HVAC Hacks: Tips for Novices and New Homeowners

Your Furnace's Heat Exchanger Explained

The heat exchanger is one a gas furnace's most important yet least understood of all its components. Unfortunately, a failure to recognize the exchanger's role can easily lead to breakdowns and other potentially hazardous problems that require HVAC maintenance. If you would like to learn more about the part played by your furnace's heat exchanger, read on. This article will provide a handy introduction to the subject.

The Basics

The basic operating principle of a gas furnace is quite simple: gas flows into the combustion chamber and is ignited, giving off heat in the process. Yet the blower system can't simply send this heated air directly up into your home, since it contains carbon monoxide and other potentially harmful exhaust byproducts.

Instead, the heated air that is generated inside of the combustion chamber is directed into the heat exchanger. The exchanger is a closed system of metal tubing around which air from inside your home is allowed to circulate. The heat thus passes through the exchanger to the air, while the dangerous exhaust fumes remain isolated. Ultimately this exhaust is routed safely out of your home through the flue.

Head And Corrosion

The nature of the heat exchanger means that it is regularly subjected to large changes in temperature, becoming quite hot while your furnace is running, only to cool off once more when the furnace is idle. As the metal heats up, it naturally expands, then contracts again as it cools. Repeated over and over, this process will eventually lead to metal fatigue--in other words, the metal will become weakened and thin. Eventually, cracks can open up along the seams.

A heat exchanger is also exposed to a number of corrosive substances that can be found in the exhaust. As time goes on, these substances will cause rust to begin forming inside of the exchanger. There isn't much that can be done to prevent this; it is simply part of the life-cycle of the heat exchanger to expose itself to such corrosion. Eventually, between the heat and the rust, it will be necessary to have a heat exchanger replaced with a new one.


Those who ignore a heat exchanger nearing the end of its serviceable life put themselves at grave risk. Should holes or cracks form in the walls of the exchanger, toxic gases will begin escaping into the air around your furnace. Eventually this air will be drawn into the system and sent up into your home. Having your heat exchanger inspected on a regular basis is of the utmost importance.