I know the multiple vents in a set-up like that are supposed to be staggered. I don't know the specifics between the fireplace and furnace. My guess is additional distance may be required. Bob H. or someone knowing will most likely see this soon.
You aren't a bad dog from KC are you? Never mind, inside Board joke.

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Accurate Inspections & Consulting, Inc.
847/736-9975
Markus Keller