Hello all, I have been scouring the forums for a while an I have not found an answer to my question so far... I am looking to install an Insert into my Zero clearance fireplace... The firplace is a Temco (not sure of spelling at the moment) 42" pre fab metal firplace... I have more width and depth available than any insert on the market can fill, I am just a little limited on hight due to my brick facing which reduces the number of approved units I can purchace... I do realize that I need an insert that is approved for installation into a pre-fab unit... However my question is this, What is the difference between a pre fab approved and a masonry only stove??? I have been to several local dealers looking at a few different stoves an I see no difference... You look at 2 different stoves from the same manufacturer inside and out, one is not pre fab approved but the other is (maybe one is larger or smaller than the other) and they are made exactly the same way with the exact same materials... The firebox on the approved model isn't any further from the bottom or the back of the unit than the non-approved, there is no extra "insulation" in the approved unit, they are made from the same thickness steel, they have the same sized fire brick... What gives??? I was just wondering, I am still buying an approved unit I was just curious... Thanks for any help you can provide...