Most people understand that smoke goes up a chimney. Removing smoke from the fireplace is what chimneys were originally designed to do. Most houses are not heated by fireplaces anymore, BUT often a chimney flue has another use – as a vent of toxic carbon-monoxide gas from the furnace or hot water tank. Newer High-Efficiency Furnaces vent out the wall, in which case you may now want to have your chimney removed.