Furnace carbon monoxide is the one heating problem that can hurt you before you know it exists. Carbon monoxide (CO) has no color, smell, or taste. Every gas furnace makes some of it, and a healthy furnace sends all of it up the flue. A cracked heat exchanger, a blocked vent, or a dirty burner changes that. In Dallas-Fort Worth most furnaces sit in the attic or a hallway closet, idle from March to November, and then run hard during the first cold snap in late November or December. That is when a hidden problem shows up.
In short
- A cracked heat exchanger, a blocked flue, dirty burners, or backdrafting are the main furnace carbon monoxide causes.
- Watch for a yellow or flickering flame, soot, rust streaks, and headaches or nausea that ease when you leave the house.
- If a CO alarm sounds, get everyone outside, call 911 from outside, and do not go back in until the fire department clears the home.
- Put a CO alarm on every level and near every bedroom, test monthly, replace units every 5 to 10 years, and book a furnace inspection every fall.
How a gas furnace produces carbon monoxide
A gas furnace burns natural gas inside a sealed metal chamber called the heat exchanger. House air passes over the outside of it while exhaust travels through the inside and out the flue. When combustion is complete and the exhaust path is intact, only a trace of CO is produced and all of it leaves the house. Five things break that chain:
- Cracked heat exchanger. Metal expands and contracts thousands of times over a furnace’s life. Fine cracks let exhaust mix with the air going to your rooms. This is the most serious furnace carbon monoxide cause, and it is common in units past 15 years.
- Blocked flue or vent. Bird nests, a collapsed liner, or a vent joint that pulled apart in the attic send exhaust back into the house.
- Incomplete combustion. Too little air or too much gas makes a yellow, lazy flame and far more CO than a clean blue one.
- Dirty burners. Dust and rust disrupt the flame pattern. Attic furnaces in DFW collect a lot of dust.
- Backdrafting. A strong kitchen hood or bath fan in a tight house can pull exhaust back down the flue.
The EPA notes that homes without gas appliances usually measure 0.5 to 5 parts per million (ppm) of CO; a poorly adjusted gas appliance can push a room past 30 ppm. See the EPA’s carbon monoxide page for more.
Furnace carbon monoxide warning signs
Signs in people
Low-level CO exposure feels like the flu without the fever: headaches, dizziness, nausea, shortness of breath, confusion, and sleepiness. Two patterns point to CO instead of a virus: several people feel sick at once, and symptoms fade after a few hours away from home. Pets often show it first.
Signs on the equipment
- A burner flame that is yellow or orange instead of steady blue
- Soot or black streaks around the burner compartment or cabinet
- Rust streaks on the flue pipe, or a vent joint that has separated
- Heavy condensation on windows near the furnace while it runs
- A pilot light that keeps going out, or short cycling with no clear cause
CO has no smell, but the conditions that create it often do. Our guide to common furnace odors covers those. Raw gas (a rotten egg odor) is a different emergency; see who to call for a gas leak.
What to do right now if the CO alarm sounds
- Get everyone, including pets, outside immediately. Do not stop to open windows.
- Call 911 from outside or from a neighbor’s home.
- Do not re-enter until the fire department has checked the home and says it is safe.
- Tell responders if anyone has symptoms.
- Do not run the furnace again until a licensed technician has inspected it.
Alarms sold in the U.S. are built to sound at about 70 ppm within 1 to 4 hours, and within minutes at 400 ppm. The NFPA’s CO safety page gives the same guidance.
Detector placement and replacement
Put a CO alarm on every level of the home and outside each sleeping area. For an attic furnace, add one in the hallway below the attic access. Test every alarm monthly, replace batteries once a year, and replace the whole unit every 5 to 10 years per the date on the back. A working alarm is the only furnace carbon monoxide warning you will get at night.
The annual inspection that prevents furnace carbon monoxide
A fall tune-up catches most of these problems before the first freeze. Here is what a technician should check:
| What the tech checks | Why it matters for CO |
|---|---|
| Heat exchanger (visual and camera) | Finds cracks and hot spots before exhaust can leak into the airstream |
| Combustion analysis at the flue | Measures CO in ppm and oxygen to confirm clean, complete burning |
| Burners and flame sensor | Dirty burners cause yellow flames and high CO output |
| Flue pipe, vent connectors, draft | Confirms exhaust leaves the house and nothing is blocked or disconnected |
| Gas pressure and temperature rise | Wrong pressure means a wrong air-to-fuel mix; most furnaces call for a 40 to 70 F rise |
| Air filter and blower | A clogged filter overheats the heat exchanger and is a leading cause of cracks |
Between visits, change the filter every 1 to 3 months. Our furnace maintenance tips and preventative heating maintenance posts cover the homeowner side; our heating services and HVAC maintenance pages cover the rest. Club Membership schedules the fall visit for you.
When a cracked heat exchanger means replacement
A cracked heat exchanger cannot be patched. On a unit under 10 years old and still under parts warranty, an exchanger swap can make sense. At 15 years or older, replacement is almost always the better call. Any technician who condemns a heat exchanger should show you the crack on camera and shut off gas to the unit. Pricing is quoted after inspection. Our guide on when to replace your furnace walks through the decision.
Furnace carbon monoxide FAQ
Can an electric furnace or heat pump produce carbon monoxide?
No. Only fuel-burning equipment produces CO. In an all-electric home the risk comes from a garage, a generator, or a gas water heater or range.
Why does my CO alarm chirp once a minute?
A single chirp about once a minute means a low battery or an expired unit. Four loud beeps in a row is the CO alarm pattern. If the unit is past its 5 to 10 year date, replace it.
How often should a gas furnace be checked for CO?
Once a year, in the fall before the first cold snap. A proper furnace carbon monoxide check includes a combustion analysis and a heat exchanger inspection, along with filter and burner cleaning.
Who do I call if I smell gas but the CO alarm is quiet?
Leave the house, then call 911 and your gas utility from outside. The Railroad Commission of Texas oversees gas pipeline safety in Texas. Once the utility makes the home safe, a licensed contractor can repair the leak.
Keep your home safe this heating season
The Right Choice Heating and Air inspects, repairs, and replaces gas furnaces across Dallas-Fort Worth, including Dallas, Fort Worth, Arlington, Plano, Frisco, Carrollton, Farmers Branch, and Irving. Every fall tune-up includes a combustion analysis and heat exchanger check. To book a fall inspection, call (214) 251-4015 or schedule an appointment online.