Two weeks have passed, and I am confident a problem was solved.
The problem? A water heater was shutting itself off. It was observed short cycling as well. The heater was located about 5 ft. to the side of an open outside door, with a clear aisle to it, and a clear, short, nearly straight vent pipe.
Yet, the hot exhaust would not go up that vent pipe. There was no draft.
Another duct, leading into that corner from outside, had been closed off. Duct opened, and -voila!- draft restored, problem solved. It appears that other HVAC items … kitchen exhaust fan, air conditioning, etc … had created a negative pressure – enough to choke the water heater.
So …. take this as a lesson …. there really IS a need for those two openings in the mechanical room!