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A heat extractor mounts on the stovepipe and extracts additional heat from flue gases beyond what normally comes from the stovepipe or the chimney. You can use it for either fireplaces or wood-burning stoves. As it extracts heat, it cools the flue’s gasses and could cause the flue to smoke, so it’s best to install an extractor with more capacity than is necessary for the size of the fireplace. The downside is that an extractor could work against the efficiency of a good wood-burning stove. As the flue gases cool, combustion is reduced and the stove gives off less heat.