Insert, cassette or inset fireplaces are made in a compact shape with a flat top so that they can be slided into an opening in the chimney. The frame of the appliance covers the opening. These appliances can be simply removed from the chimney later the same way without structural work. Built-in inserts are fitted with a convection box and often have fans as well. They are typically used in the replacement market and installed in existing chimneys, but can also be used in new build projects. All normal maintenance work can be carried out via the door opening. No structural work is needed for this.