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Carbon monoxide is the reason annual furnace inspections aren't optional — they're a safety measure. In Morris County and across North NJ, we respond to service calls every winter where a homeowner has persistent headaches, or a CO detector has alarmed, and the source turns out to be their HVAC system. This guide covers how it happens, how to detect it, and what to do.
How HVAC systems produce carbon monoxide
Carbon monoxide is a byproduct of incomplete combustion. Any time natural gas, oil, or propane burns without enough oxygen — or when something interrupts the combustion process — CO is produced instead of the carbon dioxide that results from clean combustion. Your furnace or boiler is a combustion appliance, which means it has the potential to produce CO under certain failure conditions.
Blocked or obstructed flue. The combustion gases from your furnace — including any CO produced — exit through a flue pipe to the outside. If that flue is blocked by a bird nest, debris, or a structural obstruction, combustion gases have nowhere to go and back up into the living space. This is most common when systems fire up for the first time in fall after sitting all summer. Blockages can happen gradually enough that no single event triggers a CO alarm, but the problem compounds over time.
Backdrafting. Most older furnaces are "natural draft" systems that rely on the buoyancy of hot combustion gases to draw them up the flue. If indoor air pressure is negative — which can happen in tightly sealed modern homes, or when exhaust fans are running — the furnace flue may backdraft, pulling combustion gases back into the house rather than expelling them. High-efficiency condensing furnaces (90%+) use induced-draft fans that create positive pressure in the flue, making them much less susceptible to backdrafting.
Incomplete combustion from a dirty or maladjusted burner. A burner that hasn't been serviced in years may have partially clogged ports, a corroded heat exchanger, or an improperly adjusted fuel-air mixture. Any of these conditions reduces combustion efficiency and increases CO production. A clean, properly adjusted burner flame is blue with a small yellow tip. A primarily yellow or orange flame is burning incompletely.
The cracked heat exchanger — the most common HVAC CO risk in NJ
The heat exchanger is the component inside your furnace that separates combustion gases from the air that gets blown into your home. Combustion happens inside the heat exchanger. Air from your home passes over the outside of it, picks up heat, and gets distributed through your ducts. As long as the heat exchanger is intact, combustion gases — including any CO — stay contained and exit through the flue.
When a heat exchanger cracks, that separation breaks down. Combustion gases can mix with the air being distributed to your living space. This is the most common HVAC-related CO risk in Morris County homes.
Heat exchangers crack for predictable reasons. Metal fatigue from years of heating and cooling cycles, restricted airflow that causes overheating (a clogged filter is a contributing factor), and simple age all cause metal to develop cracks over time. Furnaces with heat exchangers that are 15 to 20 years old are at meaningful risk. The cracks are often invisible to the naked eye — which is why inspection requires a trained technician with the right tools, not just a visual look inside the cabinet.
A cracked heat exchanger is not a fixable problem. The furnace must be replaced. It's an expensive finding, but a licensed technician who tells you this is protecting your family's safety, not upselling.
NJ CO detector requirements and proper placement
New Jersey law (the Carbon Monoxide Alarm Act) requires working CO detectors in all residential buildings, including single-family homes, on every floor with sleeping areas. Detectors must be within 10 feet of each bedroom door.
For a typical two-story Morris County home, this means at minimum: one detector on the first floor near the furnace area, and one on the second floor hallway near the bedrooms. Many safety organizations recommend one per floor regardless of bedrooms.
On placement: CO is slightly lighter than air but in practice disperses fairly evenly throughout a space, unlike smoke which rises. Most CO incidents happen at floor level during sleep when people aren't moving. Place detectors at knee to breathing height — not up near the ceiling like smoke detectors, and not on the floor. A detector placed on a nightstand or wall at 4 to 5 feet is in the right location for the bedroom sleeping situation.
Replace CO detector batteries annually or when the low-battery signal sounds. Replace the entire unit every 5 to 7 years — CO detectors have a limited sensor lifespan and an old unit may no longer alert accurately even if it appears to function.
Warning signs your system may be producing CO
Don't wait for the detector to alarm. These patterns warrant an immediate service call:
Yellow or orange burner flame. Look through the inspection window on your furnace at the burner flame. A healthy natural gas flame is predominantly blue. Yellow or orange indicates incomplete combustion, which means CO is being produced.
Soot or black marks around the furnace, particularly around the heat exchanger access panel. Soot deposits mean combustion gases are escaping the heat exchanger or combustion chamber. This is not normal and indicates a serious problem.
Excessive condensation on windows during heating season. Some condensation is normal in winter, but heavy condensation throughout the house can indicate combustion gases are entering the living space and adding moisture along with CO.
Flu-like symptoms in household members that improve when they leave home. Headaches, nausea, dizziness, and fatigue are the classic CO poisoning symptoms at low levels. The key distinguishing feature is that symptoms improve when people are away from home — something true flu does not produce. If multiple family members report this pattern, treat it as a CO emergency.
What to do if your CO detector alarms
Leave the house immediately. Everyone, including pets. Do not stop to investigate the source. Do not try to find and shut off the furnace before leaving. Get outside to fresh air.
Call 911 from outside. The fire department will respond and measure CO levels. They have the equipment to safely assess what's happening before anyone re-enters.
Only after the fire department has cleared the home should you re-enter. Then call Air2Cool to inspect and identify the source before you run the heating system again.
Annual furnace inspections are your primary defense against CO exposure. Our tune-up service includes heat exchanger inspection, combustion analysis, and flue inspection. See our heating repair services and preventative maintenance plan.
Need HVAC help in North NJ? Call Air2Cool at (201) 787-5657 or request a free estimate. Same-day service available across Morris County and North NJ.