Furnace Carbon Monoxide: Warning Signs and What to Do

• Carbon monoxide (CO) is colourless and odourless — your furnace can leak it silently while your family sleeps, which is why early warning signs matter.
• If you suspect a CO leak, leave the building immediately — do not stop to investigate, grab belongings, or troubleshoot. Call 911 from outside.
• A cracked heat exchanger is the most common furnace-related cause of CO poisoning, and Ohio River valley freeze-thaw winters accelerate exactly this kind of damage.
• CO detectors are a backup layer of protection, not a substitute for annual furnace inspections.
• Louisville HVAC Pro’s technicians inspect heat exchangers, flue systems and burner assemblies to catch CO risks before they become emergencies.

Your furnace could be producing carbon monoxide right now, and nothing in your home would look, smell or sound any different.

Carbon monoxide poisoning from residential furnaces is not a rare edge case. It happens when ageing equipment develops cracks, when flue pipes get blocked, or when a system that seemed fine last winter quietly crosses a threshold it cannot come back from. This guide exists because homeowners in Louisville, Fairdale, and across Jefferson and Bullitt County deserve a straight answer about what to look for and exactly what to do if they find it.

What Makes Carbon Monoxide So Dangerous

Carbon monoxide kills by replacing oxygen in your red blood cells. Your body cannot tell the difference between CO and oxygen at the point of absorption, so the gas binds to haemoglobin and prevents your blood from delivering oxygen to your brain and organs. This happens whether you are awake or asleep, and it happens faster at higher concentrations.

CO Is Invisible, Odourless and Fast-Acting

Unlike natural gas, which utilities deliberately scent with mercaptan so you can smell a leak, carbon monoxide has no added odorant. It produces no visible smoke at the levels that come from a malfunctioning furnace. By the time a person feels noticeably ill from CO exposure, their ability to make decisions — including the decision to leave — is already compromised.

• Around 70 ppm: headache, fatigue and nausea within a few hours
• Around 150–200 ppm: dizziness, disorientation and loss of consciousness
• Around 400 ppm: life-threatening within roughly three hours
• Around 800 ppm: death within two to three hours
• Above 1,600 ppm: death in under an hour

These numbers matter because a furnace with a cracked heat exchanger can push CO into living spaces at concentrations that move through those thresholds faster than symptoms become obvious — especially in a home sealed up tight for winter.

Who Is Most at Risk in Your Home

The EPA identifies unborn babies, infants, elderly people, and individuals with anaemia, heart disease or respiratory conditions as especially vulnerable to carbon monoxide. That said, CO at high enough concentrations is dangerous to any person or animal. A healthy adult can be incapacitated just as surely as someone with a pre-existing condition — it simply may take a higher concentration or a longer exposure.

Furnace Warning Signs That CO May Be Leaking

There is no single alarm that goes off when your furnace starts leaking CO — unless you have a working detector positioned correctly. What you often get instead is a combination of subtle signals that are easy to dismiss individually. Taken together, they paint a clear picture.

Physical Symptoms That Appear Only at Home

One of the most telling patterns of CO exposure is symptoms that follow your location. If you feel better after leaving the house and worse after returning, that is a serious red flag. This pattern differs from ordinary illness, which follows you regardless of where you are.

Pay close attention if several people in the household are experiencing similar symptoms at the same time. A shared virus is possible, but shared symptoms that ease when everyone leaves the building and return when they come back inside point directly to an indoor air quality problem — and CO is at the top of that list.

If that table describes your household, do not wait. Leave the home, get fresh air, and call 911 from outside. Tell the dispatcher you suspect carbon monoxide.

Visual Clues on Your Furnace

Your furnace will often show physical evidence of a combustion problem before symptoms appear. These are not cosmetic issues — each one indicates that combustion byproducts may be escaping where they should not.

Burner flame colour is one of the most immediate indicators. A properly burning gas furnace produces a steady blue flame. An orange or yellow flame, or one that flickers and wavers, indicates incomplete combustion, which produces more CO than a clean burn.

• Soot or black streaking around the furnace cabinet, flue pipe connections, or heat exchanger panels
• Excessive condensation on windows during furnace operation, which can indicate incomplete combustion venting
• Visible rust on the heat exchanger or flue components
• An unfamiliar smell when the furnace runs — not gas, but a faint stale or stuffy odour that appears with the heat
• The furnace cycling on and off more often than usual, which can indicate a safety switch responding to heat exchanger stress

None of these signs should be ignored or assumed to be minor. A cracked heat exchanger — the metal component that separates combustion gases from the air your family breathes — will not heal itself, and it is the single most common structural cause of furnace-related CO leaks.

Behavioural Signs in Pets

Animals, particularly birds, are more sensitive to carbon monoxide than humans. If a pet bird becomes lethargic or dies without obvious cause during heating season, that is an urgent warning. Dogs and cats will also show signs of CO exposure — lethargy, vomiting, loss of coordination — before some humans notice symptoms, simply because they spend more time close to floor level where CO can accumulate.

Do not dismiss a pet acting strangely during winter, especially if the behaviour coincides with furnace operation. Get everyone — people and animals — outside and call 911.

CO Poisoning Symptoms You Should Never Ignore

Carbon monoxide poisoning mimics several common conditions, which is exactly why it goes unrecognised until it becomes severe. Knowing the symptom progression gives you the information you need to act before exposure reaches a dangerous threshold.

Low-level, long-term exposure is particularly deceptive. A slow leak from a heat exchanger with a hairline crack may cause weeks of low-grade headaches, fatigue and disrupted sleep before anyone connects it to the furnace. Many families assume they have caught a bug or are simply run down.

Moderate exposure moves faster and feels more acute. Symptoms that might be confused with flu — nausea, dizziness, shortness of breath — appear **without fever**, which is a distinguishing detail. If your household is unwell without fever and the furnace is running, treat it as a CO situation until proven otherwise.

• Low-level: persistent headache, mild fatigue, slight nausea, shortness of breath on exertion
• Moderate: severe throbbing headache, drowsiness, disorientation, rapid heart rate, vomiting
• High-level: inability to think clearly, loss of muscle control, loss of consciousness, convulsions, death

Why Low-Level Exposure Is the Hardest to Catch

Low-level CO exposure feels like a dozen other things. A dull headache that starts an hour after you get home, fatigue that does not match your activity level, a faint nausea that comes and goes — all easy to attribute to stress, poor sleep or a mild cold. The critical detail at this stage is the pattern, not the individual symptom. If these symptoms appear consistently at home and ease after an hour or more outside, do not wait for them to get worse.

Low-level exposure over days or weeks can cause cumulative harm even when no single episode feels like an emergency. CO detectors are specifically designed to alarm before the average healthy adult would feel significant effects — which tells you something important about how subtle early exposure really is.

Moderate and Severe Exposure

At moderate concentrations, symptoms escalate quickly and become harder to push through: a severe throbbing headache, pronounced dizziness, disorientation, vomiting, and a rising heart rate as the body tries to compensate for reduced oxygen delivery. At this level a person may still be mobile, but their judgement is already impaired — which is exactly when they are most likely to make the dangerous decision to lie down instead of leaving.

At high concentrations, CO poisoning becomes a life-or-death event within minutes. Loss of muscle control means a person cannot physically leave even if they understand they need to. Unconsciousness follows, and without rescue and immediate medical treatment, death occurs. The guidance is unambiguous: get out first, then call for help. There is no version of this situation where staying inside to investigate is the right call.

What To Do the Moment You Suspect a CO Leak

If your CO detector alarms, if several people in your home are showing symptoms, or if you have any reason to believe your furnace may be venting combustion gases into your living space, the protocol is the same. It is not complicated, but it has to happen in the right order.

Do not open windows. Do not turn appliances off. Do not go back for a phone charger or the dog’s medication. The urge to do something productive before leaving is natural, but CO impairs the brain’s ability to assess risk even at moderate levels. By the time you feel the need to investigate, your judgement about whether it is safe to stay may already be compromised.

**Every person and animal leaves the building immediately. That is step one, and nothing happens before it.**

Once You Are Outside

Call 911. Tell the dispatcher you suspect carbon monoxide and give your address. Do not re-enter the home for any reason until emergency responders have cleared it. If anyone is showing symptoms of poisoning — confusion, vomiting, loss of consciousness — say so immediately so they send medical response alongside fire services. Emergency personnel carry equipment to measure CO concentrations room by room and can identify the source safely.

What To Tell the Emergency Room

If anyone in your household is symptomatic, go directly to an emergency room. Tell the treating physician you suspect carbon monoxide poisoning and, if you can, give them a timeline: how long you were in the home, when symptoms started, and whether they eased after leaving. A blood test measuring carboxyhaemoglobin can confirm CO exposure, but it is time-sensitive — the longer the gap between exposure and testing, the less reliably it shows elevated levels. Mention the furnace specifically and how recently it was running.

When To Call a Technician Instead of 911

Call 911 any time people or animals are showing symptoms, or when a CO detector has alarmed. This is not a situation for a technician first. Emergency responders need to confirm the space is safe before any service work begins. Fire department personnel will ventilate the structure, identify the source where possible, and give the all-clear before you or any contractor re-enters.

Once the building has been cleared, that is when you call us at (502) 553-8534. At that point a full inspection of the heat exchanger, flue pipe, burner assembly and venting system is needed before the furnace is operated again under any circumstances. Do not simply air out the house and turn the furnace back on — the condition that caused the leak has not been repaired.

CO Detectors: What They Can and Cannot Do

A carbon monoxide detector is genuinely important safety equipment, and every home with a gas furnace should have one installed and maintained. But understanding what a detector actually does — and what it does not — prevents a false sense of security.

CO detectors measure cumulative exposure over time, not instantaneous concentration. Most residential detectors are engineered to alarm at around 70 ppm sustained over one to four hours, or at higher concentrations for shorter periods. A slow, low-level leak may therefore accumulate for some time before the detector reaches its alarm threshold. It will still alarm before most healthy adults suffer severe effects — but it is not a guarantee that no exposure has occurred.

A Detector Is a Backup, Not a Safety Net

A detector does not prevent CO from being produced. It does not stop a cracked heat exchanger from leaking. It measures what is already in your air. Families who rely on a detector as their primary protection and skip annual furnace inspections are accepting a risk they may not fully appreciate.

Detectors also fail. Sensors degrade — most CO detectors have a functional lifespan of five to seven years — and a detector past its service life may not alarm at all. Check the manufacture date on yours. If it does not have one, or if it is more than seven years old, replace it before this heating season.

How To Choose and Place a CO Detector

Look for detectors listed to the UL 2034 standard, which sets minimum performance requirements for residential CO alarms. Combination smoke and CO detectors are fine as long as they are listed for both functions. For a home with a gas furnace in the basement, install at least one detector on every level including the basement, and one within ten feet of each sleeping area. Plug-in models with battery backup handle power outages — which are exactly the conditions when people run alternative heat sources that create additional CO risk.

Preventing Furnace CO Leaks in the First Place

Detection is the last line of defence. Prevention is the one that actually keeps CO out of your house, and for a gas furnace it comes down to combustion and venting being verified by someone with the instruments to measure them.

What an Annual Inspection Actually Checks

• **Heat exchanger integrity** — visual inspection with a mirror or borescope, plus combustion analysis, looking for the cracks that let flue gases into the supply air
• **Combustion analysis** — a calibrated analyser reads CO in the flue gas and confirms the burn is complete rather than producing excess CO
• **Draft and venting** — verifying the flue actually pulls combustion gases out, with no disconnected sections, corrosion, or blockage
• **Burner condition** — dirty or misaligned burners produce the yellow, wavering flame that signals incomplete combustion
• **Safety controls** — testing that the rollout switch, high-limit switch and pressure switch actually shut the burner down when they should

That last point is worth dwelling on. Those safety switches exist specifically to stop the furnace when combustion or airflow goes wrong. A switch that has failed closed provides no protection at all, and there is no way to know without testing it.

The Louisville-Specific Risks

Two local conditions raise CO risk here. The first is the Ohio River valley’s freeze-thaw pattern — a furnace that cycles repeatedly as temperatures swing between the teens and the fifties puts far more thermal expansion and contraction through the heat exchanger than one running steadily in sustained cold. That cycling is what cracks heat exchangers, and it is why heat exchanger inspection matters more here than the mild winter reputation suggests.

The second is flue blockage. On high-efficiency furnaces the PVC exhaust often terminates low on an exterior wall, where snow, ice, or even a bird’s nest can obstruct it. On older homes with masonry chimneys, deteriorating flue liners and nesting material do the same thing. A blocked flue has nowhere to send combustion gases except back into the house. It is worth walking outside and physically looking at your exhaust termination after any ice storm.

The Water Heater Nobody Checks

If your furnace and water heater share a masonry chimney, and the furnace is later replaced with a high-efficiency unit that vents through PVC, the water heater is left venting alone into a flue sized for two appliances. That oversized flue may not draft properly, and a water heater that cannot draft will spill combustion gases into the room. This is a genuinely common oversight during furnace upgrades, and it is one of the specific things our technicians check when assessing a replacement.

Frequently Asked Questions

Can a furnace leak carbon monoxide without the detector going off?

Yes. CO detectors alarm on cumulative exposure — typically around 70 ppm sustained over one to four hours — so a slow leak can put CO into your air for a while before the alarm threshold is reached. Detectors also fail as their sensors age, and a unit past its five-to-seven-year lifespan may not alarm at all. That is exactly why a detector is a backup layer rather than your primary protection, and why an annual combustion inspection matters.

What does a cracked heat exchanger smell like?

Usually nothing at all, which is the danger. Carbon monoxide itself is completely odourless. Some homeowners report a faint stale or stuffy smell when the furnace runs, but that is an unreliable indicator and its absence proves nothing. Visual signs — soot streaking, rust, a yellow or wavering burner flame — and the location-linked symptom pattern are far more useful than smell. If you smell rotten eggs or sulphur, that is a natural gas leak rather than CO, and the response is the same: leave immediately and call 911 or your gas utility from outside.

Is it safe to run my furnace if my CO detector went off once and then stopped?

No. A detector that alarms and then stops has still detected carbon monoxide. Do not treat silence as an all-clear — the concentration may simply have dropped below the alarm threshold while the underlying fault remains. Leave the building, call 911 from outside, and have the furnace inspected before it runs again. Airing the house out addresses the symptom, not the cause.

How often should a gas furnace be inspected for carbon monoxide risk?

Once a year, before the heating season starts. In practice that means late summer or early autumn in Louisville — ahead of the first cold snap, when scheduling is easier and a problem found is a problem you have time to fix. If your furnace is more than 15 years old, has been short-cycling, or has had any repair involving the burners or venting, an inspection is worth doing regardless of when you last had one.

Do electric furnaces and heat pumps produce carbon monoxide?

No. Carbon monoxide is a byproduct of combustion, so an all-electric furnace or heat pump produces none. However, homes with electric heating often still have other fuel-burning appliances — a gas water heater, a gas range, a fireplace, or an attached garage — and those carry their own CO risk. Every home should have CO detectors regardless of what heats it. A running vehicle in an attached garage is a well-documented source of household CO poisoning that has nothing to do with the furnace.

If you want a heat exchanger and combustion inspection before this heating season, Louisville HVAC Pro’s serves homeowners throughout Louisville, Jefferson County, Bullitt County and Southern Indiana — call (502) 553-8534.