I received a response to this question from the state of OR Building Codes Division. Please understand that this answer is relative to this exact problem, and that input should be gathered from the local AHJ.
"While this isn’t a duct, it is a penetration between the garage and the occupied space that should meet R305.2.
Also, the pertinent sections of Chapter 24 of the ORSC that aren’t being met are:
G2407.2 and G2407.4 for combustion air. Obvious that there isn’t adequate combustion air; the occupied space is at a negative pressure to the garage, especially with the fireplace not operating (the pilot light might reduce some backdraft by drawing heat/vent air flow up the fireplace vent)
G2427.3: Positive flow required for any vent. The “home” is obviously at a different pressure than the garage, and the system is back-flowing into the home. The maximum “negative pressure” (draw) of the water heater and furnace will be about -0.05”. The interior of the home can have bath fans, kitchen fans, temperature difference between the warm home and the outdoors (creating stack effect pressure similar to that created by the vented flue), wind effect, etc. that can easily overpower the draft up the water heater to the roof vent outlet, especially when just the pilot light of the water heater is the only heat source in the vent system.
The system is not meeting code. Are you still in contact with the customer? They should be made aware of the possible hazards; we hope they have working carbon monoxide detectors. I’m unaware of a method to equalize the pressure between the garage and the living space, since a duct/opening can’t be made between the living space and garage (per ORSC R305.2)."