I'm about to frame in a Lennox gas burning fireplace and the instructions say the depth of the framing must be 16 in. and "it is important that this dimension be exact". I'm going to have 24 in. deep cabinets on both sides of the fireplace. Because of this, the fireplace face will probably be something like 26 in. from the rear wall. Why the 16 in. exact instruction? Shouldn't it say 16 in. minimum? Is there a reason I can't have 10 extra inches of empty space between the fireplace and the back wall?
Not familiar with the exact unit, but since you said Lennox gas fireplace, my thoughts go to the idea that it may be a high efficiency, direct side wall vent unit? If so, that dimension would be critical for the length of the factory furnished pipe connecting the firebox to the exterior vent cap. If it is a vertically vented unit, I can't think of any reason why the depth of the enclosure could not be more than mininum.