When your fireplace is running, be it burning wood or gas, in most cases it is drawing in outside air to burn and go up your chimney. Here's the problem, the fireplace draws in air from the room to burn the fuel. However, most of the heat escapes up the chimney. That hot air, which just went up the chimney, is replaced by cold outside air seeping in around the cracks in doorways, windows, and exterior walls. Hot air leaves your home and is replaced with cold outside air, that's the same thing an air conditioner does.
Fireplaces when designed and built properly, can contribute to a home's heating system.
Wood burring stoves draw combustible air in from the outside through a special duct and the exhaust flue routes through the house to radiate its heating energy. Use Solar Power to heat or cool your home. In the winter, use the sunshine in the mornings and afternoon to draw in the warmth. Open the drapes on eastern and southern windows of your home. At night and on overcast days, close your drapes and shades, and keep your northern windows covered.