Moved to Fireplace forum.
Putting a Napoleon Wood stove Insert into an existing masonry fireplace with a 1980 era heatilator box. The box has three 3" tubes . Looks like removing the middle tube will allow space to run a 6" stainless steel pipe down the chimney flue and connect to the wood stove insert. The chimney has existing clay lining and everything is in good shape. The heatilator box is feed with cool air from two bottom openings and releases into the room over the fireplace via the three pipes. Looks like the throat with one of the pipes removed will be just about 6" and a touch more. Any tips or surprises to look out for?
Moved to Fireplace forum.
Get a level II inspection first but cutting those tubes can lead to a house fire and is against code. You can torch the unit out then have a mason rebuild the fireplace as a code compliant masonry fireplace but once you destroy the integrity of that steelform fireplace, it no longer meets the code requirements for a fireplace, voids the listing of the steelform fireplace and could cause excessive heat build up which could lead to an unfriendly fire.
Hire a pro to do the level II and consult.
As for factory built fireplaces you should never install a wood insert into those units.
Keep the fire inside the fireplace.