To clarify you have two flues, therefore you have two chimneys already. One for gas appliances and one for other use.
First question is do you use the fire place and want more heat out of it than what you are getting??
Second question why the basement?
If you don't use the fireplace and/or you want to provide greater heat than what a fireplace provides. I would suggest closing the fireplace off, easy DIY project and attaching a thimble into the masonry chimney, and vent a wood stove out that chimney. Extend your hearth pad out easy DIY project to accommodate a stove. And done for a fraction of the cost and headache of putting it in the basement.
Also today I was really thinking pretty hard about my existing chimney and pre existing flue for my gas furnace because now I have a direct vent furnace and the old flue is capped. However, I do believe that the hot water tank is running into the T that used to be the flue for the old gas furnace. So the tank and furnace both went into this T and up into the same flue. Because while I was on my roof looking down my fireplace chimney, I noticed that I have two flues comming out of my chimney but only one is for the fireplace. Perhaps someone in this thread could shed some light on this one???
First question is do you use the fire place and want more heat out of it than what you are getting??
Second question why the basement?
If you don't use the fireplace and/or you want to provide greater heat than what a fireplace provides. I would suggest closing the fireplace off, easy DIY project and attaching a thimble into the masonry chimney, and vent a wood stove out that chimney. Extend your hearth pad out easy DIY project to accommodate a stove. And done for a fraction of the cost and headache of putting it in the basement.