My waterheater is located in the basement of a two story house. I also have a wood burning fireplace in the basement that has an outside air intake. The waterheater is vented with a 3" duct that is to a "y" connection on a 6" vent duct that runs to the roof. The 3" waterheater duct is orphaned as the 6" duct was once also attached to our old furnace.

Issue is that when I burn a fire in the fireplace, I get a backdraft down through the water heater 3" duct. I have to open a basement window to re-establish venting but as soon as I close the window, the backdraft comes back.

Should I put a booster fan on the 6" duct or should I run a 3" or 4" duct through the middle of the 6" duct? Or???