Great info, Sylvan. The question left is: If there is no specification for vent height to prevent wind pressure from forcing down or up as the wind makes an aerodynamic effect over the building, then why is a return bend or mechanical check valve necessary?
The impression is that the wind will target the vent pipe to go down the pipe and not create a pressure on the roof. Another impression is that the NFPA is incorrect in specifying the height requirement for a chimney, even though thay have found that a Swiss cap or a power vent can malfunction from wind pressure on a low termination.
The difference in the solutions stem from the differences between combustion venting and water seal. The furnace will depend upom milinches of pressure to flow vent products, the trap will depend upon the weight of inches of water to keep a seal. The trap may work at pressures 100 times the air pressure that would disturb combustion venting, so the solutions can be different.
It is good that a 2nd year plumbing apprentice is asked this, as he will confront this again in a different and lighter pressure when he discovers that he is also required to be a heating expert among his other plumbing skills. It is unfortunate that many states have no licensing or education prerequisites for heating contractors; in this case a plumbing apprentice has an advantage.
As for Nick Hydro, I think you are right. He would put a can on a short chimney or vent and come back many times until he finally got it right – if ever.