Rinnai RUC98i Tankless problem

northil

My water heater has been working fine for the past 9 months. Suddenly about a week ago I noticed that the unit that was normally quiet was making boiling noises, even after the call for hot water was over. Note you can clearly hear the boiling noises coming through the concentric vent which is completely new. The boiling noises continue for a few minutes after each time the unit is used as well as during. Two days ago I finally started hearing a beeping noise and went to look at the unit while the hot water was running and I noted a CODE 10. We also started experience a sudden loss of hot water after only a few minutes of running. If we turn the hot water faucet completely off and then turn it back on we get a few more minutes of hot water before it turns cold again. I was sure that there was no blockage in the concentric vent line. I even disconnected the line from the top of the unit and i could clearly feel good airflow and saw no blockage in either the top of the unit or in the line. Furthermore when I removed the cover from the unit and turned on the hot water, you could clearly hear the boiling noise loudly and then again another CODE 10. It's driving me crazy! Does anyone know what the problem could be? I cannot see anything wrong. I checked the water inlet screen and it was nearly clean but i rinsed it anyways and the gas pressure is 3/4 pipe and strong. The fan does sound like its running but there is a loud boiling sound.