Sta-Rite SR200NA, about two years old quit on me. When I started the heater, I received an AFS error and heater would not ignite. I checked the air flow switch and it was working fine. After checking everything over I found there was water in the exhaust vent. I disconnected the heater, poured the water out, hooked everything back up and the heater works. My question is, where did the water come from? Is there an internal leak, or could enough water filled up through the vent to where it stopped some exhaust flow?