Can anyone tell me if a flexible metal vent pipe is allowed on a gas fired water heater as shown in the attached photo. I haven’t come across this one before. Thanks.
They are allowed. You just have to make sure that they are of the proper type and not a laundry vent connector.
That one looks like it is of the proper type. The Type B flex that I’ve seen here has stamping on the ends.
What Russel said and it shouldn’t run downhill like it appears to.
I agree that it shouldn’t run downhill, but from my perspective I don’t think it does. At least not enough to be an issue. There might be a little bit of a “droop” in the line, but IMHO it’s not enough to obstruct the flow. :mrgreen:
A little bit of a droop in the line can, indeed, interfere with proper drafting of the water heater, which then could result in lower efficiency of the water heater, backdrafting, and excessive levels of carbon monoxide. Gas combustion is not something that you want to mess around with.
Do you think, from the pic proved, that there is enough “droop” to be a concern?
Yes, I do.
If you really want to have fun sometime, mess with the installation on your own water heater and then take some carbon monoxide readings of the different types of installations. It will be eye-opening.
Just be sure when you duct tape it back together (as in the picture) you do it properly. :roll: 
That doesn’t appear to be duct tape in the picture. It looks more like the high-temperature metal tape that is approved for such uses.
I actually like it better than relying just on the male-female fitting and screws since I see it as a secondary fail-safe; it helps hold the screws in place so that they don’t back out due to vibration or gravity (or Little Johnny playing with his first set of tools).
And I wonder why they put a hot water handle (red) on what hopefully is the cold-water line. They should have used blue, but that’s something that I don’t call out. Just noteworthy to me.
Besides being bad at carpentry, some plumbers are also color blind? 
In my neck of the woods, our AHJ calls that flexible material a “connector”, which is disallowed, as it appears installed.
I assume the flex is rated B’vent material and connects the elbow to ridgid B’vent running through a chase area up and through the roof. We can, from the draft diverter, use a single-wall elbow and then convert to any approved double-wall (B-type) vent material to the outside. We aren’t allowed to mix materials after the DD connection.
