Up, up and away. That is how vents should go. Sadly, this one goes up, over, down, over, up, and hopefully out. There was very low overhead clearance. There were solutions, but this installer didn't find them.

What's the difference, you might ask. Lots!

If vents don't go steadily upward, they won't draw properly. Several things can result. One is carbon monoxide venting into the room. If the room is underneath somebody's apartment, that's not so good. It's also a dangerous room to hang out in, should anybody be inclined to keep a big commercial water heater company.

Another problem has to do with soot. Tanks that don't draw well may begin to soot up, which can both exacerbate the situation -- sort of a downward spiral -- and also create a fire hazard.

A third issue is corrosive condensation that can destroy the vent itself.

One other thing: Where two vent sections feed into a common pipe, the sums of the squares of their diameters should be roughly equivalent to the square of diameter of the common pipe. Too much, too little, not so good.