Carbon monoxide can’t be seen or smelled, so the absence of an odd odor doesn’t tell you whether your home is safe. Furnace safety depends on working alarms, safe habits, and attention to changes in your heating system.
Since 2006, we’ve maintained and repaired heating systems for Central Valley homeowners. A gas furnace is designed to vent combustion gases outdoors, but equipment or venting problems demand prompt attention.
Why Carbon Monoxide Requires More Than Your Senses
Carbon monoxide is a colorless, odorless gas that can cause sudden illness and death when inhaled. It can build up indoors without providing the warning people often expect.
CO poisoning symptoms often resemble the flu, but without a fever. Watch for headache, dizziness, weakness, upset stomach, vomiting, chest pain, or confusion, especially when symptoms begin or worsen inside the home and improve after leaving.
- Infants, older adults, and people with heart disease, anemia, or breathing conditions can become ill more quickly.
- People who are asleep or impaired by alcohol or other substances can fail to recognize symptoms before exposure becomes life-threatening.
- More than 400 Americans die each year from unintentional, non-fire-related CO poisoning, according to the Centers for Disease Control and Prevention.
Don’t rely on how you feel as a detector. Symptoms require emergency action, while a working CO alarm provides a separate warning.
How a Furnace Can Become a Carbon Monoxide Source
A properly operating gas furnace burns fuel and directs combustion byproducts outdoors through its venting system. Carbon monoxide forms during incomplete combustion, meaning the fuel doesn’t burn fully.
Worn, poorly adjusted, or poorly maintained combustion equipment can become an indoor CO source. A flue can create problems when it’s blocked or improperly sized, or when connections leak or come apart. Back-drafting is another concern: exhaust gases move into the home instead of outdoors.
Visible Changes Matter
Soot near the furnace, corrosion around vent connections, water staining near a flue pipe, or a change in heating performance are reasons to arrange a professional assessment. Don’t try to judge combustion or venting safety from flame appearance alone.
The heat exchanger transfers furnace heat to household air while separating it from combustion gases. Damage or visible concerns around this area require evaluation by a trained technician rather than a temporary adjustment or continued furnace use.
Place & Maintain CO Alarms Correctly
California requires carbon monoxide devices in dwelling units with a fossil fuel-burning heater or appliance, a fireplace, or an attached garage. Under the 2025 California Residential Code, effective January 1, 2026, alarms belong outside each separate sleeping area and on every occupiable level of the home, including basements.
An alarm in one distant location might not give sleeping family members timely notice. Coverage should match the layout of the entire home, not just the room where the furnace sits.
Build these checks into your household routine:
- Test each smoke and CO alarm monthly.
- Replace removable batteries annually, and follow the manufacturer’s instructions for sealed or hardwired units.
- Check the manufacturing date and replace the entire alarm at the end of its listed service life.
- Keep alarms in place, and don’t remove batteries to silence a nuisance alert.
A carbon monoxide alarm warns of dangerous CO levels. It doesn’t identify whether the source is a furnace, water heater, fireplace, attached garage, or something else. It also doesn’t prove that a furnace is operating safely or replace a professional inspection.
What to Do When an Alarm Sounds or Symptoms Start
Leave the building for fresh air immediately if an alarm sounds or anyone develops possible CO poisoning symptoms. Take people and pets with you, then contact emergency services from outside.
Don’t silence the alarm and continue using the furnace while trying to find the cause. Don’t re-enter until emergency responders determine the home is safe. If they identify a possible equipment or venting issue, have the affected system evaluated before returning it to normal use.
Don’t Troubleshoot First
Opening windows, resetting an alarm, or turning off the furnace must not delay getting everyone outside and contacting emergency services. Protect people from continued exposure before attempting to identify the source.
Pair CO Alarms with Annual Furnace Maintenance
Alarms and annual maintenance serve different purposes. An alarm warns when carbon monoxide reaches a dangerous indoor level, while a maintenance visit checks for furnace conditions that contribute to combustion or venting concerns.
We recommend routine furnace maintenance every fall, before heavier heating use begins. During a typical visit, we check:
- Burners and flame patterns
- Safety controls and electrical connections
- Airflow and thermostat communication
- The heat exchanger area for visible concerns
Older venting systems found in some Stanislaus County homes deserve particular attention during a heating assessment. Regular service provides useful information about vent connections, combustion components, and safety controls before a performance change becomes an urgent heating problem.
Winter Furnace & CO Checklist
Before relying on your furnace for the season:
- Confirm that alarms are installed where California’s current rules require, tested monthly, and within their listed service life.
- Make sure everyone in the household knows to leave immediately, get fresh air, and contact emergency services when an alarm sounds or symptoms appear.
- Watch for soot, corrosion, water staining, repeated shutdowns, or a new change in furnace operation.
- Arrange annual maintenance before periods of heavy heating use.
We offer a Free Diagnostic and 24/7 emergency availability for furnace maintenance in Modesto and urgent heating concerns. Call Comfort Experts Heating & Air Conditioning at (209) 222-2779.