Upon arrival tenant told me that she was without hot water for 2 days and the error code on the tankless water heater was showing 79. I removed the cover plate to see the inside of the water heater and noticed that there was moisture inside the unit. I dried the unit with towels and hairdryer for a while. After drying, I reset the tankless and turned it back on. The unit was functioning correctly again and producing hot water into the home again. It’s hard to say exactly where the water was coming from. I spoke with Rinnai customer support to figure out what is causing the moisture. From the information I gathered from them is that it’s a process of elimination to figure out the problem. What could be causing the is a O ring that has gone bad. If the issue occurs again then the next step would be to order the replacement parts.