REPLACING a zero clearance fireplace with a wood burning insert

I have recently removed my DESA 36" prefab zero clearance fireplace and I am installing a fireplace insert. I had a mason friend come in and build up the area behind the hearth with block, and then we finished it off with firebrick. I now have fire brick under, and at least 8" masonry around the sides. The prefab was using double wall(12" and 8") flue pipe. My question is, can I just extend the existing pipe down to the fireplace insert, or do I need to use triple wall, or am I way off here? What I DON'T want to do is burn my house down. Please let me know if anyone out there has done this, or attempted this and let me know your ideas. Thanks in advance.

Travis

Don't do what you're doing then. Install an EPA rated Zero Clearance.

Travis,

You need to use the exact flu system that your new insert was designed for. Most modern, EPA approved, high efficiency, stoves and inserts use a 6" diameter flu system. I am referring to the diameter of the pipe that the combustion gases actully flow through (the inside pipe).