The vent for this NG water heater slopes downward, about 3/4" in 1’ length before it goes into the larger vent from the NG furnace, which does a have sharp upward slope. Then larger vent then feeds immediately into a masonry chimney. Is this downward slope off the water heater appropriate?
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Forgot to load my picture the first time! 
No, it’s not right…it should have an upward grade of 1/4" per foot. Looks they ran out room to slope properly and didn’t bother to re-do the tie-in in the proper location so they would have enough room for proper grade.
Absolutely not. . .
Ditto my man Jeff P.
Heated gas only flows uphill.
The biggest issue with reverse slope vent connectors is that condensation/moisture/debris can build up at a low point. It doesn’t appear to be a huge issue with that one assuming there are no low points between the elbow and the connection to the larger (furnace/boiler?) vent connector (hard to tell from such a small pic), and the larger one has correct pitch back to the equipment. But that doesn’t make it right, and I would still write up the concern.
JMO & 2-nickels … 
Thanks for the replies, everyone!
I too, was not sure that it was a “big” deal since it was such a short run and it was close to the larger vent. That was the reason that I even posed the question. My “feeling” was that it was probably getting adequate draft.
Since this does deal, however, with a potentially lethal situation, I did write it up.
Even though determining adequate draft is beyond an inspection, adequate draft is another concern … but at least in my mind not as big an issue as creating a low point since it’s not a long run and/or severe pitch down to the larger vent connector from the pic/description (would be a significant concern in my mind if that was the case). But again it doesn’t make it right, and I think you made the correct call to write it up.
JMO & 2-nickels … 
Thanks, Robert.
I agree, additionally the low slope interferes with proper draft which creates a condition where back drafting is that much easier. It wouldn’t take too much negative pressure to cause it, might be as simple as operating a clothes dryer and a bath fan at the same time.
These days when I inspect homes with natural gas I automatically add a note cautioning that it would be wise to replace one of the smoke detectors in the sleeping area with a combo Smoke/Carbon Monoxide detector.
I do the same if the home has any fuel burning appliance, gas, oil, wood, etc.
More of a concern with a long vent connector or severe downward pitch after the elbow … but again, it doesn’t make it right, and is also poor practice.
Especially the CO detector if there is a sleeping room on the same level as the equipment … and new smoke detectors throughout is always a good recommendation if the system is suspect/lacking.
JMO & 2-nickels …
If the hot gas will NOT GO downhill…where does it go in this situation? 
Since you put the smiley face after your statement, Tony, I’m not sure if you were being serious or not.
The exhaust gases can possibly backdraft out into the room, the living space of the house. The exhaust vent on a water heater is not sealed at the top of the heater, there is a gap between the heater and the vent. So if the venting isn’t working/drafting properly, fumes/carbon monoxide can easily backdraft into the room.
There is exactly how I would have worded it to my client rather than cite code…
Thank you for helping me make my point…
I must be missing something, I didn’t see that anyone here was citing code. I certainly didn’t cite code to my client. I roughly worded it just like I worded it here, but with a little more polish, I hope .
Didn’t quote code per se…but the way you explained it will help others who come across this thread that aren’t sure how to word it…
I’m not trying to be an arse here…just trying to help.
Since you posted yours I’ll post one of mine.
Tony I have found that citing code is often the only way to get the point accross. The code most frequently quoted is the “Code of the West”, in the section about stuff not being right. Using the above as an example my exact quote would be “That aint right”.