Remember, codes are a minimum std. If this is installed in a confined space but with the return 10ft 1 inch away from the vent collar or draft hood, you still have grounds to advise them to:
a) address the confined space with makeup air per the code OR BETTER
b) move the return even farther away such as outside the confined space.

Appliances can't read code books. Even with the code required MUA, the return could overpower the vent. Fans are very powerful---atmospheric vents are very, very weak.

This is a situation that warrants a more comprehensive system evaluation including a Worst Case Depressurization Test, duct balancing & sealing and combustion analysis.

Bob

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