Furnace Heat Exchangers – What You need To Know?

Most people do not give their furnace a second thought until it breaks. Then, it is a race to look up everything you can and try to figure what went wrong.

A critical part of your furnace that you should know about and monitor is the heat exchanger. It is vital to ensure that your furnace is functioning correctly. 

A furnace heat exchanger is a device that keeps the combustion process of generating heat separate from the air you breathe for safety reasons. 

If you want to know more about how a heat exchanger works in your furnace, why they are so important, and the different types that exist, give this article a read.

Knowledge is power, and you might be able to avoid some problems altogether if you keep a watchful eye out. 

What Is A Furnace Heat Exchanger?

The heat exchanger in your furnace separates your breathable air from the combustion process involved in keeping your home warm and cozy.

This is crucial for avoiding carbon monoxide poisoning! If your furnace’s heat exchanger malfunctions, it puts everyone in the household in danger. 

The heat exchanger spans the furnace from the burner assembly to the place where the chimney vent connects. The heat exchanger comprises that metal corridor and a separate chamber to allow for a safe release of hot air throughout your house.

Heat exchangers are not unique to furnaces; you can also find them in refrigerators, vehicles, and pools.

How Do Furnace Heat Exchangers Work?

A furnace heat exchanger works by exchanging the heat created within the furnace with the cold air sucked in from your house’s interior.

The combustion process makes the air warmer, and the resulting byproducts need to be kept separate from your air, or it will become toxic to breathe. The whole point is to make your house nice and warm, not poisonous. 

The air warms because when a higher temperature gas interacts with a lower temperature gas, the heat transfers from one to the other respectively as they naturally move towards equilibrium.

Once the air temperature has been swapped, the warm air is pumped out into your house, keeping you at your preferred temperature. 

For a furnace heat exchanger to work, it has to be completely sealed. The combustion byproducts are gases, which means they could easily escape through even the tiniest crack.

The steps involved in a gas-powered forced-air HVAC system, the most common type nowadays, proceed as follows:

  • You set the furnace’s thermostat for a specific temperature.
  • The furnace detects that your house’s temperature has dropped below your set limit.
  • The furnace turns on and starts the combustion process. The hot gases rush into the heat exchanger’s metal chamber and start warming up the chamber walls.
  • Your air ducts suck in the cold air throughout the house and run the air through the heated chamber. As the air travels along the walls, the heat is transferred into the air. 
  • The now warmed air is pumped back out throughout your house.
  • The whole time, the combustion gases are kept out of your house’s air.
  • The unwanted combustion gases then get blown out of a vent that directly connects the heat exchanger to the outside. 

Some people choose to use high-efficiency furnaces that actually use two heat exchangers. Their process is somewhat different.

When an additional heat exchanger is used, it draws the combustion gases through it to generate more heat for your house so that nothing is wasted. The combustion byproducts are still expelled outside of the house afterward. 

Types Of Heat Exchangers

The metals used most often in heat exchangers are titanium, stainless steel, and copper because they are great heat conductors. The entire point of a heat exchanger is to conduct heat from the walls to the air, so you always want to use metal.

Heat exchangers can be made with a wide variety of materials and components. A few of the most common elements include:

  • Plates
  • Shells
  • Adiabatic wheels
  • Tubes
  • Fins
  • Coils

Shell And Tube Heat Exchanger

A heat exchanger can either connect to the burner assembly through small coils and exit to the flue pipe, called a shell and tube heat exchanger, or it can be a plate heat exchanger. The former method is known as a shell and tube heat exchanger.

A shell and tube heat exchanger holds the two gases in separate spaces which are inside of a shell. The gases can transfer heat, but nothing else mixes.

The pathways inside are usually made of the same metals that we mentioned above because they are so good at transferring heat.

Gas flows from one side to the other inside of the tubes while the second gas runs through the shell. As it does so, the metal tubes heat up, and that heat is transferred to the gas just inside the cover.

A shell with a larger tube bundle, meaning the number of tubes inside of the shell, has a higher-efficiency heat transfer because the more tubes there are, the more warmth can be generated within a single shell using the combustion gases that your furnace generates.

Every tube has a turbulator inside that stops any kind of buildup from blocking the way.

Baffles are also used within the shell to increase the amount of heat that is transferred. The gas in the cover has to move around the baffles, small barriers, which means that the gas has to go over the tube bundles multiple times, giving them more opportunities to absorb the heat from those heated tubes. 

Single-Phase Heat Exchanger

A single-phase heat exchanger keeps your airflow constant.

The gas heated by the combustion process goes into the tubes, flows through, and then exits the tubes into a vent outside the house. The cold air sucked in from the house goes into the shell, flows around the pipes, warms up, and then exits into your home. 

Two-Phase Heat Exchanger

In contrast to a single-phase heat exchanger, a two-phase heat exchanger causes a phase change during the process.

For those of you who do not remember your chemistry, a phase change means that hot air (steam) enters the tubes and liquid water comes out the other side. 

An exchanger can also be single or multipass, which refers to how many times the air passes through the shell before being released.

Multipass systems are more efficient than single-pass because the air is shunted back and forth, via the baffles, across the tubes, which gives the air more time to absorb the heat before being released into your home. 

U-Tube Heat Exchanger

The tubes are continuous and bend into a U-shape, giving this heat exchanger its name. All of the pipes are held in place by a single tubeplate that anchors them to the shell.

That U shape makes the heat exchanger into a multipass one, which is more efficient. The form also lets the heat expansion happen without expansion joints since the bend floats freely.

The only major downside to a U-tube heat exchanger is that it is difficult to clean, but you will be hiring an HVAC professional to do any cleaning and maintenance, so you will not have to worry. 

Fixed Tube Heat Exchanger

A fixed tube heat exchanger has two tube sheets that are anchored into the shell. Because this is the easiest type of heat exchanger to make, it also is the least expensive.

If you are looking for a new heat exchanger and are on a budget, this might be the right type for you. However, cost-effectiveness comes with a different kind of cost.

Since the tubes are firmly attached to the walls of the shell, they are not able to freely expand and contract.

They work best when the temperature between the gas inside the tubes and the cover is not drastically different. If they are, the pipes might expand, and that can cause damage and the heat exchanger to crack or burst.

In addition, like the U tube heat exchanger, the fixed tube heat exchanger’s tubes cannot be cleaned since they are firmly sealed within the shell. 

Floating Head Heat Exchanger

The newest and best version of a heat exchanger, the floating head heat exchanger combines the best of both worlds.

A fixed tube sheet holds one end of the tubes firmly inside the shell, while the other end is controlled with a floating tube sheet and can freely expand and contract. In this model, the pipes do not need to bend to be held aloft inside of the shell.

With the floating head heat exchanger, the tubes are easily accessible and can be cleaned as part of the routine maintenance.

You also do not have to worry about a large temperature difference between the gases since the tubes can withstand the heat expansion and contraction.

This is the most expensive type of heat exchanger, but it is incredibly efficient. 

Plate Heat Exchanger

A plate heat exchanger works by having a bundle of thin plates stacked on top of each other. The stacked plates make passages through which gas can flow.

The plates can be stacked through a variety of methods. Rubber gaskets hold them together, and bolts are tightened to form a waterproof seal. 

They can be brazed, welded, or bolted so that another passage underneath allows for the other gas to pass through without mixing with the combustion byproducts.

The plates are held in alignment by supportive bars located on the top and bottom of the heat exchanger.

One benefit is that these heat exchangers are smaller than the shell and tube ones. 

How Do Plate Heat Exchangers Work?

The plates in a plate heat exchanger are usually made of highly conductive metals like titanium, steel, nickel, aluminum alloy, or graphite. The corrugations in the plates give the gases more surface area in which to transfer the heat and make turbulence.

There are various methods of corrugation which provide unique benefits. 

Each plate has its own gasket in order to keep the gases separated and ensure each gas flows down the plate stacks that it is squeezed into. This separation is essential.

The plates vary according to the gas being pumped through and alternate so that each successive stack is the opposite variety of gas. 

For example, the combustion-heated air starts at the top and flows down in every odd-numbered plate, while the cold air from the house begins at the bottom and flows up in every even-numbered plate to maximize heat transfer.

The gaskets can also be configured to allow for different airflows, which change the heat exchanger’s efficiency. 

They can be configured for either single-pass flow or multipass flow, which was addressed above. Rubber gaskets aren’t the only type of sealant used in plate heat exchangers, however.

Plate brazed heat exchangers use copper brazing, which lets the pressure increase without causing problems. 

Welded plate heat exchangers are even better for higher pressures, but the nature of the plates being welded together is that they cannot be adequately cleaned.

Semi-welded plate heat exchangers use both welded and non-welded plates together to give you the best of both worlds in a single heat exchanger. 

Advantages And Disadvantages Of Plate Heat Exchangers

Despite their widespread use, it is essential to know the advantages and disadvantages of plate heat exchangers so that you can decide whether it is right for your furnace and your home.

You should always consult with a professional HVAC installer to ensure that the heat exchanger you choose is compatible with your furnace. 

When compared to shell and tube heat exchangers, in many ways, plate heat exchangers are superior, but not in every case.

In cases when the temperature difference between the cold air in your house and your furnace-heated air will be vastly different, a shell and tube heat exchanger is preferable. 

AdvantagesDisadvantages
Small and compactIf the rubber gaskets are excessively compressed, they can warp and leak.
More efficient at heat transferRubber gaskets are somewhat temperature sensitive, so high temperatures shouldn’t be used.
More surface area available to gasesPlate pathways are narrow, so you may need additional power to maintain airflow.
Requires less maintenance than shell and tube heat exchangersGases with significantly different temperatures don’t transfer as well as shell and tube heat exchangers.
Easy to repair Less efficient because heat can escape

Furnace Heat Exchanger Cost

If you decide to replace your furnace’s heat exchanger, you will need to hire an HVAC professional. You should not attempt to do the replacement yourself if you aren’t trained.

Additionally, if you try to repair the heat exchanger yourself, you may end up voiding the manufacturer’s warranty or injuring yourself.

Your furnace is not the place to embark on a personal DIY quest, no matter how handy you are. While you can hire someone to repair your heat exchanger, it may be more budget-friendly to replace the entire thing.

Furnace heating being repaired by a repairman

It often takes a professional about nine hours to dismantle the furnace, remove the old heat exchanger, install the new one, and reassemble it. 

Rates will vary depending on the HVAC company you choose and the location in which you live. You should contact multiple HVAC companies in order to compare their rates and find the one that is right for you and your home. 

Some people also choose to replace the whole furnace when the heat exchanger needs to be replaced, so consider that if you are selecting a new heat exchanger and are not sure whether it would be compatible with your current furnace.

Benefits Of Heat Exchangers

The benefits should be evident in how the combustion gases are kept separate from your breathable air.

Without a heat exchanger, it would not take long to poison your household with carbon monoxide ultimately. With this in mind, it is always smart to have a carbon monoxide detector in your house.

That way, if anything breaks or goes wrong, you and your family will be immediately alerted once the levels of carbon monoxide become dangerous, and you can evacuate the house before anyone is seriously injured.

However, to avoid that situation, you should frequently check your furnace’s heat exchanger(s) to ensure that they are functioning correctly. 

Caring For A Heat Exchanger

Your furnace’s heat exchanger is a vital component to keeping your house warm and safe. As such, you want to ensure that your heat exchanger survives intact for as long as possible.

If you’re going to take extra precautions, hiring an HVAC specialist to come out once a year and perform routine maintenance on the heat exchanger can go a long way towards keeping you safe. 

Routine Maintenance 

Routine maintenance involves the specialist checking for any kind of damage and repairing that damage, or advising you that the damage cannot be repaired, and helping you pick out and install a new heat exchanger before any harm comes to your household.

This kind of maintenance can be invaluable for older heat exchangers. 

They will also check to see if any of the vents are blocked or closed. That kind of air blockage can lead to rupturing the combustion chamber and leaking the combustion byproducts into your air.

Normal wear and tear for your furnace’s heat exchanger can also lead to these kinds of problems. The average lifespan of a heat exchanger is ten to twenty years

Your HVAC professional will use an infrared camera to check if there are any tiny cracks or fractures that might otherwise be invisible to the human eye. This is crucial to maintaining your home’s safety.

If there is a problem, you may have to replace the heat exchanger or, sometimes, the entire furnace depending on the problem’s severity. 

Changing The Air Filter

The manufacturer’s specifications say how often you should change the air filter. Generally, you should change it every three to six months, depending on how much use your furnace gets.

You may have to change it more often when it is cold. Also, remember never to shut any of your vents. Some people choose to close vents to save money, but it can be dangerous. 

Man changing air filters

What Are The Common Problems With These Heat Exchangers?

As you may have guessed, many things can go wrong with a furnace heat exchanger, especially as time goes on. These are the most common problems that you should watch out for when it comes to your furnace’s heat exchanger.

If you notice any of these issues, you should immediately have a new one installed.

Sealing Break

A break in the sealing of the combustion chamber can be a serious issue. Any kind of crack or chip can allow the dangerous combustion byproducts to escape because gas doesn’t require much of a puncture to get out.

If any damage happens to the sealing, the carbon monoxide can easily flow into your house and taint the previously-breathable air.

Metal Fatigue

Metal fatigue, or when your heat exchanger fails because of constant heating and expansion of the metal walls, followed by the cooling and contracting.

Although the metal is strong, after enough times of being heated and cooled, the metal will eventually give out. 

Poor Installation

Poor installation is always a problem. Ensure that you trust the HVAC installer you hire and have the heat exchanger checked at least once a year by another professional to ensure proper installation.

Poor Design

Poor design by the manufacturer can’t be helped by all of the most trustworthy installers in the world.

Talk to your installer before putting in a heat exchanger and get a professional opinion to make sure that you are choosing a well-reviewed and highly rated quality heat exchanger for your furnace. 

This is especially important if you live in an area of the world that is frequently cold since you will likely be running your furnace more often. 

Improper Maintenance 

Improper maintenance can shorten the lifespan of any device. Follow the suggestions that we gave above, and you shouldn’t have to worry that anything you did contribute to how long your heat exchanger remained adequately functioning. 

In Conclusion

A furnace heat exchanger is a device that separates the combustion byproducts from the air you need to be able to breathe in your home. Ensure that your heat exchanger is working so that you won’t ever have to worry about carbon monoxide poisoning.