It's a big energy hog...sucking all of the combustion air (and already conditioned air most likely) out of the structure (and through cracks). Whether it is allowed or not, you'd have to read the installation manual.
If they did not perform a combustion analysis, the difference in having the pipe to supply the air verses being open to the structure could also have the fuel mix off. And, that could vary based on if there were anything else running at the same time (say a furnace, the clothes dryer, the exhaust vents, the stove vent hood, windows open, etc.).