Ask the Expert: What Is a Heat Exchanger and How Does It Work?

Nathan WallaceNathan Wallace is a Senior Service Technician and expert in HVAC maintenance and repair at Standard Heating & Air Conditioning, where he has worked since 2001. Nathan is happy to share his knowledge about how your furnace and heat exchangers work. 

Heat Exchangers in Your Home

Many homeowners don’t know what or where a heat exchanger is until something goes wrong with theirs. When your furnace is in good working order, it’s easy to forget that the heat exchanger is the largest component of the furnace. Like all man-made devices, time and use take their toll on heat exchangers, and can result in cracks or holes forming. When this happens, the result can be a furnace that produces incomplete fuel combustion and unacceptable levels of carbon monoxide (CO), a potentially unsafe condition. Carbon monoxide entering your home—either through poor venting or leaks in the heat exchange—can cause serious illness or even death.

There are many different types of heat exchangers. Their purpose is to safely transfer heat, and they have many applications, including space heating, refrigeration and cooling, power stations, chemical plants, and other processes. This blog is about the types found in a forced-air furnace, like the ones in many Twin Cities homes.

How Does a Heat Exchanger Work?

Heat exchangers, metal shells and tubes, work by transferring heat from one place to another. When a furnace burns natural gas or propane fuel, its exhaust/combustion by-products (also known as flue gas) enter and travel through the heat exchanger. The hot flue gas heats the metal as the gas makes its way to the exhaust outlet of the furnace. As this is happening, the hot metal heats the air circulating over the exterior of the heat exchanger.

Primary Heat Exchanger

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

Secondary Heat Exchanger

If you have a furnace that is considered high-efficiency (90%+ efficient), it contains both a primary and secondary heat exchanger. As the combustion exhaust leaves the primary heat exchanger, it travels into a secondary heat exchanger where more heat is released from the flue gas and water vapor begins to form. This change of state from water as a vapor to a liquid releases latent heat in the secondary heat exchanger, bringing the furnace to an even higher level of efficiency. This is why high-efficiency furnaces are sometimes referred to as condensing furnaces. Secondary heat exchangers are generally constructed from stainless steel or a coated steel material capable of withstanding heat, moisture and acid.

Health Risks

Because the heat exchanger contains the flue gas inside of itself, it is important there are no holes, cracks or other deterioration. This type of deterioration that permits leakage and mixing of flue gas with the air being heated can result in incomplete combustion and the formation of carbon monoxide and other harmful by-products. Although your furnace may not immediately leak carbon monoxide into the living space, high CO levels make it unsafe to operate. Something as simple as the chimney getting plugged or damaged exhaust pipe can result in a very dangerous situation.

Detect Heat Exchanger Problems

Unless your furnace is malfunctioning or your carbon monoxide detector is going off, it’s nearly impossible to know if your heat exchanger has developed problems without direct inspection or testing for CO. This is why regular maintenance and inspections are very important. An inspection and a combustion analysis/CO test is the best way to determine whether the furnace is operating safely.

Visual Inspection

Some heat exchangers can be inspected visually. Others require specialty tools to examine the unit more closely. Our technicians are equipped with a camera with a flexible shaft that can see into inaccessible areas of your furnace to ensure a thorough inspection. We sometimes find internal issues in heat exchangers that would otherwise be hidden and appear in good condition from the outside.

Contact us today to schedule a maintenance check or tune-up.