TL;DR: Carbon monoxide is colorless, odorless, and tasteless, and your gas furnace is the most common source of it in a Spring, TX home. This weekend, overnight lows settle into the 50s and the heat is on at night for the first time this season, which is exactly when a neglected furnace can start leaking CO into the air your family breathes. This guide covers what CO is, how it gets into your home, the symptoms that get mistaken for the flu, where your detectors belong, what to do the moment an alarm sounds, and the one annual visit that catches it first. If your furnace has not been checked since last winter, book heating repair in Spring, TX or a fall tune-up before the cold nights ahead.

Every winter, families across the country are hurt by a gas they cannot see, smell, or taste. Carbon monoxide kills more than 400 Americans a year in unintentional poisonings not linked to fires, and sends more than 100,000 people to emergency rooms, according to the CDC. The furnace in your closet or attic is the appliance most likely to be the source, and the risk peaks exactly now, in the first weeks of heating season, when systems that sat idle for seven months fire up again. The good news is that CO poisoning is almost entirely preventable: a working detector, an annual service visit, and knowing what to do when the alarm sounds are all it takes.

What carbon monoxide actually is

Carbon monoxide, or CO, is a gas produced whenever fuel burns. Your gas furnace burns natural gas to make heat, and in a healthy system the combustion gases, including CO, travel up the flue and out of the house. The danger starts when combustion goes wrong or the exhaust path fails, and CO drifts into the living space instead. Because it has no color, no smell, and no taste, you cannot detect it with your senses. A healthy adult can breathe low levels of it for hours without realizing anything is wrong, and by the time symptoms appear, the situation is already serious. That invisibility is the whole story of CO. Everything in this guide exists because you cannot rely on your nose or your eyes.

How CO gets into a Spring home

The furnace is the first place to look, and there are four ways it lets CO into the house:

• A cracked heat exchanger. The metal wall between the flame and the air that blows into your rooms can crack after years of heating and cooling cycles. Once it cracks, combustion gases mix with the air your family breathes. We covered this failure in detail in our guide, Cracked Heat Exchanger: What It Is, What It Looks Like, and Why We Shut Systems Down Over It.
• A blocked flue or chimney. Bird nests, debris, rust, and collapsed liners can block the exhaust path. When combustion gases cannot get out, they back up into the house.
• A failed draft inducer or venting problem. The inducer fan pushes exhaust gases out through the flue. If it fails, or the vent pipe has gaps or the wrong slope, CO leaks at the joints.
• Dirty burners or wrong gas pressure. Burners that burn unevenly, or gas pressure set too high or too low, produce more CO than a clean, correctly tuned flame.

The furnace is not the only source in the house. Gas water heaters, gas stoves and ovens, fireplaces, and gas clothes dryers all burn fuel, and any of them can produce CO when they are faulty or poorly vented. A car left idling in an attached garage can push CO into the house through the connecting door, and a portable generator is the single deadliest CO source in American homes: the Consumer Product Safety Commission links generators to more CO deaths than any other consumer product. Read more about the systems we service on our gas furnaces page.

The symptoms that get mistaken for the flu

The CDC lists the most common symptoms of CO poisoning as headache, dizziness, weakness, upset stomach, vomiting, chest pain, and confusion. The trap is that they feel like the flu, without a fever, and they arrive in heating season, which is also flu season. Three patterns tell them apart:

• Several people, or pets, in the same home feel sick at the same time
• Symptoms start after the heat comes on, or after a fuel-burning appliance runs
• Symptoms ease when you leave the house and return when you come back

That last pattern is the classic warning sign. Headaches, nausea, or dizziness that fade when you step outside are telling you something about the air inside. Take it seriously. The people most at risk are infants, older adults, pregnant women, and anyone with chronic heart disease, anemia, or breathing problems. And the cruelest part of CO poisoning is that people who are asleep can die before they ever notice symptoms, which is why the detector in the hallway outside the bedrooms matters more than any other safety device in the house.

Your CO detectors: where they go and how to keep them working

A working CO detector is the only reliable warning most homes get, and placement decides whether it can do its job. The National Fire Protection Association and the Consumer Product Safety Commission recommend a detector on every level of the home, and outside each separate sleeping area, close enough that the alarm can wake you at night. Interconnected alarms are best, because when one sounds, they all sound. Keep each detector at least 15 feet away from the furnace or other fuel-burning appliance, so normal operation does not cause false alarms, and keep them out of high-humidity rooms like bathrooms and away from drafty windows and doors, where moving air can delay detection.

Buying the detector is only half the job. Test it once a month by pressing the test button and listening for the alarm. Check the batteries when the clocks change in spring and fall, the habit the CDC recommends. Use a battery-operated model or a plug-in model with battery backup, so it keeps working during a power outage. Replace the unit on the schedule in the manufacturer’s instructions, because the sensors wear out over time, usually within about 10 years. And know two things your detector cannot do: a smoke alarm does not detect carbon monoxide unless it is labeled as a combination smoke and CO alarm, and a CO alarm does not detect natural gas. A rotten-egg smell is a different emergency, covered in the next section.

If the alarm sounds, do this

Do not try to find the source. Do not open windows first. Do not silence the alarm and go back to sleep. The steps are simple and they are the same ones the CPSC and the CDC give:

1. Get everyone out, including pets, and into fresh air right away. Leave the door open behind you if it is quick to do so.
2. Call 911 from outside. Do not call from inside the house. Tell the dispatcher you suspect carbon monoxide.
3. Do not go back inside until emergency responders say the air is clear.
4. Get medical care and tell the doctor you suspect CO. Poison Control is also available at 1-800-222-1222 for advice.

A gas smell is a different emergency with the same leave-first rule. Natural gas is given a rotten-egg odor on purpose, so a sulfur smell near your furnace means a possible gas leak. Get everyone out without touching light switches, the thermostat, or anything else that could spark, then call 911 or your gas utility from outside, and stay out until responders say it is safe.

Prevention: the annual service that catches it first

The CDC advises having your heating system, water heater, and any gas, oil, or coal-burning appliance serviced by a qualified technician every year, and heating season is the time to do it. On a heating visit, the technician measures CO in the flue gas with a combustion analyzer, inspects the heat exchanger with a camera scope, checks the burners and gas pressure, and verifies the venting and safety controls. Those are checks no homeowner can do with household tools: you cannot measure combustion gas levels without an analyzer, you cannot see inside a heat exchanger without a scope, and you cannot verify gas pressure without a manometer. A furnace that lights then dies, short-cycles, or will not start at all deserves the same professional look before the cold settles in. Our guide to a heater that won’t turn on after sitting idle walks through those symptoms from the homeowner’s side.

If your system is 15 years or older and has needed several repairs, ask the technician for an honest assessment while the visit is happening. The signs it’s time to replace your heating unit will help you weigh the call, and a fall tune-up on a regular schedule is how the inspection gets done every year without you having to remember it. Here is what to look for in a maintenance plan if you want that coverage.

The winter habits that create CO risk

Most CO poisonings do not come from a broken furnace. They come from ordinary habits in cold weather. Keep these five rules, and you remove the risk at its source:

• Never heat your home with a gas stove or oven, even for a short time
• Never run a generator inside the home, basement, or garage, even with doors and windows open. Keep it outside, at least 20 feet from windows, doors, and vents
• Never leave a car idling in a closed attached garage, even for a minute
• Never burn charcoal indoors, and never use a portable gas camp stove inside
• Keep flues, vents, and chimneys clear, and never block a furnace vent to save space

Get the furnace checked before the cold settles in.

Heating season is here, and the furnace that kept you warm last winter deserves a check before it carries another one. A working CO detector, a yearly service visit, and knowing the alarm steps are a small investment against a risk you cannot see or smell. KAC Express is open 7 days a week from 7 AM to 10 PM, with same-day service when you call before 2 PM, $50 off your first repair, free second opinions, and 400+ five-star Google reviews from your neighbors.