What Is a Heat Exchanger? How It Works, Cracks and CO Risk

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A heat exchanger is a component that moves heat from one fluid or gas to another without letting the two mix. In a home, the one people usually mean is the furnace heat exchanger: a set of sealed metal chambers or tubes that get hot from burning gas or propane on the inside while the blower pushes house air across the outside, so your rooms get warm air and the combustion gases leave through the flue. Because that metal wall is the only thing separating combustion byproducts, including carbon monoxide, from the air you breathe, a cracked or corroded heat exchanger is treated as a safety issue. If you are asking what is a heat exchanger because a technician mentioned one or an inspection report flagged it, this guide explains how it works step by step, the main types, the terms on a label or quote, typical cost and lifespan ranges, maintenance, common problems, warning signs and what an inspector can and cannot check. It summarizes EPA, CDC, CPSC and ENERGY STAR guidance current as of 2026; anyone with symptoms of carbon monoxide exposure should get to fresh air and call 911, and a licensed HVAC technician should diagnose any suspected heat exchanger defect.

Part of our Heating, Cooling & Ventilation guide — start there for the full picture →

What a Heat Exchanger Does in a Home

Heat exchangers are everywhere in a house once you know what to look for. The principle is always the same: two streams separated by a conductive wall, with heat flowing from the hotter side to the cooler side.

• Gas or propane furnace. Combustion gases inside, house air outside. This is the main subject of this guide.
• Boiler. Combustion gases on one side, heating water on the other.
• Water heater. A flue tube through the tank or a coil in a tankless unit transfers burner heat into the water.
• Air conditioner and heat pump coils. Refrigerant on one side, air on the other. The indoor and outdoor coils are heat exchangers too.
• Heat-recovery and energy-recovery ventilators. A core lets outgoing stale air warm or cool incoming fresh air without mixing the airstreams.

The furnace version gets the most attention because it is the one with a direct safety role. If the barrier fails, combustion gases can find a path into the airstream the blower sends through the house.

How a Furnace Heat Exchanger Works, Step by Step

On a typical modern gas furnace, a heating cycle runs in this order:

1. The thermostat calls for heat. The control board starts the sequence.
2. The inducer fan starts. This small fan pulls air through the heat exchanger and pushes exhaust into the flue, proving draft before ignition. A pressure switch confirms it is working.
3. The igniter lights the burners. Flames fire into the openings of the heat exchanger tubes or chambers.
4. The metal heats up. Hot combustion gases travel through the inside of the heat exchanger, transferring heat to the metal walls.
5. The blower starts. After a short delay, the blower pushes return air from the house across the outside of the hot heat exchanger.
6. Warm air goes to the rooms; exhaust goes outside. Heated air moves through the supply ducts, while combustion gases exit through the vent pipe or chimney.
7. Safety switches stand guard. A high-limit switch shuts the burners off if the heat exchanger area overheats, and rollout switches trip if flames escape the burner area.
8. The cycle ends. Burners shut off, the inducer purges remaining gases, and the blower runs a little longer to pull the last heat off the metal.

Each of those components has its own failure modes, and several protect the heat exchanger directly. Our guide to the furnace inducer motor explains how the draft side of that sequence works and fails.

Main Types of Furnace Heat Exchangers

The design depends mostly on the furnace’s efficiency class and age. The comparison below summarizes what a homeowner is likely to see.

A standard-efficiency furnace has one heat exchanger and sends hot exhaust up a metal flue. A condensing furnace adds the secondary heat exchanger, which cools exhaust enough that it can vent through plastic pipe and produces water that must be drained.

Key Terms on a Label, Report or Quote

• AFUE. Annual Fuel Utilization Efficiency. ENERGY STAR explains that certified furnaces have higher AFUE ratings and higher-efficiency blower motors than standard models. Condensing furnaces, with their secondary heat exchanger, are the ones that reach the high-AFUE range.
• Input and output Btu/h. The data plate lists how much fuel energy the burners use per hour and how much heat reaches the air.
• Temperature rise. The data plate also lists an acceptable range for how much warmer supply air should be than return air. Running above that range usually means too little airflow, which overheats the heat exchanger.
• Combustion analysis. A technician’s test of flue gases, including CO, oxygen and temperature, to check how cleanly the burners are running.
• Draft. The movement of exhaust out through the vent. Poor draft can push gases back into the furnace area.
• Red tag. A technician or utility shutting the gas off to an appliance considered unsafe, often with a tag on the gas valve, until it is repaired or replaced.
• Heat exchanger warranty. Manufacturers often warranty the heat exchanger longer than other parts, but labor, registration and transfer rules vary, so the paperwork matters.

Cracked Heat Exchangers and Carbon Monoxide Risk

A crack or hole in the heat exchanger means the barrier between combustion gases and house air is compromised. That is why technicians and inspectors take it seriously, even though a crack does not always produce measurable carbon monoxide in the living space. Pressure relationships, crack location and burner condition all affect what happens, and conditions can change from one cycle to the next. The safe approach is to treat a confirmed crack as a reason to stop using the furnace until it is repaired or replaced.

EPA describes carbon monoxide as an odorless, colorless and toxic gas that can kill before people are aware it is in the home, and lists leaking chimneys and furnaces, back-drafting furnaces, and worn or poorly adjusted combustion devices among indoor sources. CDC notes that gas- and oil-burning furnaces are among the household items that produce CO. Our dedicated guide to a cracked heat exchanger covers diagnosis, repair versus replacement and what to ask a technician in more depth.

Why heat exchangers crack

• Overheating from poor airflow. Dirty filters, closed or blocked registers, undersized ducts and weak blowers keep heat in the metal. Repeated overheating stresses the steel.
• Metal fatigue. Every cycle heats and cools the metal. Over many years, expansion and contraction can open cracks at bends, seams and welds.
• Oversizing and short cycling. A furnace that is too large runs short, frequent cycles, which adds thermal stress and moisture.
• Corrosion. Condensation inside a standard furnace, leaking humidifiers or AC coils above the furnace, and chlorine or other chemical vapors drawn into combustion air can all rust or eat through metal.
• Over-firing. Gas pressure or orifices set too high push more heat into the exchanger than it was designed for.

Warning signs to watch for

• A carbon monoxide alarm sounding, especially when the furnace is running.
• Soot or scorching around the burner area or the furnace cabinet.
• Flames that lift, waver or change shape when the blower starts.
• Rollout or limit switches that trip repeatedly.
• Rust flakes on the burners, water in the furnace, or visible corrosion.
• Unusual odors when the heat runs.
• Household members with headaches, dizziness, weakness, nausea or confusion that improve after leaving the house. CPSC lists those among CO poisoning symptoms; CDC adds that CO symptoms are often described as “flu-like.”

If a CO alarm sounds or anyone has symptoms, follow CPSC’s advice: get outside to fresh air immediately, then call 911. Do not try to diagnose the furnace first.

How Technicians Check a Heat Exchanger

No single test proves a heat exchanger is sound, so experienced technicians usually combine several methods and document what they find. Asking which methods were used is a fair question when a crack is reported.

1. Visual inspection. With the burners and sometimes the blower removed, the technician looks at accessible surfaces for cracks, holes, rust-through and separated seams. Bends and burner inlets get the closest look.
2. Camera inspection. A small inspection camera fed into the tubes or chambers shows areas that cannot be seen directly. Photos or video give the homeowner something concrete to review or show a second technician.
3. Flame observation. If the flame pattern lifts, rolls or changes color when the blower starts, air may be pushing through an opening in the heat exchanger.
4. Combustion analysis. An analyzer in the flue measures CO, oxygen and other values. A sudden change in readings when the blower comes on can point to a breach.
5. CO in the supply air. Measuring carbon monoxide at supply registers can help, though a clean reading does not by itself rule out a crack.

If a technician red-tags a furnace for a cracked heat exchanger, ask for the photos and readings in writing. A second opinion is reasonable before paying for a replacement, provided the furnace stays off in the meantime.

Typical Costs and Lifespan

As a rule of thumb, a furnace heat exchanger is meant to last the life of the furnace, commonly somewhere around 15 to 25 years with good airflow and annual service. Cracks are more likely late in a furnace’s life, though airflow problems or poor installation can cause them much earlier. Typical national ranges, which vary by brand, model and warranty status, look like this:

Because labor is a big share of a heat exchanger job, many owners of older furnaces choose replacement instead. Our furnace replacement cost guide explains what drives those quotes.

Maintenance Basics

Most of what protects a heat exchanger is simple and routine:

• Annual heating service. ENERGY STAR’s maintenance checklist recommends having a contractor check the heating system in the fall, including gas connections, gas pressure, burner combustion and the heat exchanger. It notes that a dirty burner or cracked heat exchanger causes improper burner operation and can make equipment operate less safely and efficiently. CDC likewise advises having heating systems and other fuel-burning appliances serviced by a qualified technician every year. A furnace tune-up covers these items.
• Filters. ENERGY STAR advises inspecting, cleaning or changing filters monthly. A clogged filter is one of the most common causes of overheating.
• Open registers and returns. Closing many supply registers or blocking returns with furniture restricts airflow across the heat exchanger.
• Clear combustion and vent paths. Keep paint, pool chemicals and cleaning products away from the furnace, and keep intake and exhaust terminations clear of snow and debris.
• CO alarms. CPSC recommends battery-operated or battery-backup CO alarms on every level of the home and outside sleeping areas. CDC advises replacing detectors according to the manufacturer’s instructions or every five years. Our roundup of the best carbon monoxide detectors compares options.

Front Range Notes

Long heating seasons. Front Range furnaces run from fall into late spring, so they accumulate more heating cycles than furnaces in milder climates. That makes annual service and filter changes especially worthwhile.

High-altitude setup. Gas appliances at elevation typically need adjustments set out in the manufacturer’s installation instructions. A furnace that is not set up correctly for its altitude may burn poorly or run hotter than intended, which affects both CO production and heat exchanger stress. Ask the installer how the unit was configured for your elevation.

Humidifiers and AC coils above furnaces. Dry winter air leads many homeowners to add whole-house humidifiers, and most Front Range furnaces carry a cooling coil on top. A leaking humidifier or a clogged coil drain dripping into the furnace can corrode the heat exchanger over time.

Tight homes and combustion air. Energy upgrades that tighten a house can change pressure balances around older atmospherically vented appliances. A technician can check draft after major air sealing work.

What a Home Inspector Checks

Under the ASHI Standard of Practice, a home inspector opens readily openable access panels and inspects installed heating equipment, vent systems, flues and chimneys, and distribution systems. The same standard specifically says the inspector is not required to inspect heat exchangers, or to determine the adequacy of combustion air. Most of a heat exchanger is hidden inside the furnace, and a full evaluation usually requires partial disassembly, cameras or combustion testing.

What an inspector typically does instead:

• Runs the furnace with normal controls and observes the burner flame through the viewport where possible, including when the blower starts.
• Looks for soot, scorching, rust, water and signs of flame rollout.
• Checks the vent connector, flue and chimney for slope, gaps, disconnections and corrosion.
• Notes the furnace age from the data plate and service records.
• Checks for CO alarms where required and may use a CO meter near the appliance.

When anything suggests a combustion problem, the report recommends evaluation by a licensed HVAC technician. A full furnace inspection by a heating contractor goes further than a general home inspection can. For sorting out which specialist to call, see our heating and cooling hiring guide.

When to Call a Pro

Call an HVAC technician promptly if a CO alarm sounds after everyone is safely outside and responders have cleared the home, if you see soot, rust or flame changes, if safety switches keep tripping, or if a technician has mentioned a possible crack and you want a second opinion with photos or combustion readings. If you are buying a Front Range home with an older furnace, ask for service records and consider having a heating contractor evaluate it before closing.

Want a careful look at the furnace and venting in a home you own or are buying? Contact our team to schedule an inspection.

References

• Carbon Monoxide’s Impact on Indoor Air Quality — U.S. Environmental Protection Agency
• Carbon Monoxide Poisoning Basics — Centers for Disease Control and Prevention
• Carbon Monoxide Information Center — U.S. Consumer Product Safety Commission
• Maintenance Checklist — ENERGY STAR
• Furnaces — ENERGY STAR
• ASHI Standard of Practice for Home Inspection — American Society of Home Inspectors

Concerned about the furnace in a Front Range home you are buying? Get in touch with our team to arrange an inspection.