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Heating systems are a mystery to most builders, even those who talk a good game. When the moment of truth comes and they're face to face with an old forced hot air furnace in a dank, dusty corner of a customer's basement, the best they can do is find the manufacturer's stamped label and read the capacity rating. But it takes more than Btu's to make a forced hot air system work efficiently, especially when adding on to an existing system to heat an addition. One of your responsibilities as G.C. for an addition or renovation is to figure out if the existing heating system can handle the extra ductwork. The more you