but those thing are very reliable. The controller board could be bad (horrors). After a bit of poking around and inspection I discovered that there was some crud deposits around the precision orifice.
It did not seem like the orifice was blocked that much but since I could not find any other problems I cleaned the orifice and reassembled the heater. That solved the problem.
Since then I have had to clean the orifice about once a year. Something in the propane is forming the crud. They also recommend cleaning the water lines by running vinegar through the heater with a pump for about an hour. I have done that once. It did not seem like there was very much lime build up though.