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Where is my furnace heat exchanger and what does it do?

Where is my furnace heat exchanger and what does it do? These are good questions that many homeowners ask. The first part of the question is easy to answer, and the second part is interesting because the Furnace Heat Exchanger is one of the major gas furnace components. Sun-Aire Comfort Systems will correct any problem that causes your Furnace Heat Exchanger to be inefficient or dangerous.

Where Is My Furnace Heat Exchanger?

The accompanying image of a furnace shows the location of a heat exchanger ( in a gas fired furnace. The furnace that you are looking at is referred to as an upflow furnace, and the heating air leaves the top of the furnace and goes up into the system supply air duct. Furnaces are also manufactured as
downflow and horizontal. The different orientation descriptions refer to the direction that the heating air leaves a furnace.

When the furnace front panels are removed, the furnace heat exchanger is not visible. The heat exchanger is behind a panel, and the reason for that will be discussed below. Although the heat exchanger is covered, there is an easy way to determine its location. External to the furnace is a steel gas pipe, generally 1/2″ in diameter, that continues to the inside of the furnace. This gas pipe will be connected to the furnace gas valve. The gas valve in turn is connected to the burners.

The burners in the furnace produce a flame that goes into the heat exchanger. Therefore the heat exchanger is adjacent to the gas valve and burners, in the same section of the furnace. Generally the heat exchanger and burner section are located in the section where the air exits the furnace. An upflow furnace has its heat exchanger in the top compartment of the furnace, a downflow in the bottom compartment, and a horizontal where the air exists the furnace.

What Does a Furnace Heat Exchanger Look Like?

Older furnaces use the Clam Shell  type as the primary heat exchanger. The clam shell is produced by stamping two pieces of metal that are mirror images of the other, so they can be joined and then fastened at the seams. These seams must be joined in such a way that they will not leak combustion gases. Each of the clam shells comprises a section or chamber. As these chambers are joined together is a manifold of sorts, the potential Btuh heating capacity is increased.

Newer furnaces may use the Serpentine  type as the primary heat exchanger. The accompanying picture shows the serpentine tubes, which appear as pipe that is bent in a specific pattern for the furnace it will be installed in. As the number of serpentine tubes increase, so does the potential Btuh heating capacity of the furnace. At the bends there are creases in the metal, which strengthen the heat exchanger.

High efficiency Condensing Furnaces employ a Secondary Heat Exchanger . The secondary heat exchanger removes additional heat from the combustion gases and water vapor leaving the primary heat exchanger.

What Does My Furnace Heat Exchanger Do?

By definition a heat exchanger will transfer heat to a cooler medium. This transfer is based on the Laws of Thermodynamics, in which heat will automatically transfer from an object with the higher temperature to an object with the lower temperature. The heat of an object is measured by the activity of its atoms. The atoms in a hot object move more rapidly than atoms in a cold object.

In a furnace heat exchanger, as with all heat exchangers, the warmer and colder mediums are separated and do not directly touch. The heat exchanger is manufactured from thin metal, and will be mild carbon steel or stainless steel depending on the type of furnace. Some heat exchangers are a hybrid such as steel with an aluminized coating. During the furnace heating cycle the combustion gas will travel through the heat exchanger and exhaust out of the furnace. As the combustion gases exhaust to the outdoor atmosphere they heat up the metal that separates them from the heating air being supplied to the house.

In the sequence of operation that comprises the furnace heating cycle, the blower motor will be turned on and push the return air pass the heat exchanger. As the cooler return air passes over the hot heat exchanger its temperature increases between 30*F to 65*F, depending on the furnace make and model. As the return air increases in temperature, the heat exchanger combustion gases drop in temperature and exhaust to the atmosphere.

Return air and supply air temperatures are measured by furnace technicians and compared to the data plate located inside the gas valve portion of the furnace cabinet. Using the Carrier Infinity model 59MN7A as an example, the continuous return air temperature cannot be lower than 60*F or higher than 80*F. Maximum temperature raise  for a 100,000 Btuh 59MN7A is between 45*F to 75*F. Using the mid-range temperatures for the return air and the temperature rise, the calculation shows a return air temperature of 70*F may be heated to 130*F, an increase of 60 degrees. The increase in temperature comes from the heat transfer at the heat exchanger exterior surface.

To ensure that the heat exchanger has a long and efficient life, it must operate within the engineering parameters of the manufacturer. If the return air temperatures are too cool, or the temperature rises above the maximum allowed temperature, the heat exchanger can be damaged. A malfunctioning exchanger requires repairs to be made to the heater. During your annual furnace tune up, these temperatures should be checked.

Return air temperatures below 60*F will cause the combustion gases to rapidly condense and cause a water problem in the heat exchanger. If the heat exchanger is uncoated mild carbon steel, the water will cause rusting, and eventually cracks and holes. In addition to the inefficiency caused by low return air temperatures, the resulting water can lead to complete failure and emissions of carbon monoxide into the home.

Now that you know where your furnace heat exchanger is and what it does, arrange for an annual furnace tune up. If you live in the Springfield, Missouri area, schedule maintenance for your furnace by calling Sun-Aire Comfort Systems at (417) 352-0975. You can also schedule an appointment on line by going to our website

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