Our home fireplace has a heater. The device is an insert that pumps air through a thermobox and heats the room and house. There was a leak, which also caused smoke to enter the room. On the advice of a chimney sweep I bought this product and it sealed all the leaks between the metal liner and the stone surfaces, this prevented smoke from escaping. It was very easy to spread without leaving a mess, so the finished product looked like the Heat-Oater was made specifically for it!