You can do everything right with your furnace and still end up with a cracked heat exchanger and a carbon monoxide risk in your home. For a lot of Austin homeowners and property managers, that diagnosis comes out of nowhere during a routine tune-up or home inspection. One day the heat is working, the next day a technician is telling you the unit is unsafe and should be shut down.
That kind of news hits hard. You might be worried about your family’s safety, nervous about tenants in a rental, and suspicious that someone is just trying to sell you a new system. At the same time, you have probably read enough headlines about carbon monoxide to know this is not something to ignore. What you rarely get is a clear, mechanical explanation of what a heat exchanger crack really is and how it puts your home at risk.
We have been working on heating systems in Central Texas since 1972, and our HVAC team has seen how heat exchangers in Austin-area homes actually age and fail. In this guide, we will walk through what your heat exchanger does, why it cracks, how those cracks can leak carbon monoxide, and what a responsible repair or replacement decision looks like. Our goal is to replace guesswork and fear with real information and a clear plan.
Don't let a broken heater ruin your day—call (512) 710-1032 now for fast assistance, or contact us online for a free estimate.
What Your Furnace Heat Exchanger Actually Does
To understand why a crack matters, it helps to picture what is happening inside your furnace when the heat is on. In a gas furnace, the burners ignite and produce a flame that burns a fuel and air mixture. That flame does not heat your indoor air directly. Instead, it heats the metal walls of the heat exchanger, which is a sealed metal chamber or series of tubes hidden behind the burner area.
On the other side of that metal, the blower pushes room air across the outside of the heat exchanger. The metal gets hot from the flame, and the blower air picks up that heat as it passes over the surface, then moves through your ductwork into the living spaces. Your indoor air and the combustion gases never touch each other. The only thing separating them is the metal of the heat exchanger and its welds and seams.
Inside that chamber, the flue gases contain a mix of carbon dioxide, water vapor, nitrogen compounds, and some amount of carbon monoxide. Those gases are supposed to leave the home through a flue pipe or vent. As long as the heat exchanger stays fully sealed, your family only receives warm air, not combustion products. Once that metal splits, rusts through, or separates at a seam, there is a physical opening that can let those combustion gases into the moving air stream.
In Austin, we see all the common furnace layouts, from closet units to attic furnaces serving two-story homes in communities around Lago Vista and the greater metro area. No matter the layout, the heat exchanger’s job is the same. It is the safety barrier that keeps the burning side of the furnace and the breathing side of the house completely separate.
Why Heat Exchangers Crack In Austin Homes
Heat exchangers do not usually fail because a homeowner pressed the wrong thermostat button or skipped one filter change. They crack because of how metal behaves when it is repeatedly heated and cooled. Every time your furnace fires, the heat exchanger heats up, expands slightly, then contracts again as it cools. Over thousands of cycles, that expansion and contraction works the metal the same way bending a paperclip back and forth will eventually cause it to snap.
This process is called metal fatigue. The stress tends to concentrate in specific areas, such as welds, corners, tight bends, and places where different pieces of metal are joined. When the system short cycles, meaning it turns on and off frequently for short runs, those cycles add up much faster. Short cycling can come from an oversized furnace, thermostat issues, or airflow problems like dirty filters and restricted ducts.
Corrosion is another major factor. In high-efficiency condensing furnaces, flue gases are cooled enough that water vapor condenses inside a secondary heat exchanger. That condensate can be slightly acidic. If it is not draining properly, or if the materials were not designed to handle long-term exposure, it can eat away at metal surfaces from the inside. Even in non-condensing furnaces, poor combustion can produce more corrosive byproducts that accelerate rust and thinning.
Installation and environment play a role too. In the Austin area, many furnaces sit in hot attics that can reach extreme temperatures in summer. That means the furnace cabinet and heat exchanger go from very hot attic conditions in the off-season to repeated heating cycles in winter, which increases total thermal stress. If the unit was not installed with proper clearances or airflow, parts of the exchanger can run hotter than they were designed to, which also shortens life.
Over more than five decades in Central Texas, we have seen patterns. Certain styles of older equipment tend to crack in predictable spots. Attic-mounted furnaces that are oversized for the home are more likely to short cycle and develop cracks earlier. None of that is about a single homeowner doing something wrong. It is about materials, design limits, climate, and long-term stress adding up inside a piece of metal you never see.
For honest work and upfront pricing on heater repair, dial (512) 710-1032 today, or connect with our team online.
How A Crack Lets Carbon Monoxide Into Your Living Space
Once you understand that the heat exchanger is a sealed barrier, it becomes easier to picture what happens when there is a crack. When the furnace starts, the burners light and the flue gases fill the inside of the exchanger. A moment later, the blower comes on and pushes room air across the outside of the exchanger and into the supply ducts. That moving air creates pressure on the outside surface of the metal.
If there is a crack or hole in the heat exchanger wall, the pressure difference between the blower air and the flue gases can push air through that opening. In many situations, that means a mix of combustion gases can be drawn or blown into the air stream that is supposed to be clean. The actual direction of flow depends on the specific pressures and temperatures at that moment, but the bottom line is that the separation between breathing air and exhaust has been broken.
Carbon monoxide is especially concerning in this situation. It is colorless and odorless and cannot be detected by smell or sight. In a tight, energy-efficient home, such as many newer builds around Austin, there is less natural leakage, so any CO that gets into the ducted air can linger and accumulate more than it might in an older, draftier structure. Depending on where supply vents and returns are located, some rooms may see higher concentrations than others.
Low levels of carbon monoxide exposure over time can lead to headaches, fatigue, nausea, and confusion that people often write off as a virus or “winter blues.” Higher levels can cause loss of consciousness and can be fatal. CO detectors are an important safety net, but they are typically designed to alarm above certain thresholds and over certain time periods. They cannot keep a cracked heat exchanger safe to operate. They only tell you when levels have become dangerous enough to trigger the device.
In our work across the Austin area, we have seen furnaces with visible cracks that had not yet set off any detectors, as well as homes where detectors did alarm because of a leaking exchanger. That is why the HVAC industry treats a confirmed heat exchanger crack as a serious safety defect. It is not the size of the crack you can see that matters, it is the fact that the only barrier between exhaust and breathing air is no longer intact.
Signs Of A Heat Exchanger Crack And What They Really Mean
Most homeowners and property managers want a simple checklist: if they see one thing, they have a crack, and if they do not see it, they are safe. Unfortunately, it does not work that way. Some cracked heat exchangers show very few visible or obvious symptoms. Others create a cluster of clues that point strongly to a problem. Understanding what those clues really mean helps you avoid both false reassurance and unnecessary panic.
One common pattern is a furnace that starts, runs briefly, then shuts itself down repeatedly. Modern furnaces use safety switches and sensors to monitor temperature and airflow. If the unit sees temperatures that are too high or other readings outside the normal range, it may shut off the burners as a protective measure. That can happen with a cracked heat exchanger, but it can also happen from other issues, such as a clogged filter or failing blower motor.
Visual signs around the furnace can also raise suspicion. Soot or scorch marks near the burner area, evidence of flame rollout where flames appear to move out of the burner compartment, or flames that noticeably change shape when the blower turns on can indicate combustion or airflow disturbances. In some cases, those disturbances are related to cracks or separations inside the heat exchanger that change how air and gases move.
Health and safety symptoms matter too. If CO detectors are alarming when the furnace runs, or if people in the building consistently get headaches, dizziness, or nausea when the heat is on, that is a red flag that should not be ignored. Those symptoms do not prove a cracked exchanger, but they do tell you something in the combustion or venting system needs immediate attention.
At the same time, a little surface rust or discoloration on the outside of the heat exchanger area does not automatically mean the metal has failed. We see a lot of online photos that make any rust look catastrophic. In reality, it is the location, depth, and pattern of corrosion or cracking that matters. This is why a proper inspection is essential. Our technicians document suspected cracks with photos or video whenever access allows and walk you through exactly what was found, instead of relying on vague warnings or pressure tactics.
Trust the experts with over 50 years of experience by calling (512) 710-1032, or book your appointment online right now.
When To Call Lantz Home Services For A Heat Exchanger Evaluation
A cracked heat exchanger is not a minor nuisance. It is a mechanical failure inside the one component that keeps combustion gases out of your breathing air. The right time to call for a professional evaluation is any time that barrier might be compromised. That could be when another company has flagged a crack and you want a clear, documented second opinion, when CO detectors have been alarming while the furnace runs, when your furnace is older and has never had a detailed internal inspection, or when you manage rentals and need reliable documentation for safety and liability.
When you contact Lantz Home Services in the Austin area, we start with straightforward scheduling and communication so you know who is coming and when. On site, our technician performs the kind of visual and test-based evaluation described in this guide, focusing on the heat exchanger, combustion, and venting. If we find evidence of a crack, we show you what we see, explain what it means in plain language, and document it with photos or notes stored in our ServiceTitan system for your records.
From there, we walk through your options. Because we are a family-owned company that has been serving Central Texas since 1972, we are careful about the recommendations we make. We know these decisions affect not just your budget, but your family’s safety and comfort or your tenants’ wellbeing. Our role is to give you clear information and practical options, whether that is proceeding with a warranted heat exchanger replacement or planning a full system upgrade that fits your home and long-term plans.
A cracked heat exchanger is not about blame. It is about how metal, heat, and time interact inside a furnace that quietly serves your home year after year. Once you understand that, the most important step is simply taking action. If you are in the Austin area and need a furnace or heat exchanger evaluation, or a second opinion on a diagnosis you are not sure about, we are ready to help.