I thought I got a "Good Deal" on the internet for a gas fireplace insert then I tried to put it in my basement and found out every fireplace manufacturer has their own configuration vent pipe, different sizes and thread pattern.This is not what you get at the hardware store. The interior heavy gauge pipe (4" a pipe within a pipe) is stainless, the outside heavy gauge pipe (7") is galvanized, there are no folded seams both are machine welded throughout which makes it "Direct Vent" . The exterior is a supply for the fire and the interior pipe is the exit for the hot gasses. The fire box is separated from the air plenum that surrounds the firebox and this is where you get the heat. This is the same principle as the new "Efficient Furnaces" but here you get to see the fire.
Pros:Easy to work with. All modular, goes together like Lincoln Logs. Heavy gauge.
Cons:Heavy Gloves are a must. The large exterior can be difficult to twist without a "Strap Wrench". Way expensive $.