A nonmechanical way to address this problem is to build a fireplace with a slanted firebox, which inhibits warm air from escaping to the outside. ZERO-CLEARANCE How to Build a Fireplace - Zero ...

Unless are an experienced do-it-yourselfer (DIY'er), your best bet is to select a zero-clearance fireplace ... Also, look for an outdoor combustion air feature. Several fireplace manufacturers ...