A fireplace "insert" is intended to fit into a pre-existing masonry fireplace. I assume you mean a metal factory built "zero clearance" wood burning fireplace for new construction The wood burning fireplace "duct" you mention is a chimney whether it is masonry or metal. You can add gas to the firebox or put a gas log kit in it but the chimney must still be designed for a wood burning fireplace and sized for the fire box opening (unless you want to pay for it to be tested by UL). You need to either try harder to get the chimney offsets to work or use a metal factory built gas fireplace already tested by UL with a B-vent through the roof. Factory built gas fireplaces are made in two types: Direct vent and B-vent. The Direct Vent unit has a double-walled metal vent pipe that can terminate on an outside wall or through the roof. It is a closed system so the fireplace gets combustion air from outside through the outer wall of the vent and the firebox has a fixed glass enclosures. The B-vent unit has a single-walled vent that can only terminate through the roof. The fireplace takes combustion air from the room so it can be open to the room. Many of the units have folding glass doors. Carefully read the instruction for installing the vent. Consider an optional outside air supply duct to the firebox.

I had similar questions about my fireplace. We built 8 years ago and I paid the extra for the gas setup. Recently I wanted to use wood to help reduce heating costs. My fireplace is a Lennox unit with outside air supply and the blower setup. I figured that seeing as my house is open-concept the wood heat would work out well. I also wasnt sure what would need to be done to convert it. I wound up taking all my install paperwork to the company who installed it, as they are 5 minutes from my home. Their service manager talked with me and explained that I did in fact have a woodburning fireplace with a gas-log setup installed. He also told me that by code they have to install it with the proper chimney and tolerances for a wood fireplace. So all I had to do was remove the 'gas guts'. Ive been burning for a few months now and its SO nice:P Dave

We had a wood burning fireplace with insert in and changed it to a gas insert.....gas heat when up the chimney as bad as the wood heat did. So we changed it to electric insert and that is what we have now. Our firebox was square so I am thinking you might need a peice of black metal to make the archway sqauare. Also on the log side, put the peice of black metal there and put doors on it for storage. By the way, we had glass doors on our fireplace.

We had a gas insert added to our wood burning fp. One of the best investments in the house that we've made. We use it all winter long. If the next owners of our house don't like it, they can just pull out the insert, and use it as a wood burning fp again. We've used ours during power outages for heat. The blower doesn't work during outages, but the flames do (as does the remote control). It still warms up the FR and the rest of the ground floor of our raised ranch/bi-level type house. I think this is the model we have: Regency Horizon HZI390E But it appears they might have changed the model numbers/styles on their site (linked below). http://www.regency-fire.com/Products/Gas/Gas-Inserts.aspx