Advertising Disclosure: This site may receive compensation for service connections made through this page. Content is editorially independent.
Before opening any furnace cabinet or burner compartment, a technician will kill the breaker at the disconnect to de-energize the system — this is not a homeowner step. Refrigerant and sealed-system work is also federally regulated under EPA Section 608; certification is legally required.
Carbon monoxide is colorless, odorless, and undetectable without a CO detector. A cracked furnace heat exchanger is one of the most common residential CO sources. At 70 ppm, symptoms appear in hours; at 400+ ppm, exposure is life-threatening within 3 hours. Place CO detectors on every level and near bedrooms. Per the canonical 2026 cost guide, heat exchanger repair runs $600–$4,000; for furnaces over 15 years old, full replacement is usually more cost-effective.
Gas and oil furnaces are remarkably efficient ways to heat a home, but they depend on safely burning a combustible fuel. When everything works correctly, combustion exhaust is sealed away from your living space and vented harmlessly outside through a flue pipe. When one critical component fails — particularly the heat exchanger — the consequences can be fatal within hours.
When an air conditioner breaks, you suffer a sleepless, sweaty night. When a gas furnace breaks in one specific way, you and your family may never wake up. According to the U.S. CDC, more than 400 Americans die every year from unintentional carbon monoxide poisoning unrelated to fires, and over 100,000 visit an emergency department. Malfunctioning heating systems are among the most common sources of residential CO exposure. And the months of January, February, and March — when furnaces run continuously — see the highest death rates.
This guide covers exactly how CO enters your home from HVAC equipment, how to recognize it before symptoms become serious, what detectors you need and where to place them, and what to expect when a provider inspects your system.
What is Carbon Monoxide?
Carbon monoxide (CO) is produced by the incomplete combustion of fossil fuels — including the natural gas, propane, or fuel oil that powers your furnace. When combustion is complete and fuel burns fully with adequate oxygen, the main byproducts are carbon dioxide (CO2) and water vapor — neither of which is immediately dangerous at typical levels. But when combustion is incomplete due to insufficient oxygen, poor ignition, or a failing heat exchanger, carbon monoxide is the result instead.
CO is entirely odorless, colorless, and tasteless. You cannot see it, smell it, or taste it. Without a working detector, it is physically impossible for human senses to detect CO until the gas has already begun affecting your bloodstream.
Here's why CO is so dangerous at the biological level: CO binds to hemoglobin — the protein in your red blood cells that normally carries oxygen — with approximately 200 times greater affinity than oxygen itself. This means your blood preferentially soaks up CO and forms carboxyhemoglobin (COHb), which cannot carry oxygen. As COHb levels rise in your bloodstream, your organs become starved of oxygen even though you are still breathing normal air. The heart and brain, which need the most oxygen, are affected first.
How Does It Leak From a Furnace?
Understanding the leak mechanism helps you understand why annual inspections matter. Inside a gas furnace, the burner assembly ignites fuel to create a flame and heat. The hot exhaust gases from this combustion — including CO, CO2, water vapor, and nitrogen oxides — are contained inside a sealed metal component called the heat exchanger.
The heat exchanger is essentially a series of curved metal tubes or clamshells. The furnace blower pushes your home's indoor breathable air over the outside surface of these hot metal tubes. The air picks up heat through the metal walls and is then pushed through your ductwork and into your rooms. The breathable indoor air and the toxic combustion gases are never supposed to make contact — the metal walls of the heat exchanger keep them permanently separated.
The problem is that this metal is under enormous thermal stress. Every heating cycle, the heat exchanger expands as it heats up and contracts as it cools down. Over years — typically 15 to 25 years on an aging furnace — this constant expansion and contraction creates micro-fatigue cracks in the metal. These cracks can be hairline-thin and completely invisible to the naked eye.
Once a crack exists, the pressure differential between the combustion chamber and the blower airstream can push CO-laden exhaust gases through the crack and directly into the airstream that flows through your vents. The gas is then distributed to every room in the house simultaneously.
Other furnace-related causes of CO leaks include:
- Blocked or cracked flue pipe: A disconnected, corroded, or bird-nested flue vent can back-draft combustion gases into the living space instead of exhausting them outside.
- Improper furnace sizing: An oversized furnace short-cycles frequently, which causes the heat exchanger to crack faster due to accelerated thermal stress.
- Dirty burners: Fouled burners create incomplete combustion, increasing CO production even with an intact heat exchanger.
- Inadequate combustion air: A tightly sealed, energy-efficient home can starve a furnace of the oxygen it needs, leading to incomplete combustion and elevated CO output.
What Other Appliances Produce Carbon Monoxide?
While furnaces are the most common source of serious residential CO events, they are not the only ones. Any fuel-burning appliance can produce CO if it malfunctions, is improperly vented, or is used incorrectly:
- Gas water heaters: Like furnaces, water heaters have flue vents that can corrode, disconnect, or become blocked. A water heater in a closet with inadequate combustion air is a common source of low-level chronic CO exposure.
- Gas ranges and ovens: While designed to vent into the kitchen (ideally with a range hood), a malfunctioning burner or blocked igniter produces elevated CO. Never use a gas oven for home heating.
- Attached garages: A car left running in an attached garage — even briefly, even with the garage door open — can push significant CO levels into an adjacent living space through shared walls, door gaps, or ductwork.
- Portable generators: Never operate a generator inside a garage, basement, or near any window or door. Generators produce extremely high CO concentrations and are responsible for a disproportionate share of CO fatalities during power outages and storms.
- Gas fireplaces and inserts: A partially blocked chimney or a cracked firebox can allow combustion gases to backdraft into the room rather than venting outside.
- Charcoal grills and camp stoves: Never burn charcoal or operate a camping stove indoors. Even in a well-ventilated garage, these devices produce dangerous CO concentrations rapidly.
What Are the Symptoms of Carbon Monoxide Exposure?
The danger of CO poisoning is directly related to both the concentration in the air and the duration of exposure. Higher concentrations cause incapacitation and death faster. Lower concentrations cause insidious, flu-like symptoms that build gradually — often while you sleep. The table below shows the relationship between CO concentration (measured in parts per million, or ppm), symptoms, and the time to serious harm.
| CO Level (ppm) | Symptoms | Time to Serious Harm | Action |
|---|---|---|---|
| 35 ppm | Headache and dizziness after prolonged exposure | 8+ hours | OSHA workplace action level — detectors alert |
| 70 ppm | Headache, fatigue, nausea within 1–2 hours | 2–3 hours | Most residential CO detectors alarm at this level |
| 150–200 ppm | Severe headache, disorientation, loss of consciousness | 2–3 hours | Evacuate immediately |
| 400 ppm | Life-threatening within 3 hours | Under 3 hours | Emergency evacuation |
| 800 ppm | Dizziness, convulsions, death possible | Under 1 hour | Emergency evacuation — call 911 |
| 1,600 ppm | Death likely | Under 15 minutes | Immediately life-threatening |
| 3,200+ ppm | Rapid incapacitation and death | Under 5 minutes | Immediately life-threatening |
A critical diagnostic clue: CO poisoning symptoms improve when you leave the house and return when you go back inside. If multiple people — or pets — suddenly develop headaches, nausea, or fatigue simultaneously while the furnace is running, treat it as a possible CO event and evacuate. Pets, especially birds, are more sensitive to CO and may show symptoms before humans do.
If your CO alarm is sounding a continuous alarm, or if multiple household members are experiencing sudden headaches, dizziness, or nausea while the furnace runs: get everyone outside immediately, call 911 from outside the home, and do not re-enter until emergency responders clear the building.
Which Carbon Monoxide Detector Should I Buy?
A functioning CO detector is the single most important piece of safety equipment in a home with any gas or fuel-burning appliance. A basic unit costs $20—$50 at any hardware store. The investment is trivial compared to the risk of operating without one.
Types of CO Detectors
- Plug-in detectors: Easy to install, can be moved between rooms. Look for models with a battery backup so they work during a power outage. Best for renters and as supplemental units.
- Battery-powered detectors: Fully portable and work during outages. Require battery checks and replacement. Good for bedrooms and areas without convenient outlets.
- Hardwired detectors: Wired into the home's electrical system, often interconnected so all alarms sound together. Most common in newer construction where they are code-required. Require battery backup.
- Combination smoke/CO detectors: Cover both threats in a single unit. Cost-effective if you are replacing smoke detectors anyway. Make sure the unit is UL-listed for both functions.
Where to Place CO Detectors
- Outside every sleeping area: The most critical placement. Place a detector in the hallway immediately outside every group of bedrooms so the alarm will wake you if CO accumulates while you sleep.
- On every level of the home: Include the basement — especially if your furnace, water heater, or other fuel-burning appliances are located there — and any finished attic space.
- Near attached garages: Cars left running even briefly in attached garages can force CO into living spaces.
- Away from fuel-burning appliances: Do not place a CO detector within 15 feet of your furnace, gas range, or water heater. Minor startup emissions from these appliances can trigger nuisance alarms.
Smoke detectors last 10 years, but the electrochemical sensors inside CO detectors degrade over time. Typical service life is 5 to 10 years depending on the model — check the manufacture date and the printed "replace by" date on the back of the unit (newer UL 2034-compliant detectors are often rated for 7 to 10 years). An expired CO detector may look fine and even beep when you test it, but its sensor may no longer respond accurately to CO gas. When the unit reaches its replace-by date, replace the entire unit immediately — not just the batteries.
What Should I Do When My CO Alarm Goes Off?
The critical distinction: a continuous, piercing alarm pattern (typically 4 fast beeps, a pause, then 4 fast beeps — repeated) means CO has been detected. This is not a low-battery chirp (a single, occasional chirp). Treat a full CO alarm as a genuine emergency every time, without exception.
- Do not try to find the source or investigate. Your only priority is getting everyone out. Do not waste time looking for the furnace switch, calling a technician, or trying to ventilate.
- Evacuate immediately — everyone, including pets. Get out of the house into fresh outdoor air. Move to a neighbor's home, your car in the driveway (not the garage), or the sidewalk.
- Call 911 from outside. Fire departments have CO meters and can measure the actual ppm levels throughout your home. They will also identify the source. This call is free and appropriate — firefighters respond to CO alarms regularly.
- Seek medical evaluation. If anyone felt unwell before the alarm sounded — headache, nausea, confusion — go to an emergency room. Carboxyhemoglobin must be measured with a blood test and may require oxygen therapy.
- Do not re-enter the home. Wait until emergency responders have measured CO levels throughout the home and explicitly declared it safe. This may require opening the home for ventilation and disabling the furnace.
- Have your heating system professionally inspected before restarting it. An HVAC provider should inspect the heat exchanger, flue vent, and combustion system before the furnace is used again. Operating a furnace with a cracked heat exchanger after a CO event can immediately re-introduce gas into the home. Our furnace warning signs guide explains how to identify heat exchanger problems before they reach this point.
Many CO poisoning fatalities involve families who silenced a CO alarm thinking it was malfunctioning or the batteries were low. CO detectors are designed to alarm at levels well below immediately dangerous concentrations specifically to give you time to act. If the alarm activates, assume it is real until proven otherwise by a professional measurement.
Connect with a local HVAC provider for a combustion safety inspection.
Call Now — (844) 582-1795Disclosure: We are a referral service and may receive compensation for qualified calls. Calls may be routed to an independent provider network and may be recorded. Pricing and availability vary by provider and location.
What Does a Professional CO Inspection Include?
When you call for a CO safety inspection or a furnace tune-up, a qualified HVAC technician will go well beyond a visual check. Here is what the process typically involves:
1. Combustion Analysis
The technician uses a combustion analyzer — a handheld device inserted into the flue pipe — to measure the actual gases being produced during burner operation. It reads CO content, oxygen levels, carbon dioxide, stack temperature, and combustion efficiency simultaneously. A properly functioning gas furnace should produce essentially zero CO in its exhaust (well under 50 ppm). Elevated readings immediately point to incomplete combustion or a compromised heat exchanger.
2. Heat Exchanger Inspection
Visually inspecting a heat exchanger is difficult because most of it is hidden inside the furnace cabinet. Technicians use several methods to find cracks:
- Smoke pencil or smoke puffer test: With the burners lit and the blower running, the technician introduces a thin stream of smoke near the heat exchanger. If there is a crack, the pressure difference will draw the smoke through it — providing visual confirmation.
- Scope inspection: A flexible inspection camera is fed into the heat exchanger to examine the interior surfaces for cracks, corrosion, or warping that could indicate structural failure.
- CO detector probe: A calibrated CO sensing probe is placed in the supply air stream at the air handler to detect any CO crossing from the combustion side to the air side.
3. Flue Vent Inspection
The technician inspects the full flue pipe run from the furnace to where it exits the home. They look for disconnected joints, corrosion holes, inadequate slope (back-pitched sections trap water and corrode faster), and any blockages such as bird nests, debris, or collapsed flex sections near the termination point.
How Much Does CO-Related HVAC Repair Cost?
If a professional inspection reveals a CO source, the repair options and costs vary widely depending on what failed and the age of the system.
| Issue Found | Typical Repair | Typical Cost Range | Notes |
|---|---|---|---|
| Cracked heat exchanger (newer furnace) | Heat exchanger replacement | $1,500 — $3,500 | Parts + labor; may require sourcing OEM component |
| Cracked heat exchanger (older furnace) | Full furnace replacement recommended | $3,000 — $7,000 | If furnace is 15+ years old, replacement often more cost-effective |
| Disconnected or cracked flue pipe | Flue repair or re-connection | $150 — $600 | Simple reconnection is low cost; full flue replacement is higher |
| Blocked flue (bird nest, debris) | Flue cleaning and capping | $150 — $400 | Add a flue cap/screen to prevent recurrence |
| Dirty burners causing incomplete combustion | Burner cleaning / tune-up | $80 — $200 | Often included in annual maintenance visits |
| CO detectors — purchase and install | New detector units | $20 — $60 per unit | Hardwired installation by electrician costs more |
| Annual combustion safety inspection | Furnace tune-up with combustion analysis | $80 — $200 | Best prevention; typically catches issues before they become dangerous |
All cost figures are approximate and vary significantly by region, provider, and furnace make and model. Get estimates from at least two providers before authorizing major repairs.
The Repair vs. Replace Decision for a Cracked Heat Exchanger
When a heat exchanger cracks, many homeowners face a difficult choice. A heat exchanger replacement on a 5-year-old furnace may be a sound investment — the furnace may have 15+ years of useful life remaining. But on a 20-year-old furnace with efficiency ratings in the 60–70% AFUE range, spending $2,500 on a heat exchanger replacement may not make financial sense when a new high-efficiency furnace (90–98% AFUE) can be installed for $3,500—$5,000 with a new warranty and meaningfully lower monthly gas bills.
The general industry guidance: if the cost of the repair exceeds 50% of the cost of a new system, and the furnace is more than 15 years old, replacement is usually the better value. A provider can help you model the numbers for your specific situation.
Some homeowners are tempted to continue operating a furnace with a cracked heat exchanger while they wait for parts or financing. This is extremely dangerous. Once a crack is confirmed, the furnace should be shut off and remain off until the repair or replacement is completed. A space heater and extra blankets are a far safer temporary solution.
How Can I Prevent Carbon Monoxide Exposure?
Most CO events are preventable. The combination of working detectors and regular professional maintenance eliminates the vast majority of risk.
What You Can Do
- Install CO detectors on every level: Including the basement, outside all sleeping areas, and near any attached garage. Test detectors monthly and replace units older than 7 years.
- Schedule annual furnace maintenance: Every fall, before heating season begins. A combustion analysis and heat exchanger inspection as part of the tune-up catch developing problems before they become dangerous. See our HVAC maintenance guide for what a tune-up should include.
- Never run a generator indoors: Keep portable generators at least 20 feet from any window, door, or vent opening — even during power outages.
- Never use your oven for heating: Gas ovens are not designed to heat living spaces and produce significant CO when used this way.
- Inspect your flue termination seasonally: Look at the exterior pipe cap each fall before the heating season. Clear any visible debris, insect nests, or vegetation that might have grown near the outlet.
- Know the signs of chronic low-level exposure: Persistent headaches that clear up when you leave the house, winter fatigue that seems weather-related, and nausea without a clear cause are all worth investigating if you have gas appliances and aging detectors.
Urgency Guide: What Your Situation Calls For
CO alarm sounding continuously / household members or pets showing symptoms (headache, nausea, confusion, dizziness) / confirmed CO exposure by first responders. Evacuate immediately and call 911. Do not re-enter. Do not restart the furnace.
CO detector chirping or alarming intermittently / your furnace is 15+ years old and has never had a combustion safety inspection / you notice a yellow or orange burner flame instead of blue. Contact a provider within a few days — do not wait until symptoms appear.
Furnace is running well, but it has been more than 12 months since the last tune-up / you are installing CO detectors for the first time / you moved into a home and don't know the furnace service history. Schedule a preventive inspection before the next heating season.
Frequently Asked Questions
Carbon monoxide has no smell, no color, and no taste — it is completely undetectable by human senses. This is precisely what makes it so dangerous. You cannot tell it is present until it has already begun affecting your bloodstream. A working CO detector is the only reliable way to know if CO is accumulating in your home.
The timeline depends on the concentration of CO in the air. At 70 ppm, headaches and fatigue may develop within a few hours. At 150–200 ppm, you may lose consciousness within 2–3 hours. At 400 ppm and above, CO exposure becomes life-threatening within 3 hours. At very high concentrations (1,600 ppm or more), death can occur in under an hour. This is why CO detectors are non-negotiable — by the time you feel symptoms at lower levels, higher concentrations may already be accumulating.
Heat exchanger replacement typically costs between $1,500 and $3,500 for parts and labor. For furnaces over 15 years old or where replacement parts are difficult to source, many HVAC professionals recommend replacing the entire furnace instead — a new unit typically runs $3,000—$7,000 installed but comes with a full warranty and improved efficiency. Costs vary by region, furnace brand, and labor rates.
Yes, though it is rare. A new furnace can develop a CO leak if it is improperly installed, has a blocked or inadequate flue vent, or if the combustion air supply is restricted. Annual tune-ups by a qualified technician — which include a combustion analysis and flue inspection — are recommended even for newer furnaces to catch any developing issues early.
CO is very close in density to regular air (slightly lighter), so it distributes fairly evenly throughout a room rather than pooling at the floor or ceiling. Most manufacturers recommend mounting CO detectors at about chest height — roughly 5 feet from the floor. Ceiling or plug-in outlet placement is also acceptable per most building codes. Follow the placement instructions that come with your specific detector model.
Smoke detectors sense airborne particles from combustion — they detect fire. Carbon monoxide detectors use electrochemical or metal-oxide-semiconductor sensors to detect CO gas in the air — the invisible byproduct of incomplete combustion. The two threats require different sensors, which is why it is important to have both types in your home. Combination smoke-and-CO detectors are available and cover both hazards in a single unit.
Connect with an independent HVAC provider in your area to inspect your heat exchanger and flue system.
Call Now — (844) 582-1795Disclosure: We are a referral service and may receive compensation for qualified calls. Calls may be routed to an independent provider network and may be recorded. Pricing and availability vary by provider and location.
Local HVAC Service Areas
Cool Call Pro connects homeowners with independent HVAC technicians nationwide. Find a pro in Chicago (IL), Minneapolis (MN), Denver (CO), or your city. Browse state HVAC guides: Illinois, Minnesota, and Colorado.