It was not getting hot and this unit was showing a code 12?

When a code 12 occurs with the Rinnai RU199iN it is indicating that a flame failure has occurred on the system. There are a few reasons that this may be happening. Such as, loose gas connections and fittings, debris and dirt clogging in the fan blower and gas burner, moisture in the tubing that connects the gas combustion chamber to the valve, or incorrect gas pressure supply for your hot water unit. If you have any further questions we recommend that you give us a call. Thanks!