Kenneth Wilson

Heat exchangers transmit heat between two or more temperature gases at a constant rate. The transferring or transmitting procedure takes place via a solid separator, which prevents gas mixing, depending on the type of heat exchanger used. Other design features like construction materials and components, heat transfer methods as well as flow configurations help to classify the types of heat exchangers accessible and categorize them. A variety of these heat exchangers are created and made to be used in both heating and cooling processes when they are used in a wide range of industries.

This article looks at furnace heat exchangers, a type of air to air heat exchanger, and explains how they work to help keep buildings warm.


The function of the heat exchangers is to allow higher temperature gases to interact with a low-temperature gas that enables the heat transfer to balance. This transfer of heat leads to a drop in the hotter gas temperature and an increase in the cooler gas temperature.

Various kinds of components can be used in heat exchangers, as well as various materials used to make them. The components and materials employed depend on the kind and purpose of the heat exchanger.

Some of the most typical components used to build heat exchangers include shells, tubes, and spiral tubes (scoils). Because of their high thermal conductivity, metals are suitable and often used for the construction of heat exchangers, like in the case of copper, titanium, and stainless steel heat exchangers.


Each furnace has a heat exchanger, which is an important aspect of the heating process. In a sealed chamber, furnaces combust fuel to produce heat. This combustion process generates heat energy from the fuel. The gas mixture alone, however, is not safe to breathe and is not suitable for heating the house by itself. They are also known as "flue gases." That’s where the heat exchanger comes in.

A heat exchanger for a furnace is a thin metal shield that sends hot air out of the furnace into the living space of your house through air ducts. The air blows across its surface on the other side, heating the breathable air quickly while the chamber heats the exchanger with flue gases. This warm air is then spread around the house or building's ductwork.

Furnace heat exchangers may consist of tubes or coils which connect to the burner on one side and a vent or flue pipe on the other, or they can be plate heat exchangers made up of multiple thin corrugated plates. Each pair of plates forms a channel that one gas can flow through, and the pairs are stacked and attached—through bolting, brazing, or welding—to produce a second pass between pairs that the other gas can flow through.

To function correctly and to block flue gases from escaping the combustion chamber, the heat exchanger must be fully sealed. An incorrectly screened exchanger might lead to a mixture of hazardous gases and air. Once the flue gas heat has been delivered to the heat exchanger walls, they are blown out of the heat exchanger and vented out of the building. Flue gas is vented onto a second heat exchanger in the high-efficiency condensing furnaces, where extra heat is taken out from them and used to further heat the building.


This part contains the hottest flue gas, found closest to the burners of a furnace. As a result, the flame and heat subject it to the most stress, which can cause cracking and heat stress with time. Furnaces that are 70-80% efficient usually have one heat exchanger.


If you have a high-efficiency furnace (90%+ efficiency), it is equally equivalent to a primary and secondary heat exchanger. As the combustion exhaust leaves the primary heat exchanger, it enters a secondary heat exchanger in which additional heat is released from the flue gas. In the secondary heat exchanger, the transfer in the state from the water as a vapor into a fluid releases latent heat and makes the furnace even more efficient.

As a result, high-efficiency furnaces are mostly referred to as condensing furnaces. Secondary heat exchangers are usually made of stainless steel or coated material, which can withstand heat, moisture, and acid.


The performance and longevity of a furnace heat exchanger can be improved through regular or annual furnace maintenance performed by a professional HVAC technician. The heat exchanger is checked for cracks during a maintenance tune-up and, when minor cracks are detected, the technician will repair them to prevent additional damage and security issues. (Related: Six Common Reasons Why Your Heat May Not Be Working And Estimated Repair Costs)

Other issues that potentially contribute to future heat exchanger damage are also checked during maintenance by professionals. Normal wear can lead to damage to the heat exchanger, but problems with airflow commonly cause them. Closed or blocked air vents inside a home or building and unclean air filters reduce the airflow of the heat exchanger, leading to overheating of the component, leading to cracks. During furnace inspection, it is possible to find clogged vents and other blockages affecting airflow through the duct system.

Protect your heat exchanger by changing the air filter to the specifications of the manufacturer. The majority of air filters last for 3-6 months. Filters that have low MERV values, however, usually require replacement every month or so. During colder months, a filter furnace may require a change as a result of constant use of the heating system.

Keep the air vents open in your home. Keep supply and return vents clear of furniture, rugs, and other objects. Do not shut vents to save money on heating bills because this could produce problems in airflows that would cost the heat exchanger and other system components costly damage.

Certain heat exchangers can be visually inspected. Others need specialized instruments to further inspect the unit. Our technicians have a camera with a flexible shaft to inspect thoroughly inaccessible parts of your furnace. In heat exchangers, we can detect internal difficulties which are hidden and appear in good shape from outside.

Kenneth Wilson
September 14, 2021

Kenneth Wilson

Retired contractor. Currently residing in Southwest Florida. Now in semi-retirement, I write and manage this blog focused on helping home owners make savvy decisions when it comes to finding contractors and getting their projects done. I also operate remodeling design service for homeowners.

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