Question: We have a Heat 'n Glo 4000INS gas fireplace insert. We also have a second floor family room with an outside wall that needs heat! What do we need to construct in order to use the above insert? We realize that we need to build an insulated structure for the venting but do we also need a firebox of some sort to hold the insert? Answer: You do have to install a fireplace in order to install an insert. Assuming that model is approved for ZC (zero clearance) fireplaces, you'd have to install one of these complete with chimney...then install the insert into it with a full aluminum liner the same size as the vent on the insert.