Over the past year, hot water keeps going out. Pilot easy to ignite. Gas kicks in. Hot water for a few hours or even a day but then shuts off. Once I noticed pilot was still lit, but flame was gone. FINALLY had gas valve replaced, and repairman showed me white corrosion on tube in old gas valve, but problem still occurring. Enclosed metal vent goes through wall to furnace and up through chimney. Could something have fallen into chimney/vent and be blocking it? Nothing I have read on any websites addresses this. Most say to replace thermocouple and valve. One site says there might not be enough air in combustion chamber, insufficient combustion air supply,or inconsistent gas supply, or clogged vent system but doesn't address HOW to fix . Who fixes that? Why didn't repairman look at that?