I would start with a chimney sweep. He can inspect your chimney and clean it and suggest any repairs if necessary. If you have major flue problems, sometimes a stailess steel flue liner is a good solution, but there are other options. I have done some research on vent free gas use and I am not convinced that it is safe or smart, besides it introduces way too much water vapor into your house and this leads to condensation problems. Here is a good article that really made me stop to think... http://www.jlconline.com/jlc/archive/energy/vent/index.html