Everything You Need to Know About Heat Exchangers

Furnace heat exchangers might not be something you need to know about in everyday life, but when it comes time to service or upgrade your heating system, it can be useful to know a few basics about these systems.

Knowing about heat exchangers can help you decide whether it’s worthwhile to pay for maintenance on an older furnace or boiler, assist you in choosing a new heating system for your home, and make it easier for you to compare the warranty terms offered by different brands.

This guide will explain what heat exchangers are, how they work, why they’re important, and what new technologies are available, as well as cover important points you need to know to make informed decisions for your home and HVAC system.

What Is a Heat Exchanger in a Furnace and How Does It Work?

In the simplest terms, the heat exchanger is the component inside your furnace or boiler that creates the heat that warms your home. Without a heat exchanger, your furnace wouldn’t be much more than an expensive metal box.

Here’s a slightly more in-depth explanation of how boiler and furnace heat exchangers work:

When your thermostat registers that your home is too cold, it sends a message to the furnace to turn on.

The heat exchanger is connected to the burner at one end, and that’s where the combustion process begins. Inside the heat exchanger, gas combusts and creates heat. This heats the surface of the heat exchanger.

The furnace then brings in cool air from your home using the air return ducts. The blower then blows air across the surface of the heat exchanger, thereby heating the air. That heated air is then distributed throughout your house using the ductwork and vents.

At the other end of the heat exchanger, the system is connected to a flue that safely vents combustion by-products outside, keeping the air in your home clean and breathable.

Furnace Heat Exchangers: Understanding Their Importance

Boiler and furnace heat exchangers are important because of heat transfer, safety, and efficiency.

The heat exchanger transfers heat from the combustion chamber to the air circulating through your home. In other words, without the heat exchanger, you wouldn’t have heat.

But just as importantly, the heat exchanger ensures combustion gasses don’t mix with the air you breathe.

Finally, a well-designed heat exchanger maximizes heat transfer and minimizes energy waste, making it cheaper to heat your home.

Heat Exchangers Are Also Safety Devices

An important thing about a heat exchanger is that it doesn’t just produce heat—it also keeps the combustion process contained so that your home doesn’t fill with dangerous gasses like carbon monoxide.

This is one reason professional installation is so vital when you get a new furnace. If the furnace isn’t properly connected to the gas line and flue, then it could pose a serious threat to your family’s health and safety. This is also why regular furnace maintenance and inspections are necessary.

Types of Heat Exchangers and Other Applications

Heat exchangers are widely used in many industries and applications. For example, heat exchangers are common in power plants, oil and gas production, the food and beverage industry, medical equipment, chemical processing, and more.

Heat exchangers are also common in everyday applications, and you’ll find them in furnaces, boilers, refrigerators, air conditioners, car radiators, and more.

In HVAC applications, some of the most common boiler and furnace heat exchangers include:

• Tube and shell: A series of tubes inside a cylindrical shell
• Plate: Thin, corrugated metal plates stacked together
• Finned tube: Tubes with extended fins to increase the surface area for heat transfer
• Serpentine: Winding, snake-like tube design in a compact form

Importance of Heat Exchanger Efficiency

An efficient heat exchanger minimizes energy waste and fuel consumption while maximizing heat transfer.

For homeowners, that translates to lower energy bills. So the more efficient the heat exchanger, the less you’ll pay to heat your home.

Beyond that, efficient, well-designed boiler and furnace heat exchangers generally experience less wear and tear, and that can potentially mean lower maintenance bills, fewer repairs, and a longer lifespan.

Proprietary Designs and Innovations

Over the years, HVAC and other manufacturers have worked hard to perfect the heat exchanger and come up with new and improved designs, especially for furnaces. When you go shopping for a new furnace, you might come across terms like clamshell, RPJ, and more.

Let’s have a look at these terms, and the new and proprietary designs that are available from some furnace brands:

• Clamshell: A clamshell heat exchanger is one that doesn’t rely on welds to keep the joints together. This increases longevity by reducing cracking or separating of the joints. Brands that use this design include Lennox, Comfortmaker, Tempstar, and Heil.
• RPJ: This stands for Rigid Press Joint, and it’s a type of clamshell heat exchanger. You can find this component with several furnace brands, including Heil, Tempstar, and Comfortmaker.
• Crimped: This is similar to the clamshell idea, so the joints are crimped together instead of welded to prevent damage from heat stress, improve airflow, and increase durability. This design is available with select Trane and Armstrong (EHX technology) furnaces.
• Turbulators: A turbulator is a device that causes irregular motion of a fluid. When you add a turbulator to a heat exchanger, like Napoleon’s Vortex, it disrupts airflow inside the heat exchanger and recovers more heat from flue gasses.
• Stainless Steel: Some manufacturers like Trane offer stainless steel heat exchangers which may be more durable and longer lasting than aluminium ones.

The Benefits of a Secondary Heat Exchanger

When it’s time to upgrade your heating system, you’ll probably come across condensing furnaces or boilers. These are options you’ll want to consider because they’re more efficient and have less energy loss than traditional models.

Condensing furnaces and boilers have two heat exchangers. The primary heat exchanger works as described above, with combustion gasses being passed into the flue to be vented outside.

But in condensing boilers and furnaces, the combustion gasses instead pass into a secondary heat exchanger, where water vapour forms. This process releases heat, thereby heating the surface of the secondary heat exchanger, which is used to heat the air.

Secondary heat exchangers recover heat and energy that would otherwise be lost in a regular furnace or boiler, so condensing models are more efficient and can heat your home for less.

Warning Signs of a Damaged Furnace Heat Exchanger

Boiler and furnace heat exchangers are made of metal that’s constantly heating up and cooling down, so fatigue (weakening) from heat stress is one of the most common problems with these systems. Stress can cause cracks, joints to separate, and other failures.

Earlier, we talked about how heat exchangers function as safety devices, so you can see why a crack could be so dangerous. A cracked heat exchanger can allow exhaust gasses to escape into your home, potentially causing carbon monoxide poisoning and other problems.

The most obvious sign that there’s a problem with your heat exchanger is that your furnace or boiler stops working as it should. But other symptoms to look for include:

• The smell of gas (in that case, get your family out of the house and call your utility company or an expert HVAC technician)
• Cracks in the heat exchanger or furnace
• Soot buildup
• Rusty heat exchanger
• Pooled water on the floor
• Change in the flame colour
• Carbon monoxide detector goes off
• Bad odours other than a gas smell

Safety Concerns and Carbon Monoxide Risks

A boiler or furnace heat exchanger safely contains combustion gasses, which include carbon dioxide, carbon monoxide, nitrogen oxides, sulfur dioxide, methane, and volatile organic compounds.

One of the biggest safety concerns with boiler and furnace heat exchangers is a carbon monoxide leak. This can happen because of a crack/leak in the heat exchanger, metal fatigue, incomplete combustion, or improper venting. Signs of carbon monoxide poisoning include:

• Headache
• Dizziness or lightheadedness
• Nausea and vomiting
• Fatigue or weakness
• Shortness of breath
• Confusion or difficulty concentrating
• Blurred vision
• Chest pain
• Rapid heartbeat or irregular heart rhythm

Beyond that, other heat exchanger safety hazards include gas leaks, poor air quality, and fire hazard if a crack allows flames to escape the combustion chamber.

Help keep your home and family safe with regular furnace inspections, professional maintenance, and installing carbon monoxide detectors.

Repairing vs. Replacing Furnace Heat Exchangers: Factors to Consider

When facing a critical failure with a furnace or boiler, some homeowners wonder, “Can a furnace heat exchanger be repaired?”

A cracked heat exchanger shouldn’t be repaired—only replaced. For one thing, it’s difficult to access a heat exchanger to complete a repair. Further, a repaired crack might still leak carbon monoxide, and the risk is too high.

For other heat exchanger problems, such as corrosion, scaling, or mechanical damage, it may be possible to repair the problem. Talk to a local HVAC expert for an assessment and quote.

How much does it cost to replace a heat exchanger in a furnace?

The heat exchanger is one of the most expensive components in a boiler or furnace. It typically costs anywhere from $1,000 to $3,500 to replace a heat exchanger, but it can be more depending on the labour costs, type of furnace/heat exchanger, age of the furnace, etc.

Repairing a Heat Exchanger Versus Buying a New Furnace or Boiler

Choosing between maintenance and upgrading a furnace or boiler can be a difficult decision, but luckily there are some simple rules to help you make the right one.

Rule #1: Replace Older Models

Heat exchangers tend to last between 10 and 20 years, so if your furnace or boiler is upward of 10 years, then you might be better off replacing it.

And when you do upgrade, you’ll get a more efficient unit with modern technology that will reduce your energy bills and make operation a breeze.

Rule #2: Repair a Warrantied Heat Exchanger

Heat exchanger warranties typically last anywhere between 10 years and the lifetime of the unit, so if the manufacturer is going to pay for the repair or component replacement, then you might as well take them up on the offer.

Rule #3: Upgrade When Repairs Get Too Pricey

Draw the line at 50 percent: if the cost of the repair will be more than 50 percent of what you’d pay to buy a new furnace or boiler, then don’t waste the money on the repair. A heat exchanger can cost well over $1,000 to repair or replace, and that could get you well on your way to a brand new furnace or boiler.

Rule #4: Replace Cracked Boiler or Furnace Heat Exchangers

Once a heat exchanger cracks, it’s safer to replace it than repair it. Because of the cost of a new heat exchanger, you might be better off upgrading your entire heating system.

Common Warranty Terms for Heat Exchangers

Repairing or replacing a heat exchanger is costly, so you’ll want to look for brands and models that are backed by long warranties.

Furnaces and boilers sold in Canada will have at least basic warranty coverage of five to 10 years for parts and major components, including the heat exchanger.

However, some manufacturers offer superior coverage, which can include:

• 20-year to lifetime warranties
• Separate warranties for the heat exchanger
• Unit replacement warranties (the manufacturer will replace the entire furnace if the heat exchanger or another major component fails during the coverage period)

Just be aware that most manufacturers require you to register a new furnace or boiler to be eligible for full warranty terms.

The warranty coverage for a heat exchanger will depend on the manufacturer and what type of heating system you have:

Boilers: HVAC manufacturers typically warranty the heat exchangers in their boilers for 10 to 12 years.

Furnaces: Most brands warranty the heat exchangers in their furnaces for at least 20 years, and lifetime warranties are quickly becoming the norm, especially for the mid- and top-tier furnaces from most brands.

Furnace Heat Exchanger Maintenance and Inspection Tips

Most boiler and furnace maintenance should be done by a qualified HVAC professional, you can do some DIY maintenance. Here are some maintenance and inspection tips:

• Do a visual inspection every year: Look for cracks, holes, rust, discoloration, and soot buildup
• Replace the filters, usually every one to three months, depending on the type of filter
• Check the burners and flame sensor regularly—you want the flame to be blue
• Make sure air is flowing properly from the registers and that there aren’t obstructions in the ductwork
• Clean the furnace system annually: Vacuum the interior and exterior to remove dust, dirt, and debris from the blower, burner, and heat exchanger

DIY vs. Professional Maintenance

While the steps above will help keep your furnace in good shape, you still need to schedule annual maintenance and inspections from a licensed HVAC professional.

They have experience with furnace heat exchangers and can make sure the entire heating system is safe, operating efficiently, and in optimal working condition.  

What’s more, when the boiler or furnace needs repairs, they’ll have the skills and training to assess, diagnose, and repair or replace. They can even catch and fix problems early, before they become major issues.

Again, because boiler and furnace heat exchangers are essential for the safe operation of your heating system, it’s critical that maintenance is performed by a trained HVAC technician.

Conclusion

The heat exchanger is one of the most important components in a boiler or furnace. Not only does it produce heat for your home, but it also keeps you safe by containing combustion gasses.

Because heat exchangers are safety devices, you should have your boiler or furnace installed, inspected, and maintained annually by a licensed HVAC technician.

You should also clean and inspect your furnace or boiler yearly and familiarize yourself with signs that the heat exchanger is damaged. If you do suspect a problem, contact a local HVAC pro immediately to keep your home and family safe.

Depending on the issue, you may be able to repair or replace the heat exchanger, or it might be time to upgrade to a new furnace or boiler.

FAQ

What is the purpose of a heat exchanger?

A furnace heat exchanger transfers heat generated by the combustion of fuel (natural gas, oil, or propane) to the air circulated through the home. It provides heat for the furnace to circulate while also preventing combustion gasses from escaping into the air you breathe.

Can boiler/furnace heat exchangers be replaced and is it worth it?

It is possible to replace the heat exchanger in a furnace or boiler. Whether it’s worth it depends on:

• Warranty: If the system is still under warranty, you may be able to replace the heat exchanger free
• Age: If the furnace or boiler is more than 10 years old, it might be worth buying a new one, especially if you can get a more efficient system with better features
• Cost: If replacing the heat exchanger will cost more than 50% of the price of a new furnace/boiler, then buy a new furnace/boiler

What happens when a heat exchanger fails?

When a heat exchanger fails, it can cause reduced efficiency, bad smells, carbon monoxide leaks and safety hazards, water leaks, and more. Depending on the cause and severity of the failure, the furnace may produce uneven heat or no heat at all.

Can you run a furnace with a bad heat exchanger?

No, you shouldn’t run a furnace with a bad heat exchanger. It’s unsafe and poses significant risks to your health (carbon monoxide poisoning), safety (fire hazard), and the efficiency of your heating system (running it could cause further damage).

How many hours does it take to replace a heat exchanger?

It can generally take anywhere between two and 10 hours to replace a heat exchanger, depending on things like accessibility, furnace/boiler type, heat exchanger style (some are modular and designed to be removed/replaced easily), and more.

What causes a furnace heat exchanger to fail?

Many things can cause boiler and furnace heat exchangers to fail, including age, metal fatigue, overheating, corrosion, improper combustion, lack of maintenance, improper installation, incorrect sizing for the space, and damage from vibrations.