Carbon Monoxide: The Winter Safety Check Most Homeowners Skip
December 1st, 2026
Most homeowners know to change the furnace filter and schedule a fall tune-up. Far fewer ask a technician to test the heating system for carbon monoxide. That gap matters more in winter than at any other time of year, because the furnace, water heater, fireplace, and space heaters all run more often, and the windows stay closed.
Carbon monoxide is colorless, odorless, and tasteless. It is produced whenever fuel burns, and it can build up quietly inside a home when a vent is blocked, a heat exchanger fails, or an appliance backdrafts. The check that catches those problems takes a technician a few minutes with a combustion analyzer. It is also the check that most homeowners never request.
Why Winter Is Carbon Monoxide Season
Carbon monoxide calls rise in cold weather for several reasons that stack on top of each other. Windows and doors stay shut, so fresh air does not move through the house. The heating system runs for hours at a time instead of cycling on and off. Chimneys and flues can be blocked by debris, nests, or ice. Holiday cooking and baking keep the gas range busy. Generators run during storm outages. Cars warm up in garages.
A house that is sealed tight for efficiency is good for energy bills and comfortable in a cold snap, but it also gives carbon monoxide fewer ways to escape. Modern homes depend on proper venting and adequate combustion air more than older houses with loose construction did. When a flue does not draft correctly, the exhaust has to go somewhere, and it can push back into the living space.
Where Carbon Monoxide Comes From in a Middle Tennessee Home
Any appliance that burns fuel is a potential source:
- A gas furnace, especially an older unit with a cracked heat exchanger
- A gas water heater, particularly when it shares a flue with the furnace
- A gas range, oven, or cooktop, even with a working vent hood
- A wood burning fireplace or wood stove with a blocked or undersized chimney
- An unvented gas space heater or decorative fireplace
- A portable generator, grill, or camp stove used indoors or in a garage
- A car idling in an attached garage, even with the garage door open
Electric heat pumps and electric resistive heat do not produce carbon monoxide themselves. The risk in those homes comes from the other fuel burning appliances in the house, or from a generator brought indoors during a power outage. A heat pump may still have a gas furnace as backup in a dual fuel system, so do not assume an electric outdoor unit means the whole system is electric.
The Check Most Homeowners Skip
A heating tune-up is often treated as an oil change: change the filter, look at the burners, and move on. A proper combustion safety check goes further. The technician should measure carbon monoxide in the flue gases and in the supply air, verify that the flue is drafting correctly, check gas pressure, inspect the burners, and look at the heat exchanger. On a high efficiency furnace, the venting and condensate system need attention too, because a blocked PVC intake or exhaust can change how the unit burns.
Homeowners skip this for understandable reasons. The furnace starts, the house gets warm, and nothing sounds wrong. Carbon monoxide does not announce itself. A small leak can go unnoticed for weeks, and the symptoms it causes are easy to blame on a cold, a long week, or poor sleep.
Warning Signs You Should Never Explain Away
Carbon monoxide poisoning symptoms are often vague at first. Watch for a combination of these:
- Headache, dizziness, or lightheadedness
- Nausea or vomiting
- Confusion, trouble concentrating, or blurred vision
- Unusual sleepiness or loss of coordination
- Chest pain or shortness of breath
- Flu like symptoms without a fever
Two patterns are especially telling. Symptoms that improve when the family leaves the house and return after coming home point to an indoor air problem. So does the same set of symptoms affecting several people or pets at once. Infants, older adults, pregnant women, and anyone with heart or lung disease are more vulnerable and may show symptoms first.
Physical clues around the equipment matter too. Soot streaks near the furnace, a burner flame that is yellow or orange instead of blue, rust or gaps in the metal flue, excessive moisture on the windows near the furnace, and the smell of exhaust inside the house are all reasons to stop using the system and call for service.
Test the Alarms, Then Test Them Again
A carbon monoxide alarm is the last line of defense, and it only works if it is powered, placed correctly, and not expired.
- Placement: put an alarm on every level of the home and outside each sleeping area. Keep it at least a few feet away from the furnace or water heater, and out of dead air corners behind furniture or curtains.
- Testing: press the test button once a month. Test the alarm on every level, not just the one you see most often.
- Batteries: replace them at least once a year, and immediately when the low battery chirp starts. Do not remove the battery to silence a chirp and forget to replace it.
- Age: replace the alarm or the sensor according to the manufacturer, usually every five to ten years. Write the install date on the back so you do not have to guess.
- Interconnection: if the alarms are wired together, confirm that one sounding sets off the rest. That feature is only useful while it works.
Never treat a sounding alarm as a nuisance. Open a window, step outside, and call the fire department. If the alarm is a combination smoke and carbon monoxide unit, the voice or display usually tells you which hazard triggered it, but let the professionals confirm the source.
A Homeowner Walkthrough for the First Cold Week
There are safe checks you can do yourself before the next cold snap:
- Clear snow, ice, and leaves from high efficiency furnace intake and exhaust pipes outside the house. Blocked pipes can cause the furnace to shut down or to burn improperly.
- Keep the outdoor unit and all vents clear. Do not stack boxes, blankets, or furniture in front of return air grilles.
- Run a generator outdoors, well away from windows, doors, and vents. Never run one in a garage, basement, crawlspace, or screened porch.
- Do not idle a car in an attached garage, even with the door open. Start it outside or pull it out first.
- Open the fireplace damper before lighting a fire, and have the chimney cleaned if it has not been serviced.
- Use space heaters with a tip over switch, keep them three feet from anything that can burn, and plug them directly into a wall outlet.
- Do not use the oven, a grill, or a camp stove to heat the house.
- Change the furnace filter when it is dirty. Restricted airflow can overheat a gas furnace and shorten the life of the blower motor.
If you see soot, rust, a disconnected flue, or a yellow flame, stop using the equipment and call a technician. Do not tape over gaps, wrap the flue, or try to adjust the burners yourself.
Your Carbon Monoxide Alarm Sounds. Now What?
Get everyone out of the house, including pets. Do not stop to open windows on the way out. Call 911 from outside and tell the dispatcher that the carbon monoxide alarm is sounding. If anyone has a headache, dizziness, nausea, confusion, or trouble breathing, say so and ask for medical help.
Do not go back inside until the fire department or a qualified technician has tested the air and found the source. Opening windows and waiting inside can make the reading look normal while the leak continues. An alarm that sounds and then stops on its own is still an alarm that needs investigation.
What a Professional Safety Check Includes
When a JLS Mechanical HVAC technician performs a heating tune-up, the visit covers more than a filter change. The technician will:
- Test carbon monoxide levels in the flue and in the supply air with a combustion analyzer
- Inspect the heat exchanger for cracks, rust, and soot
- Verify flue draft and check the vent connector for gaps, corrosion, or blockage
- Measure gas pressure and inspect the burners and ignition components
- Check the blower, capacitors, wiring, and safety switches
- Confirm the thermostat is calibrated and the filter is correct for the system
If the check finds a problem, you get a clear explanation and a flat rate price before any repair begins. Some issues are simple adjustments. Others, such as a cracked heat exchanger, may change the repair or replace decision. Either way, you are making that decision with test results instead of a guess.
Why Older Homes Need This More, Not Less
Older furnaces and water heaters were built before some of today's safety standards, and their venting was designed for a house that leaked air. When windows are replaced, insulation is added, and a house is tightened up, the old venting arrangement can stop drafting the way it once did. Bath fans, kitchen exhaust fans, and clothes dryers can also pull enough air out of a tight house to reverse the draft on a naturally vented appliance.
That is why a carbon monoxide check is not only for furnaces that look old. A newer furnace with a blocked vent, a water heater sharing a flue with a larger appliance, or a fireplace chimney with a cracked liner can create the same problem. The equipment does not have to be failing to be unsafe.
Schedule the Check You Have Been Skipping
The best time to find a carbon monoxide problem is before the furnace runs hard, not after an alarm wakes the family at two in the morning. A fall heating tune-up with combustion testing catches venting and heat exchanger issues on a mild afternoon, when the repair can be scheduled instead of treated as an emergency.
JLS Mechanical HVAC serves Smyrna, Murfreesboro, and the surrounding Rutherford County communities. Call (615) 474-0648 to schedule a heating tune-up with a carbon monoxide safety check, or contact us online. You can also learn more about our heating services and what happens during a service call.
Posted in: Heating & AC Service