I have heard for years that a fire place actually causes more heat to be lost than it generates.

I just assumed that meant that after the fire burns down and the flue is still open that heat goes up the flue.

But I had the fireplace guy here yesterday to give me a quote on changing my gas fireplace to a wood burner and my wood burner to a gas fireplace.

He said that fireplaces draw warm air from the house up the chimney while they are still burning. That would mean that they have to be expelling tons of heat into the room while simultaneously sending heat existing from the room up the chimney.

The intuitive answer seems to be that before people had furnaces fireplaces made the houses warmer so the addition of a furnace should not change that. And I know that the room my fireplace is in feels warmer - so again intuition implies that fireplaces are good.

What do you make of this?