My heater seems to start just fine. Blower comes on for a short time on low speed. Then I can hear ignition and heat comes out of stack. After a short time, the blower kicks into high gear and everything seems fine. Then a hear something actuate, and water seems to be flowing out of the heater....this is where it gets strange. It sounds like gravel being pushed through the line, and then the flow stops. The heater seems to stop firing, and after a minute or so the fan slows down, and the process starts all over again. After usually three times I get a service error and it says "Air Flo Sw". Any suggestions?