Many of these wood fireplace design ideas have gorgeous mantels in place, and it would be terrible for the interior design aesthetic to rip those out and replace them with a contemporary style of fireplace. There are many reasons to add a fireplace insert. Homes that currently have old wood-burning fireplaces do not enjoy the energy efficiency that comes from a new wood burning fireplace insert because old fireplaces leak out the heat up through the flue and up the chimney to the outdoors, thus wasting fuel and adding to the cost of burning wood. You will see that just as your fireplace opening has specific sizes, so too does the fireplace insert. You can select the correct size of wood fireplace insert for your particular fireplace, so that the fit is perfect and you reach the best energy efficiency possible.