"Frank" writes:

My downstairs fireplace is in family room next to furnace room.
Furnace needing air can cause drawback and I need to keep door to
stairs open when burning fire. Otherwise, even when fireplace is
heated, smoke can come out when furnace comes on and I open glass
screen to add wood.

I don't think that's my problem. The furnace is located in a closet of
sorts that has a direct vent through the wall to the outdoors, at least
6 inches in diameter, so it should have lots of combustion air without
depressurizing the house. And the basement room has a hot air register
but no return air register, so the furnace should pressurize the room
slightly if it does anything.

Dave