Furnace Heat Exchanger

The heat exchanger in a furnace separates the combustion process from your breathing air. It is a combination metal chamber and passageway that starts at the burner assembly and ends approximately where the chimney vent connects to the furnace. Air is heated as it is blown across the hot metal surface of the heat exchanger. The heated air is then distributed through the house to warm the house.
The heat exchanger must have an air (and gas) tight seal to separate the gasses in the flue products inside the heat exchanger from the breathing air passing over the outside surface. This is because the flue gasses can be poisonous – such as deadly carbon monoxide – and contamination of the breathing air by these gasses pose a health risk and can be fatal
All heat exchangers fail eventually. This is because of metal fatigue. Metal when it is heated up expands, and when it is cooled off contracts. This expansion/contraction cycle is part of the normal furnace heating process. Over time this constant expansion and contraction have the same effect on a heat exchanger that bending a paperclip back and forth: it breaks. And when that happens contamination occurs and it is no longer safe.
While heat exchangers are typically manufactured to last between 10 – 20 years, many factors can accelerate the process of heat exchanger failure. These factors usually fall under the categories of poor maintenance, poor initial system design, and installation, or poor equipment design by the manufacturer. Anyone or a combination of these factors can result in a heat exchanger failing in a few short years.