Gas Water Heater, Soot and Bad Flame

Hi,
A friend who owns her own home asked me to look at her water heater. The pilot had gone out, and she called the gas company. They sent someone out who replaced the thermocouple (right term?). Not long after, she noticed a bad smell and sooty scorch marks above the burner cover. She turned it off immediately of course.
I have looked at it and read the instructions. The pilot lights and stays lit. The burner flame lights when it is turned on, but the flame is very orange and sputtery. Flames lick all around the underneath and threaten to come out the burner cover. It improves slightly if the damper is closed, but the room still smells scorched and sooty. The burner does not appear clogged, though the inside of the chamber is very sooty and caked.
This isn't behavior you like to see in a major household appliance. Should she expect to have to replace the unit, or is there a reasonable shot at repair? It is an older unit of unknown age.
Thanks, Harry
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Harry Smith wrote:

The "bad smell" is very characteristic of poor combustion. In this case, either the chimney is blocked or (likely) the heater's central flue has collapsed.
If you remove the draft hood, you can look down the flue in the heater to discover.
Flue collapse is caused by old age and/or very high tank pressure.
Jim
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Thanks Jim.
I remember you from the aircooled VW group. Speedy as ever. ;) Anyway, I placed my hand over the exhaust outlet on top of the heater and could feel hot air coming out. But judging from the amount soot on the bottom it could easily be half plugged inside. I suppose we'll just have to take it apart and see?
Harry
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my chimney cap cracked years ago, the ceramic flue liner broke and fell in blocking the flue.
check for good draft up flue, and stick a mirror in the opening can you see daylight.
we had carbon monoxide poisioning and could of died
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Resolution!
Turns out it was a relatively easy repair. I took the vent off, hosed it out (just a big of black dust), then removed the baffle, hosed it off, and scraped out the inside of the flue. Verified the intakes on the bottom were unobstructed. Now it burns much better. The damper still needs to be closed for a clean flame, but I suppose that is not abnormal?
Thanks all.
Harry
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