Every 6 to 8 months or so my water heater (A.O. Smith 'Power Shot' high-efficiency) stops heating. The first time this happened (mid 2010) a plumber friend discovered that the burner had some corrosion that had fallen onto it that was preventing the burner from firing properly. He showed me how to clean it off and replace it, and I have been performing this repair every time it's happened since.
It is an easy fix, but obviously this should not be happening. What could be causing these rust particles to fall onto the burner? Is there a repair I should look to have done? I can't see far enough into the heater to see anything above it that might be rusting out.
The heater is about 12 years old, and aside from this issue it works very well. Here is a picture of the unit in question, in case I have my terminology wrong. The small pile to the left of the assembly is the pile of rust particles I shook off the circular part.