I currently have a vent free 30,000 BTU unit that I use to augment my furnace for the basement. It works great when I turn it on, heats the finished basement right up. I use it sporadically because the vent free makes me nervous. I have a fireplace guy that wants to install a VENTED 18000 BTU unit. The basement is likely 600-800 square feet. Is 18,000 BTU big enough?