Natural gas hot water heater was only thing venting into this old chimney. It definitely has some major defects, but I called it out as a safety issue because I felt it was on the verge of collapsing. What do you think?
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For sure. That chimney is not safe by any measure.
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Man, check for Rodents of Unusual Size.
You’re not required to determine cause, just point out this hazard. As for cause my guess would be either big rodents, or a gap between the flue and the chimney, causing the cinder block to be perpetually soaked, then undergoing freeze thaw cycles.
It’s better to vent that flue away from the chimney anyway, as the chimney is too massive and uninsulated for a good draft. The hot exhaust may cool in the flue, and sink back down, leading to turbulence and backdraft.
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That’s how I see it, too.
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Recommend a licensed masonry contractor rebuild the masonry chimney and crown.
Immediate Safety hazard. Do not operate until replaced.
Foundation is not faring as well I see.
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Makes you wonder how people can let something like that go this far without repairs.
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Falling bricks can kill you! Good call.
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Catastrophic failure
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Well That thars yer problem…or at least part of it…
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William. This situation is set up to be deadly. The narrative should have been, natural gas water heater flue directly to CMU masonry block chimney.
Situations like this can have dire consequences to the occupants.
Read up on Gas Appliances and Your Masonry Chimney.
Chimney Liners Any natural gas fired equipment requires the proper flue.
You must try harder to be taken seriously. This could have tarnished your reputation and landed in in court with some very serious charges leaved at you. This is not a deficiency. This is a very serious problem that could burn down the property or ending up taking someone’s life.
Our local gas utility won’t even do routine work in a building with that condition. The local insulation people won’t weatherstrip a building like that. The only reason the occupants are alive are the serious leaks in most construction…
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I concur 100% Bryce.
That residence is a drafty building in serious need of updating.
Unfortunately, the residents become aware when the home inspection report advices them. Typically, during any negotiations or when the report is turned over the the listing agent because the prospective purchaser moved on and the report was summited as the deal breaker.
I can see a clay flue lined chimney, which is perfectly acceptable for natural draft water heaters and furnaces.
Provided, the flue is well maintained (obviously not in this case)
I see problems with these at times. 80% furnaces the exhaust often cools too quickly causing condensation and subsequent problems.
If you look closely, the clay flue has gaps where the masonry is damaged. I think the exhaust was escaping here and the moisture was causing spalling and deterioration. Or moisture was entering from above with similar effect. Or both.
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Thanks for the link regarding venting gas appliances. It was very informative. I did call this chimney out as a safety issue. I had the realtor and buyer look at it with me and I described the liability issues with a possible collapse as well as improper venting issues with the gas hot water heater.
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And perfectly unacceptable for forced draft furnaces… a topic the Nachi training hardly touches.
That said agreed 100% on cold flue acidic condensation as a contributing or primary factor here.
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Zactly, Brian… 
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They are not acceptable. Modern natural gas fired appliances produce lower temperatures combined with acidic flue gases that erode terra cotta and masonry. That chimney is a perfect example.
You forget 1 important quality, when maintained. This terra cotta liners should be sealed with fire rated mortar/refectory mortar . These liner shift and crack. I repaired many. In my neck of the woods, metal liners are required.
Bravo… and have not touch sense I arrived 11 years ago.
Good subject for Ben to take up.
Which is exactly what I said. The OP stated it had one appliance, a gas water heater.
A natural draft gas water heater may perform very well with a clay liner if it is well maintained.
Modern appliances, such as high efficiency furnaces will not do well with a clay liner.
I read your link, I took no exception to it. But not all clay liners are defective or require replacement.
May preform and do preform are two totally different realities. I can suspect 1 in 100 terra cotta lined masonry chimneys are adequate. Looked down hundreds and hundreds of chimney flues. I rebuilt them and now inspect them.