I have a recently installed E110CP that has an E29 error code issue that won't go away. Sometimes it comes up when the boiler is under heavy load (heat and DHW) and other times it seems completely random. The unit will go into "fan check" mode and then throw the error if the fan stays on too long. Rinnai already had us replace the fan once already, but that didn't change anything.
I know E29 is a ventilation issue, but the vent is a short concentric ubbink kit (about 8 feet) with 1 90 degree elbow so I don't think there's any issues with that. There's nothing outside the house to cause any obstruction.
But if it is a 29, negative pressure in venting, there may be a bad pressure/draft switch or improperly hooked up switch. Silly as it was, a new rinnai right out of the box had a plug in the condensate trap, caused quite the mess the first day the boiler run!!! ;)
Don't blame me, I voted for the old war hero and the business expert!!!!