Any device that burns fuel requires air. When a house has insufficient ventilation, fuel-burning equipment will sometimes be forced to draw air in through the exhaust vents of other devices. This recycled air often contains carbon monoxide - a dangerous poison. This process is called backdrafting.
Backdrafting occurs more often in older homes because there may be inadequate ventilation. Newer homes are bound by building codes requiring the installation of an air pipe that draws in air from outside, and therefore backdrafting is less of an issue. In any event, it is worth consulting with an expert especially when you are about to seal up cracks in your home's exterior; you need to make sure that an air pipe is indeed safely installed. The other solution, although it may require a little expense, is to house your fuel-burning systems within sealed containers that draw air from the outside.
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