Navien NCB-240 combi --- mystery fix.

[Npreiser]

Last week my Navien unit stopped providing domestic hot water and flooring heat , but also provided no errors at all. On a hard reset , during power up it would do it normal purge..etc, no errors. Calls for hot water would result in nothing. The input flow sensor did register the correct flow rate changes based on the calls. After scheduling a service call , and staring at the unit for hours, I decided to remove and clean the combustion/burner unit window(next to the flame rod). The window was totally covered with ash..etc. After I replaced the window, the unit worked ..... Can someone explain this , it makes no sense why the flame rod would suddenly start working... ..