Last Revised: 10/26/26
When outside temperatures drop and you bump the thermostat up, your furnace clicks on and you catch the familiar dusty smell as the heat starts flowing for the first time this season.
Any fuel-burning furnace produces carbon monoxide as a heating byproduct, so when something goes wrong, the most dangerous problem is one you can’t see, smell, or hear.
The vast majority of furnaces in Southeastern Pennsylvania run safely for years. But carbon monoxide problems do happen, and they’re almost always preventable. Knowing the warning signs, where your detectors belong, and what to do if one goes off can make all the difference.
At Mattioni, our technicians have been servicing furnaces across Chester, Delaware, and Montgomery Counties for more than 75 years. In this article, we’ll walk you through how a furnace can leak carbon monoxide, how to spot the signs early, and how to keep your home protected all winter.
What’s Ahead
- Can a Furnace Leak Carbon Monoxide?
- What Causes a Furnace to Leak Carbon Monoxide?
- How to Tell Your Furnace May Be Leaking Carbon Monoxide
- What to Do If Your Carbon Monoxide Alarm Goes Off
- Where to Put Carbon Monoxide Detectors
- Pennsylvania’s Carbon Monoxide Alarm Law
- How to Prevent Carbon Monoxide Problems With Your Furnace
Can a Furnace Leak Carbon Monoxide?
Yes, if it burns fuel. Natural gas, propane, and oil furnaces all create carbon monoxide (CO) as a byproduct of combustion. It’s a normal part of the process that creates heat for your home. Part of a furnace’s job is to keep those gases sealed away from your home’s air supply and send them safely outside.
Here’s how that process works in most gas furnaces:
- The burners ignite a small incoming amount of natural gas or propane inside a sealed combustion chamber.
- The direct flames and heat are contained, projected into, and conducted across a metal heat exchanger.
- Your blower motor pushes household air across the outside of the heat exchanger, where it picks up heat without ever touching the combustion gases. That clean heat is pushed through your ductwork and into your home’s living spaces.
- The draft inducer motor pulls the exhaust gases (which contain carbon monoxide) out through the heat exchanger and into the flue or vent pipe, which carries those gases out through a vertical chimney or a horizontal PVC pipe.
Carbon monoxide only gets into your living space when something in that process fails, or when the exhaust has nowhere to go.
If you have an electric furnace or heat pump, there’s no carbon monoxide directly produced. CO is only produced when burning fuel. If you have a dual fuel system, though, your backup gas furnace will still produce it. Other appliances, like gas water heaters, gas dryers, fireplaces, and gas ranges, can also pose CO risks to your home.
What Causes a Furnace to LEak Carbon Monoxide?
Four common causes of a furnace CO leak
A furnace only leaks carbon monoxide when something goes wrong, and there's almost always a specific cause.
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Cracked heat exchanger
Years of heating up and cooling down can open cracks that let exhaust mix with the air you breathe.
What helps: a yearly inspection, and never running a furnace with a known crack
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Blocked or damaged vent
Nests, debris, snow, or separated pipe sections leave the exhaust nowhere to go.
What helps: keeping vents clear, and never bypassing the pressure switch
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Incomplete combustion
Dirty burners, the wrong gas pressure, or too little air for combustion can make CO levels spike.
What helps: cleaning the burners and checking gas pressure during a tune-up
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Backdrafting
Exhaust fans, a running dryer, or a chimney left too big for one appliance can pull exhaust back indoors.
What helps: checking the draft whenever your furnace or water heater is replaced
The U.S. Consumer Product Safety Commission recommends having your heating system professionally inspected and serviced every year.
Mattioni Plumbing, Heating & Cooling
1. A cracked heat exchanger
Each time your furnace runs, the metal heat exchanger heats up and expands, then cools and contracts when it shuts off. Over the years, that’s thousands of expansion and contraction cycles.
Heat exchangers are built with materials designed to withstand this, but over many years, especially in an older furnace or one that overheats from a clogged filter, that stress can open cracks or holes in the metal. Once that sealed barrier is breached, combustion gases can mix with the air your blower sends through your home.
A furnace with a cracked heat exchanger should not be run. Our guide to cracked heat exchangers explains how they’re found and what to do next.
2. A blocked, damaged, or disconnected vent
If the line that exhaust uses to leave your home is blocked, it has nowhere to go except places it shouldn’t. Common culprits of a blocked exhaust include:
- Birds’ nests, leaves, or debris in a chimney
- A deteriorating chimney liner in an older masonry chimney
- Rusted or separated vent pipe sections
- Snow or ice covering the white PVC intake and exhaust pipes
Most modern furnaces have a pressure switch that shuts the burners down when the vent is blocked. That’s why a snow-covered pipe often announces itself as “no heat” (which protects you from the potential carbon monoxide backdraft).
Pro Tip: Never let anyone bypass a pressure switch just to get the heat back on. That switch is a life-safety device. Learn more in our guide to protecting your furnace and heat pump from snow.
What color is your gas furnace's flame?
If your furnace has a viewing window, the flame can tell you how cleanly it's burning.
WHAT YOU WANT
Crisp, steady blue
The burners are getting the right mix of fuel and air.
GET IT CHECKED
Yellow, orange, or flickering
A sign of incomplete combustion, which can raise carbon monoxide.
Gas furnaces only: an oil burner's flame is normally yellow-orange. Don't remove panels to look; leave that to your technician.Mattioni Plumbing, Heating & Cooling
3. Incomplete combustion
Clean-burning fuel, like natural gas and liquid propane, produces very little carbon monoxide. When the fuel-to-air ratio is off, CO production can spike. Dirty burners, incorrect gas pressure, or a furnace closet that’s been sealed up so tightly that the furnace can’t get enough air for combustion can all cause it.
The key symptom for incomplete combustion is a yellow or orange flame on a gas furnace instead of a crisp blue one.
4. Backdrafting
Older, atmospherically vented furnaces rely on hot exhaust rising up the chimney on its own (at atmospheric pressure). Strong kitchen or bathroom exhaust fans, a running dryer, or a fireplace can depressurize a tightly-sealed house enough to pull exhaust back down the chimney.
A related problem shows up in homes that’ve recently upgraded to a high-efficiency furnace with a dedicated PVC exhaust vent (and moved off of an atmospheric chimney). This change in exhaust could leave the water heater (which still uses the atmospheric chimney) alone on a chimney sized for two appliances, leading to poor drafting. A good installer will check for this and address it if needed.
How to Tell Your Furnace May Be Leaking Carbon Monoxide
Carbon monoxide has no color, smell, or taste. A working CO alarm is the only reliable way to know it’s in your home. Still, your furnace and your body can both give you clues.
Warning signs of carbon monoxide
- Yellow or flickering burner flames on a gas furnace instead of steady blue ones
- Soot, scorch marks, or rust streaks on the furnace cabinet, around the burner area, or on the vent pipe
- Heavy condensation on windows near the furnace or throughout the house
- Stale, stuffy air
- Your furnace keeps shutting off automatically, or the pilot light keeps going out
Early carbon monoxide poisoning can also cause headaches, dizziness, nausea, chest pain, and confusion. If several people in the house feel sick at the same time, and the symptoms fade when you’re away from home, carbon monoxide could be the cause. Get everyone outside, call 911, and tell the doctor you suspect carbon monoxide.
According to the CDC, unintentional carbon monoxide poisoning not linked to a fire kills more than 400 Americans and sends more than 100,000 to the emergency room every year. CDC data also shows CO deaths are highest in the winter months, right when furnaces in our area are working hardest.
What To Do If Your Carbon Monoxide Alarm Goes Off?
Your CO alarm is sounding.
Never assume it's a false alarm. Do these five things, in this order.
Get everyone out
Move everyone, including pets, outside into fresh air right away. Don't stop to look for the source.
Call 911 from outside
Tell the dispatcher your carbon monoxide alarm is sounding and whether anyone has symptoms.
Stay out until the all-clear
Don't go back inside until emergency responders say it's safe.
Get medical care for any symptoms
Even if someone starts to feel better, see a doctor and say you suspect carbon monoxide.
Have your appliances inspected
A licensed technician should find the source before you run the furnace again, even if the alarm has stopped.
Is it an alarm or just a chirp?
Most residential CO alarms use a repeating pattern of four quick beeps to signal carbon monoxide in your home, and a single chirp every minute usually means a low battery or that the alarm has reached the end of its life. Patterns vary by brand, so check the label on the back.
Where to Put Carbon Monoxide Detectors
The placement of your CO detector matters. The U.S. Consumer Product Safety Commission recommends CO alarms on every level of your home and outside each sleeping area.
- Don’t mount a detector right next to the furnace. Fuel-burning appliances can release a small puff of CO at start-up, which may cause a false alarm. Instead, place the alarm at least 15 feet from the furnace and water heater.
- If your furnace is in the basement, a detector at the top of the basement stairs is good placement for covering that level of your home.
- Avoid dead-air spots and drafts. Keep detectors out of corners, away from supply vents and ceiling fans, and away from open windows and doors.
- Put one inside the house near the door to an attached garage if you have one.
Where to put carbon monoxide detectors
Put one on every level of your home. If your furnace is in the basement, the top of the stairs covers that level.
BEST SPOTS
- Outside every sleeping area
- At the top of the basement stairs
- Inside, near the door to an attached garage
- At least 15 feet from the furnace and water heater, to avoid false alarms
KEEP DETECTORS AWAY FROM
Corners and dead-air spots, supply vents and ceiling fans, and open windows or doors
Sources: U.S. Consumer Product Safety Commission; CO alarm manufacturer guidance. Always follow your alarm's instructions.Mattioni Plumbing, Heating & Cooling
Test and Replace Your Detectors on Schedule
- Test them monthly using the test button.
- Replace batteries at least once a year, unless your alarm has a sealed 10-year battery.
- Replace the whole alarm when it reaches the end of its rated life, typically 7-10 years. Check the manufacture date on the back, and if you can’t find one, replace it.
Pennsylvania’s Carbon Monoxide Alarm Law: What Homeowners and Landlords Need to Know
PA’s Carbon Monoxide Alarm Standards Act was signed in December 2013, and rental properties had until mid-2015 to install the required alarms. Here’s what it means for you:
- If you’re selling a home, you must disclose information about the home’s carbon monoxide detectors on the property disclosure statement.
- If you’re a landlord, rental properties with a fossil-fuel heater or appliance, a fireplace, or an attached garage must have an approved, working CO alarm installed near the bedrooms and the fuel-burning heater. Landlords must also replace any alarm that was stolen, removed, or broken during a previous tenancy, and make sure the batteries work when a new tenant moves in.
- If you’re a tenant, you’re responsible for keeping the alarm working and replacing batteries while you live there, and for telling your landlord about any problems in writing.
- Penalties: Willfully failing to install or maintain a required CO alarm is a summary offense with a fine of $50.
- Local rules can be stricter. The act allows municipalities to adopt tougher requirements, and some townships check CO alarms as part of resale or rental inspections.
The bottom line: even if the law doesn’t require detectors in your specific situation, every home with a gas or oil furnace should have them. They cost far less than a single service call.
How to Prevent Carbon Monoxide Problems With Your Furnace
Detectors warn you about a problem. Maintenance stops it from happening in the first place.
Schedule a furnace tune-up every fall
An annual heating maintenance visit is the single best way to catch CO hazards early. Our furnace tune-up service cleans your system and inspects and tests its safety features and controls. Book your maintenance visit online now. On a gas or oil furnace, a thorough tune-up covers:
- The heat exchanger, checked for cracks, rust, and early signs of wear-and-tear
- The flue, vent connections, and chimney draft
- The burners and flame, cleaned and inspected
- The pressure switch, flame sensor, and other safety controls
Our VIP Membership makes it easy to stay on schedule year after year.
Keep Vents and Air Paths Clear
- After snow or ice storms, gently brush snow away from PVC intake and exhaust pipes. Be careful, and don’t use a shovel, since cold PVC cracks easily.
- Don’t store boxes, laundry, or paint cans near the furnace.
- Change your air filter on schedule so the furnace doesn’t overheat.
Know When It’s Time to Replace
Most furnaces last about 12 to 15 years. As they age, heat exchanger cracks become more likely and repairs get harder to justify. If your furnace is in that range and your technician finds a safety problem, our guide on repairing vs. replacing a furnace and our article on how long a furnace lasts can help you decide.
Breathe Easy This Heating Season
You shouldn’t have to wonder whether the furnace keeping your family warm is also putting them at risk. Peace of mind takes three simple steps:
- Check your detectors today. Make sure you have a working CO alarm on every level and outside every bedroom, and press the test button on each one.
- Call Mattioni. Schedule a furnace tune-up at (610) 400-8510. Our licensed technicians will clean your furnace and inspect and test its safety features and controls.
- Enjoy a safe, warm winter. Know your furnace is running cleanly, and that if anything ever does go wrong, you’ll be warned in time.
For more than 75 years, families across Chester, Delaware, and Montgomery Counties have trusted Mattioni to keep their homes comfortable and safe. Let us help you head into winter with one less thing to worry about.
