The invention provides an insert assembly to convert an existing fireplace hearth into a gas burning fireplace and a method for installing the insert assembly while conveying the appearance of a wood burning fireplace. The insert assembly comprises legs removably attachable to the base of the firebox for allowing the insert to be installed higher into the hearth. The inner walls of the firebox are colored so as to camouflage the structure of the assembly in the hearth and a decorative fender mimics a wood burning grill. Where the insert includes a heat exchanger, a guide plate is provided at the top of the insert to ensure that air exiting the heat exchange outlet at the top of the insert is directed into the room.