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Most new homes are built with forced-air heating systems. So it's no wonder that some builders use the ductwork for fresh air ventilation as well. Besides bringing fresh air into the house, integrating ventilation into a existing forced-air system can actually correct a problem that's common to tight homes with combustion appliances. It's called backdrafting and it works like this: When a house can't get enough outside air to replace that removed by exhaust-only appliances like clothes dryers and bath fans, then the interior is put under a negative pressure. To equalize the pressure, air is pulled down the chimney and through the furnace. If the furnace happens to be turned on, this can also introduce potentially lethal carbon monoxide into