that is true for chimneys that cannot draw air from the outside. Chimney's will pull a vacuum especially with a good hot fire. My chimney has 2 vents on each side of the fireplace that can be opened to draw outside air into the fireplace which reduces significantly the vacuum on the inside but it isn't perfect either. If I get a rip-roaring fire going in the fireplace and maintain it, I can get a good portion of the house heated up quite well but the far end upstairs (separate heating zone) will suffer so there is some loss.