I stumbled on this article today about replacing an old prefab ZC fireplace. It has several tips including info on the primary areas of failure in old installations. Seeing how many folks are contemplating these units, this might be helpful information.
http://www.jlconline.com/public/articles/2003/0311/0311repl.pdf
http://www.jlconline.com/public/articles/2003/0311/0311repl.pdf