Furnaces

Heat exchangers might not be something you need to know about in everyday life, but when it comes time to service or upgrade your furnace or boiler, 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 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 a furnace’s heat exchanger works:

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.

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.

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.

Common Heat Exchanger Problems and Symptoms

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

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 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

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

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.

Another thing that’s becoming more common for furnaces is lifetime replacement warranties. With this type of warranty, the manufacturer will replace the entire furnace if the heat exchanger or another major component fails during the coverage period.

Modern gas furnaces are so quiet, so convenient, and require so little attention that it’s easy to take them for granted.

But it has taken a lot of time, effort, money, and ingenuity to get to where we are today, and home heating wasn’t always as simple, safe, or efficient as it is today.

Appreciating and understanding your furnace can help you take better care of it, so let’s look at the history of gas furnaces, how we got to where we are today, and what life was like before we had these technologically advanced systems keeping our homes and families warm through the long winters.

Before the Gas Furnace There Was the Humble Fireplace

The ancestors of modern humans had only one major tool at their disposal to help them stay warm: fire. In fact, humans relied on charcoal and wood fires to keep warm up until the nineteenth century, when the first coal furnaces were made.

But in ancient homes, ventilation was understandably poor, and as a result, rooms were often filled with smoke and soot. Moreover, open fires aren’t typically very efficient at heating large spaces, so people had to stay pretty close to the fire to stay warm.

Additionally, as the size of homes continued to increase from mere single-room structures, these brick and stone fireplaces became increasingly inefficient.

One of the first stoves that could more efficiently heat larger spaces was the kang, a large masonry stove built of bricks or clay that was invented in China more than 7,000 years ago.

From Fires to Central Heating and Back Again

One of the earliest forms of central heating we know about is the ondol, a type of underfloor heating system used in Korea dating back at least 7,000 years. The system used a fire underneath a masonry floor to heat large spaces.

Another early type of central heating was the hypocaust, another wood-fired underfloor heating system used by the Romans more than 2,100 years ago.

The hypocaust fell out of use with the fall of the Roman Empire, and humans didn’t return to the idea of central heating for well over 1,000 years.

During the Middle Ages, people went back to basic hearths with open roofs that allowed smoke and soot to escape. When the domestic chimney was invented in the twelfth century, fireplaces became increasingly common.

Eventually, people in Europe and Asia started coming up with new versions of cockle stoves (masonry stoves covered in tiles) that were better at containing smoke and soot, and that had improved flue systems that allowed less cold air into the house.

Modern Developments in Central Heating Systems

Starting in the eighteenth century, people began to explore the idea of central heating more seriously, and the most common methods used hot air, hot water, hot steam, or electricity to heat homes.

• In the hot air category, a major development came when Benjamin Franklin invented the Franklin Stove in 1741. This unit was metal-lined and had a hollow baffle located near its back. The specialized stove produced less smoke and more heat, while the ductwork ensured that warmth was more evenly distributed throughout the room.
• On the steam front there was James Watt, inventor of the Watt steam engine, who developed the first working steam-based heating system for his home by attaching pipes to a boiler and running them through his house.
• Around the same time, a man named Franz San Galli invented the first hot water radiator in 1855, and this paved the way for modern radiators.
• Followed by that, Thomas Edison made a noteworthy contribution of his own: the invention of the electric heater in the early 1880s.
• But it wasn’t until 1885 that a man named Dave Lennox redefined the market with an affordable heating solution that did not use wood as its fuel. It was a coal-burning cast iron radiator, and it became the option of choice for many homeowners. Not only was this particular radiator much more affordable than others on the market, but it also made use of natural convection to distribute the heat more efficiently, similar to the Franklin Stove.

Alice Parker’s Furnace and the Rebirth of Central Heating

We have a woman named Alice Parker to thank for our warm and comfortable winters, because she was the first to patent a central heating system in America back in 1919.

Her 100-year-old design hasn’t changed much over the years: the thermostat-controlled system she invented had cool air drawn into a heat exchanger, heated with natural gas, and then circulated through the house via ductwork using the power of natural convection.

The design was a major breakthrough for home heating, not only because it was so effective, but also because it relied on natural gas instead of wood or coal. Eventually, electric fans were added to the equation to make heat distribution even more efficient and effective.

Shortly after Alice Parker’s patent, gas and oil versions of these forced-air furnaces would replace coal as the primary source of fuel for home heating—relieving homeowners of the task of manually adding coal (or wood) to the fire.

Parker’s furnace design was so effective that it still stands as the primary home heating method for homeowners today. Technological developments have made the design quieter, more efficient, and more cost-effective, but modern HVAC manufacturers are still working with the same basic design that Parker devised.

Modern Innovations that Have Improved Gas Furnaces

Even though the concept of the forced-air heat furnace has remained more or less the same since the middle of the twentieth century, there have been a wealth of innovations that have increased the efficiency, performance, and ease-of-use of gas furnaces.

Let’s look at a few of the developments that have significantly improved the modern gas furnace:

Secondary Heat Exchangers

Parker’s original furnace had a single heat exchanger that safely heated the air for your house while also keeping dangerous flue gasses separate from the air.

Some modern furnaces, called condensing furnaces, now have secondary heat exchangers that recover energy from the exhaust gasses that would otherwise be lost.

As such, condensing furnaces are more efficient, cost less to operate, and have less energy loss than traditional furnaces.

Two-Stage and Modulating Gas Valves

Another major contribution to the modern trend of high-efficiency furnaces comes in the form of two-stage and modulating gas valves.

A traditional furnace has a simple single-stage gas valve, which essentially means that your machine is either on or off. When on, the furnace runs at full power until the thermostat is satisfied, at which point the unit shuts off.

Building upon this came the two-stage option. Just as the name suggests, this variation added a new level at which the furnace could operate, depending on the environment or season. For example, the gas valve could be programmed to run at either 60% or 100%, thus operating more efficiently and more quietly.

Then came the modulating furnace. These operate in extremely precise increments. Instead of varying between just two stages, modulating furnaces can make incremental adjustments to their heating output when the thermostat calls for it.

Therefore, these units are able to continuously run at a very low setting while still maintaining a consistent temperature and using less energy.

Electronic Ignition Systems

The earliest gas furnaces relied on a pilot light to ignite the gas, and this wasted a great deal of gas and money.

By contrast, lots of modern furnaces use electronic ignition systems to light the burners. These have many advantages, including less waste, less money on wasted gas, and no maintenance problems related to the pilot light.

Programmable and Smart Thermostats

One of the earliest thermostats dates back to 1620, but modern versions began cropping up in the late nineteenth century.

In 1886, a man named Albert Butz patented the first electric thermostat. This device monitored temperature and regulated the damper to make temperature adjustments. Although he didn’t know it, Butz had created what would be developed into the modern room thermostat. The same year he got the patent, he founded a company that would eventually become Honeywell.

Before long, thermostats became programmable, but the next major development came in 2007, when a company called Ecobee began marketing the first smart thermostat.

Smart thermostats are excellent energy and money savers because they learn your habits and adjust heating and cooling settings automatically to reduce energy use.

Moreover, these devices can be controlled wirelessly through smartphones and tablets, making controlling your comfort settings a breeze.

ENERGY STAR® Certified Furnaces

One of the most important developments with modern furnaces is that they’re more efficient than ever. The most efficient furnace on the market right now has an AFUE rating of 99 percent, meaning it only wastes 1 percent of the fuel it consumes.

Moreover, it’s easier than ever for consumers to identify highly efficient models thanks to the ENERGY STAR program.

Originally launched in 1992 by the Department of Energy and the Environmental Protection Agency (EPA), ENERGY STAR has since been adopted by a number of countries, including Canada.

ENERGY STAR is essentially a certification program that celebrates appliances that are especially efficient. An ENERGY STAR furnace must have an AFUE rating of 95% or above.

Getting the Most from Any Furnace

The developments in modern gas furnace technology mean that when it’s time to upgrade your heating system, you can get a decent model on just about any budget.

Whether you want the most basic single-stage furnace or the highest-end model with all the bells and whistles, you’ll be able to find something that’s energy efficient and dependable.

Some important things to remember are to have your furnace sized properly for your home, to have it installed by a professional HVAC technician, and to have it serviced and maintained regularly. All of these things will ensure your furnace functions optimally, efficiently, and reliably, no matter what model you choose.

Buying a new furnace is a big decision because there’s so much choice on the market these days, including different brands, makes and models, available technologies, furnace types, and efficiency ratings.

But one of the most important things you’ll need to know before you buy a furnace is what size you need for your space.

Sizing a furnace isn’t just about calculating the square footage of your home. There are lots of things you have to account for when sizing a furnace. Today we’ll explain what those factors are, explore why it’s important to size a furnace properly, and talk about how you can get a furnace that’s the right size for your space, your climate, and your comfort needs.

➤ Try our new Furnace Sizing & BTU Calculator to get an instant estimate for your home!

Why Furnace Size Is So Important

It might be tempting to buy a smaller furnace to save money on the purchase price or to buy a larger one to maximize space-heating, but furnaces are like Goldilocks’ porridge: you need one that is just right, for your home.

A furnace that’s too big will cycle irregularly. This is bad because it will create hot and cold spots throughout the house, impact efficiency, increase maintenance, and reduce the lifespan of the furnace.

A furnace that’s too small will run continuously and still won’t be able to keep your home at a comfortable temperature. The increased workload will also raise your energy bills, create maintenance problems, and cut the life of the furnace short.

Furnace Size Can Maximize Energy Efficiency

Another problem with having a furnace that’s the wrong size is that it will negatively impact the efficiency of the heating system.

The AFUE (annual fuel utilization efficiency) of a furnace is calculated based on normal operation. When a furnace runs continuously or irregularly because it’s not the right size, then you won’t get the efficiency rating you paid for, and that will result in more energy use and higher heating bills.

To ensure your furnace operates at the efficiency level it’s supposed to, it has to be sized properly for your home.

How Furnace Size is Measured

When you talk about furnace size, you’re not actually talking about the physical dimensions of the unit. Rather, a furnace’s size is related to its heating output, and how much heat it can generate.

Furnace output is usually measured in British thermal units (BTUs) per hour. A BTU is a measurement of energy, and specifically the amount of energy required to increase 1 pound of water by 1 F.

A bigger furnace, such as one with an output of 120,000 BTUs per hour, will generate enough heat for a larger space. A smaller furnace will have a smaller BTU capacity, such as 40,000 BTUs per hour, and be suitable for a smaller space.

Some manufacturers have started measuring furnace size in MBH instead of BTUs. MBH is one thousand BTUs per hour, so you’re still working with the same measurement: 60 MBH is 60,000 BTUs.

Furnace Sizing Starts with Square Footage

One of the biggest things that will impact your furnace size is the size of your home, and specifically how many square feet of space need to be heated.

You can use the square footage to get an idea of what size range you should be looking at for your new furnace. From there, however, there are plenty of things that can impact the size furnace you need, including:

• The climate where you live
• The style and orientation of your home
• How many exterior walls you have
• The quality and thickness of your insulation
• The number of windows and doors, and their age
• Furnace efficiency
• The state of your ductwork

You’ll have to factor in all these elements, along with square footage, to accurately calculate furnace size.

Factors Other Than Square Footage that Impact Furnace Size

Climate

Canada is a rather large country, and the climate in winter can vary drastically depending on where you live. The climate will also help to determine what size furnace you need.

For example, a 2,000 square foot home in Ottawa would require a larger furnace than that same home in Victoria because Ottawa winters are so much colder.

Home Style, Layout, and Orientation

The design of your home is important to consider when sizing a furnace because there are so many little things that can impact heating requirements.

Orientation is one of them. South-facing homes get more sun, and in winter you can take advantage of this by letting in the light and allowing the sun to warm your home. In other words, south-facing homes might be able to get away with smaller furnaces than homes that are shaded.

The number of floors in your home will also impact BTU requirements, but maybe not in the way you think. A 1,000-square-foot home with a single floor will need a slightly bigger furnace than a 1,000-square-foot home with two floors because the second floor actually provides insulation, thereby reducing heating requirements.

Your floor plan can also affect furnace size because open-concept homes have fewer interior walls to hold heat compared to homes with lots of isolated rooms, and this can mean you need a larger furnace.

Furthermore, certain features like sliding doors or sunrooms may leak heat more readily and therefore may increase overall heating requirements, especially in that part of the home.

Insulation

Insulation is an important tool that can increase the efficiency of your home. Homes that are well-sealed and have thick insulation are more efficient, and efficient homes can typically get away with smaller furnaces.

Insulation isn’t just important in the walls: your attic, crawlspace or basement, and roof all need to be insulated to prevent heat loss.

Windows and Doors

Windows, skylights, and doors can be notorious for leaks and drafts, especially older ones that aren’t energy-efficient and properly sealed. The more glass and entryways you have, the more BTUs you’ll need to compensate for the heat loss.

By contrast, if you don’t have a lot of these features, they’re very small, or they’re well-insulated and energy-efficient, then a slightly smaller furnace might be sufficient.

Additional Heat Sources

Some homes have wood stoves, gas fireplaces, and other secondary heat sources that can reduce the demands on a furnace. If you have additional heat sources that you use regularly in the winter, then you won’t need a furnace with as many BTUs of heating output.

Ductwork

Like with windows and doors, leaky ductwork can increase heating demands throughout your house and increase the furnace size you need, along with increasing your energy bills.

It might be worthwhile to perform a visual inspection of your ductwork to see if there are any problem areas. By fixing leaks, you can improve the overall efficiency of your home and your heating and cooling systems.

Exterior Walls

Homes with fewer exterior walls have less exposure to the elements, and will likely have lower heating requirements. This can include:

• Apartments
• Condominiums
• Semi-detached homes
• Townhouses

Furnace AFUE (its energy efficiency rating)

We already mentioned how size can impact AFUE, but the AFUE of the furnace can also impact the size you need.

Let’s use a 100,000 BTU furnace as an example. At 100 percent efficiency, that furnace would indeed generate 100,000 BTUs of heat per hour. However, if the AFUE is only 90 percent, then that same furnace will only generate 90,000 BTUs of heat per hour.  

The take-home here is this: the lower the efficiency rating, the larger the furnace will need to be.

You will pay more for a higher efficiency model, but you’ll also be able to get away with a smaller furnace, and that will offset some of the cost. Furthermore, you’ll save more on energy bills in the long run with a high-efficiency furnace.

Estimating Furnace Size Based on Square Footage

We’ve already talked about the myriad factors that can impact furnace size, but you can get a basic estimate using the square footage of your home.

While you shouldn’t use this calculation to make a final decision about furnace size, you can use it to get an idea of the size range to look at when you’re shopping for a new model.

The general rule of thumb is that you need between 30 and 60 BTUs per square foot, depending on your climate.

Climate zones can help you determine how many BTUs per square foot you need. You can find more information about climate zones and insulation requirements from the North American Insulation Manufacturers Association.

Here’s a list of general BTU capacity requirements you can use as a starting point to estimate furnace size:

Let’s look at an example of how these estimates can help you.

Say you live in a cold climate like Winnipeg in a home that’s 1,800 square feet. The chart would indicate you need a furnace that’s around 108,000 BTUs. But let’s say your home is older, doesn’t have a lot of insulation, and has drafty windows. That could add an additional load to your heating requirement, so you might want to look at furnaces between 110,000 and 130,000 BTUs.

But then say you recently upgraded the insulation, installed energy-efficient windows and doors, and were looking at a furnace with an efficiency rating around 99 percent. In that case, you might want to look at furnaces between 85,000 and 100,000 BTUs instead.

These are just examples with estimated numbers to give you an idea of how the additional factors will change the furnace size you need.

Getting a Furnace Professionally Sized

The best way to ensure your furnace is the right size for your home is to have an evaluation from a professional HVAC expert.

An experienced HVAC technician will be able to accurately calculate the square footage of your home, factor in all the other elements that can impact furnace size, and give you a precise BTU capacity for your furnace.