A heat exchanger is combination of one or more chambers or “cells” where the air in your home gets heated. The heat exchanger is designed as a barrier, it’s job is to contain a 3300 degree fire*, transmit the heat to an occupied living space, and vent the toxic combustion by-products to the outdoors, without harming anyone. (*Approximate temperature of burning natural gas.).
But if your heat exchanger has a crack or hole in it your furnace must be condemned unsafe for operation as it can cause sickness, even death from carbon monoxide poisoning.