Error message 10 indicates blocked supply/exhaust. Re-filled propane tank to make sure adequate gas to unit. Flushed coils and cleaned water filter. Took off cover and blew out supply air and supply air fan, burners and around coils. Found some mud daubber wasps inside the unit. How do I take apart the vent stack to the roof so I can clean that out?

I forgot to say this is a Rinnai heater, about 3 years old installed in a country house only used on weekends. It worked flawlessly until about 3 weeks ago.