You can do this no problem and it is effective. You don't even need much added CFMs as the heat from your room and the other heating devices will push the air up and out. I've seen it done. But you need to do the following: place back draft dampers on your grow room duct and on any other ducts that are entering the same line. Otherwise you'll just push the stinky air out of the furnace or hot water heater line. You need one on your grow room duct b/c you don't want the furnace pushing into your grow room. Also, if there are any other lines that connect into the flu you may face issues. For example, if there is a first or second floor fire place that feeds into the same line you may get stink in the house. Another option you might consider is venting into your toilet stack. Very effective. All odor is driven into the sewer system. And of course a plain old carbon filter will do the trick. You can vent into an adjacent room with a decent filter...