It’s important to remember an adequately sized opening and enough room volume doesn’t ensure combustion air makes it to the burner. Don’t make this dangerous assumption.

You can only determine if air ends up at the burner when you measure multiple CO readings from the equipment’s flue gas. If those readings are stable, you have a consistent air supply to the burner.

If your draft pressure stays in range through all these tests, consider yourself fortunate. Typically, you’ll uncover at least one issue.
These tests enable you to isolate various building conditions that may be the difference between missing or diagnosing a potentially dangerous condition.