All heating systems that use oil, solid fuel, or natural gas to run need some sort of ventilation. Your gas furnace uses combustion to produce heat, but the process also leads to the development of some pretty unpleasant byproducts, including carbon monoxide—a dangerous, toxic gas.
Sometimes, we see multiple appliances using the same flue vent system for multiple appliances, but this isn’t always a possibility. In fact, we almost always recommend against it if you have an older chimney and a new high-efficiency furnace. Can your new furnace be vented through the chimney?