Furnaces and forced-air heating systems are more complicated than you may realize, and they rely on a large number of different components to produce heat and transfer the heat to raise the air temperature in the home. One of the most important parts of any gas furnace is the heat exchanger, as it works to transfer the heat produced by the gas burners to the air. In this article, we’ll explain everything you need to know about furnace heat exchangers including what they are and how they work to allow furnaces to produce heat.
Understanding the Role of a Furnace’s Heat Exchanger
The heat exchanger in a furnace is what captures the heat that flows out of the gas burners, and it also works to keep the combustion fumes from mixing with the air in the furnace. Heat exchangers are made up of metal tubes or pipes that connect on one end to the combustion chamber and the exhaust flue or chimney on the other end. To understand exactly how a heat exchanger works and what role it plays in allowing a furnace to safely heat a home, it’s necessary to look at all that happens when a furnace ignites and runs.
In most modern furnaces, the first thing that happens when the thermostat calls for heat and signals the heating system to run is the draft inducer turns on. The draft inducer is located at the top of the combustion chamber that houses the gas burners. It works to draw fumes out of the combustion chamber and into the heat exchanger tubes. This process creates a draft in the combustion chamber that pulls oxygen into the chamber. This process is necessary for the burners to stay lit and for the natural gas or propane to combust fully.
The inducer typically runs for at least 30 seconds before the furnace ignites, and this is the first thing you hear whenever your heating system turns on. The reason that the inducer runs first is to pull any latent combustion fumes out of the chamber. This is important as any fumes that remained inside the chamber could starve the burners of oxygen and lead to a slight delay between when gas starts flowing to the burners and when they actually light. This type of ignition delay can be dangerous as it allows gas to begin filling up the chamber. This can result in a small explosion when the burners ignite.
Once the burners are lit, the inducer continues to run and pull the extremely hot combustion fumes produced by the burners into the heat exchanger tubes. As the fumes flow through the heat exchanger, most of the heat flows out of the fumes into the metal tubes. This process quickly raises the temperature of the heat exchanger to somewhere between 150 and 200 degrees Fahrenheit.
Furnaces have high-temperature and low-temperature limit switches that constantly monitor the temperature of the heat exchanger. When the low-temperature limit switch detects that the heat exchanger has reached a set temperature and is hot enough, it signals the blower to turn on and start pulling cool air into the furnace. As the cool air is drawn over the heat exchanger, the heat flows out of the metal tubes and into the air. This process instantly heats the air to somewhere between 140 and 170 degrees before it flows out of the furnace and into the supply plenum. As the hot air then moves through the supply ducts, it typically cools down by around 50 degrees by the time it flows out of all of the vents.
When the furnace is running, the hot combustion fumes constantly flow through the heat exchanger. They are then vented outside through the exhaust flue. This means the metal tubes in the heat exchanger are continuously absorbing heat from the fumes and releasing the heat into the air flowing over the tubes to ensure the furnace keeps effectively heating the air.
Primary vs. Secondary Heat Exchangers
Conventional furnaces only have one heat exchanger, but high-efficiency condensing furnaces have a secondary heat exchanger located in between the primary exchanger and the exhaust flue. While the metal tubes in a heat exchanger are effective at capturing heat from the fumes flowing through them, they can still only absorb heat at a certain rate. This means that the fumes still contain some latent heat when they exit the heat exchanger, and all of the heat that remains in the fumes is wasted.
Conventional furnaces are between 80 and 89% efficient. If a furnace is 80% efficient, it means that the heat exchanger captures 80% of the heat from the combustion fumes. The rest of the heat remains in the fumes when they exit the heat exchanger, resulting in 20% of the energy the furnace consumes being wasted.
The secondary heat exchanger is what allows condensing furnaces to be more energy efficient, as it works to capture most of the remaining heat from the fumes after they exit the primary heat exchanger. This process cools the fumes down so much before they flow into the exhaust flue that water vapor in the fumes condenses into liquid, which is why this type of unit is known as a condensing furnace.
The Importance of Annual Heat Exchanger Inspections
As we said, the heat exchanger keeps the harmful combustion fumes separated from the air that flows through the furnace. If the heat exchanger is damaged, some of the fumes will flow out of the tubes and into the ductwork where they’ll then be circulated throughout the building. This is a major safety hazard as it can result in carbon monoxide flowing out of all of the vents and into every room in the home. The best way to ensure this doesn’t happen is to have a technician fully inspect the heat exchanger and look for cracks, rust, and corrosion to ensure that the exchanger is fully sealed and not leaking.
The metal tubes are welded to the top and bottom of the heat exchanger, and these weld joints can sometimes crack and allow the fumes to start leaking out. The metal always expands as it heats up and then contracts when the furnace turns off and the heat exchanger cools down. This expansion and contraction can slowly weaken the welds. This means that the heat exchanger can crack simply due to age. However, it happens more commonly due to the furnace and heat exchanger overheating.
The high-temperature limit switch is designed to shut off the furnace if the temperature of the heat exchanger ever rises above a set limit. This is important for preventing damage and the risk of a fire. The problem with overheating is that the metal expands even more, which can lead to the exchanger cracking as it cools down and contracts.
If you notice that your furnace shut off before it brought your home up to your desired temperature, there is a decent chance that it overheated and triggered the limit switch. Furnaces will generally only ever overheat when the blower isn’t bringing enough cool air in and drawing it over the heat exchanger. If there isn’t sufficient air flowing over the exchanger, the temperature of the metal will continue increasing until the furnace starts to overheat.
The main reason this happens is due to the furnace filter being too dirty and clogged to the point where very little air can flow through it. This is why it’s important to replace the filter regularly. The problem can also happen if the blower is unable to pull sufficient air into the furnace through the return air vents because it is wearing out, malfunctioning, or clogged with dust.
Our Professional Services
Aurora Pro Services Heating, Air, Plumbing, Electrical, & Roofing is always here to help if you need to have your furnace inspected or you need any other heating service. We repair and service all types, brands, and models of furnaces, and we also offer professional furnace installation and a full range of air conditioning, plumbing, electrical, and roofing services for customers in Greensboro and throughout the Triad area. For more information or to schedule an appointment for any home service, give us a call today.
