I know it goes against the whole purpose of this site, but you really should get it look at by a professional chimney sweep. The cracks may be fine the way they are or the could be bad, and there may be other problems with the system that you weren't able to see. Go to ncsg.org click on "Homeow...

That is a huge no-no. Factory built fireplaces are tested as a system. All parts work together but once you change a single piece it's un-tested and you just don't know what it will do. The second you mess with it the manufacturer will wash their hands of it. It is also against any local building co...