Yes, boilers can produce carbon monoxide, and it can be fatal. If you suspect a leak, get everyone out of the house immediately, call 911 or your local fire department, and seek medical attention for anyone showing symptoms. Working carbon monoxide alarms and a professional annual inspection are the two things that prevent this from happening in the first place.
TL;DR:
- Place carbon monoxide alarms outside each separate sleeping area and on every level; choose interconnected models meeting UL 2034 and replace units per manufacturer instructions.
- Headaches, dizziness, or nausea that ease outdoors and return indoors can signal an indoor source, while severe exposure may cause confusion or unconsciousness.
- A carbon monoxide alarm that sounds again within a day of reset signals a persistent source; call a technician instead of resetting it repeatedly.
- Visual checks can reveal soot, rust, or a yellow pilot flame, but only technicians with calibrated equipment can measure combustion gases and confirm safety.
- After snowstorms, renovations, or weatherization, check that flues remain clear; exhaust fans and tighter homes can disrupt airflow and trigger backdrafting.
Table of Contents
- How Boilers Produce Carbon Monoxide
- Signs and Symptoms of Carbon Monoxide Exposure
- What to Do Right Now if You Suspect a Leak
- What a Professional Boiler Inspection Actually Checks
- Prevention: Alarms, Testing, and a Maintenance Schedule
- Recent Boiler Recalls Tied to Carbon Monoxide
- Gas vs. Oil Boilers: How Carbon Monoxide Risk Differs
- How Proper Installation Prevents Carbon Monoxide Buildup
- Why Ventilation and Chimney Upkeep Matter for CO Safety
- Common Boiler Malfunctions That Lead to CO Emission
- Massachusetts Codes and Requirements for Boiler CO Safety
- Long-Term Health Effects of Boiler-Related CO Exposure
- What I've Learned Inspecting Boilers in Older Homes
- Get Your Boiler and Vents Checked by a Licensed Technician
- FAQ
- Sources
How Boilers Produce Carbon Monoxide
A boiler burns fuel, gas or oil, to create heat, and that combustion process should produce mostly carbon dioxide and water vapor. When combustion is incomplete, often from a dirty burner, a clogged orifice, or poor air supply, it produces carbon monoxide instead. The gas has no smell or color, so there's no warning sign unless an alarm catches it.
Venting problems are the other major cause. A flue that's blocked, disconnected, or corroded can't carry exhaust gases outside, and backdrafting can push those gases back into the living space instead of up the chimney.
Several conditions raise the risk of a boiler producing or releasing carbon monoxide:
- A cracked heat exchanger that lets combustion gases mix with household air
- Delayed ignition, where fuel builds up before the burner lights
- Missing or loose access panels that let exhaust escape into the room
- A flue blocked by a bird or animal nest, or by snow and ice after a storm
- Backdrafting triggered by a power outage, a running exhaust fan, or a recent remodel that changed the home's airflow
Older boilers and systems that haven't been serviced recently carry more of this risk simply because wear and buildup accumulate over time.
Signs and Symptoms of Carbon Monoxide Exposure
Carbon monoxide poisoning produces symptoms that feel a lot like the flu, which is part of what makes it dangerous. According to the CDC, common symptoms include headache, dizziness, weakness, nausea, vomiting, chest pain, and confusion. In severe cases, exposure can progress to loss of consciousness, coma, and death.
One pattern worth paying attention to: symptoms that ease up when you leave the house and return when your back home point toward a source inside the building rather than a stomach bug or seasonal illness.
Beyond physical symptoms, your home itself can show warning signs:
- Soot stains around a boiler, furnace, or vent opening
- Rust or water streaks on vent pipes or the chimney
- A pilot light that frequently goes out or burns yellow instead of blue
- Excess condensation on windows near the heating system
Vulnerable groups face higher danger from the same exposure level: infants, older adults, and people with heart or respiratory conditions absorb and are affected by carbon monoxide more severely than a healthy adult. A CO alarm that keeps re-triggering within a day of being reset usually signals a persistent source that needs a technician, not just a reset button.
What to Do Right Now if You Suspect a Leak
If a CO alarm sounds or you notice symptoms that seem tied to being indoors, act in this order:
- Evacuate everyone immediately. Get the whole household, including pets, into fresh air outside.
- Call 911 or your local fire department from outside the home, not from inside while you look for the source.
- Get medical care for anyone with symptoms and tell the clinician you suspect carbon monoxide exposure, since CO poisoning is frequently misdiagnosed because its symptoms are so nonspecific.
- Don't re-enter the home until fire or utility personnel confirm it's safe, and shut off the fuel supply only if you can do so safely on your way out.
- Call a licensed heating technician to inspect and repair the system before you turn the boiler back on, and notify the manufacturer if a recall is involved.
Pro Tip: Never try to "air out" a suspected CO leak by opening a few windows and waiting it out indoors. Leave first, then let emergency responders decide when ventilation is safe.
What a Professional Boiler Inspection Actually Checks
When a technician inspects a boiler for carbon monoxide risk, the process goes well beyond a visual glance. Combustion analysis uses a flue-gas analyzer to measure the actual CO and CO2 levels coming out of the appliance, along with draft testing to confirm exhaust gases are moving in the right direction and at the right rate.
Visually, a technician checks for:
- Soot buildup on the burner, heat exchanger, or flue connections
- Cracks in the heat exchanger that could let combustion gases escape
- Loose, missing, or improperly sealed access panels
- Obstructions in the vent pipe or chimney, including nests or debris
Homeowners can do some basic checks themselves, like looking for soot stains or unusual moisture near the unit, but these visual cues only suggest a probable problem; they can't confirm or rule one out. Combustion gas levels require calibrated equipment and training to read correctly.
Pro Tip: If you notice a soot ring or discoloration around a burner or flue joint, treat it as a reason to schedule testing soon, not as something to simply wipe clean. Our annual boiler service guide walks through what a full inspection visit typically covers.
Prevention: Alarms, Testing, and a Maintenance Schedule
Carbon monoxide alarms are the first line of defense, and placement matters. The CPSC and NFPA recommend installing an alarm outside each separate sleeping area and on every level of the home, with interconnected alarms preferred so that one triggering sets them all off. Alarms should meet the UL 2034 standard and be tested regularly, with batteries and units replaced according to the manufacturer's schedule.
Beyond alarms, a few habits make a real difference:
- Schedule an annual professional inspection of the boiler, venting, and chimney
- Never run a generator, grill, or gas-powered tool indoors or near a window
- Check that vents and flue outlets are clear after a snowstorm or home renovation
- Replace CO alarms at the age the manufacturer specifies, not just the batteries
A properly maintained, professionally inspected heating system is far less likely to put dangerous CO into your home than one that's gone years without a checkup.
Recent Boiler Recalls Tied to Carbon Monoxide
Recalls happen, and they matter. A 2025 CPSC recall notice covered certain Triangle Tube Prestige and Aerco Esteem gas boiler models due to a carbon monoxide poisoning hazard, with two deaths reported. Recall actions typically instruct owners to stop using the affected unit and contact the manufacturer for a free inspection or repair.
If you own a boiler affected by a recall:
- Stop using the unit immediately if the recall instructs you to
- Contact the manufacturer directly using the model and serial number
- Arrange for inspection or repair before resuming use
- Check the CPSC recall database periodically if you own an older or less common model
Keeping your boiler's model and serial number on file makes it much faster to check against future recall notices.
Gas vs. Oil Boilers: How Carbon Monoxide Risk Differs
Gas and oil boilers both produce carbon monoxide when combustion is incomplete, but the ways that happens differ enough to matter. Gas boilers rely on a consistent, precisely metered fuel-air mixture, so a dirty burner, a failing gas valve, or a cracked heat exchanger can shift that balance and quickly raise CO output. Because natural gas burns cleanly when everything is working, gas boiler problems often develop quietly, with soot and smell being minimal until the issue is advanced.
Oil boilers burn a heavier fuel and tend to produce more visible warning signs early on, like soot streaking or an oily odor, because oil combustion naturally generates more particulate matter. That makes oil-related combustion problems somewhat easier to catch by sight, but oil systems also depend heavily on a well-adjusted burner nozzle and proper draft; a nozzle that's worn or misaligned can raise CO output even with correct fuel-air ratios elsewhere in the system.
Both fuel types depend on proper venting, so a blocked or corroded flue creates the same backdrafting risk regardless of whether the appliance burns gas or oil. The real differentiator isn't the fuel so much as maintenance frequency and venting condition. A neglected gas boiler and a neglected oil boiler carry similar risk once years pass without inspection, even though the visible warning signs along the way look different. Newer high-efficiency condensing boilers, whether gas or oil-fired, use sealed combustion systems that draw outside air directly, which reduces (but does not eliminate) the backdrafting risks tied to older atmospheric-vent designs.
How Proper Installation Prevents Carbon Monoxide Buildup
Correct installation is where CO prevention really starts, since many problems that show up years later trace back to how the system was originally set up. Flue sizing has to match the boiler's output; a flue that's too narrow or too long for the appliance can't move exhaust gases fast enough, which raises backdrafting risk even on a brand-new unit.
Clearances matter just as much. Combustion air intakes and exhaust vents need the spacing specified by the manufacturer and local code, both from combustible materials and from obstructions like nearby walls or vegetation that could block airflow. EPA guidance stresses following manufacturer and local code installation instructions and having flues and chimneys inspected as part of routine safety checks, not just at install time.
Sealed combustion systems, which draw outside air directly to the burner rather than pulling it from the surrounding room, reduce the odds of depressurization-related backdrafting that can happen when exhaust fans, clothes dryers, or other appliances compete for air in a tightly sealed home. Proper installation also includes pressure-testing gas lines and confirming the vent terminates in a location where exhaust can't re-enter the home through a window or soffit vent.
A rushed or improperly permitted installation is one of the more preventable sources of long-term carbon monoxide risk, since problems built into the system from day one tend to surface only after months or years of otherwise normal operation.
Why Ventilation and Chimney Upkeep Matter for CO Safety
Ventilation systems and chimneys do the unglamorous work of carrying combustion byproducts out of the house, and when either one is compromised, carbon monoxide has nowhere to go but back inside. A chimney that hasn't been inspected in years can accumulate creosote, debris, or even animal nests that partially or fully block the flue path.
Beyond blockages, structural issues like a cracked chimney liner let combustion gases leak into the wall cavity or attic space rather than exiting outside, which can be harder to detect visually than a straightforward obstruction. Mechanical ventilation, like exhaust fans, clothes dryers, or a whole-house fan, can also depressurize a home enough to pull exhaust gases backward down a chimney that would otherwise vent properly on its own.
Routine chimney maintenance should include a sweep and inspection on a schedule appropriate to the fuel type and usage, along with checking that the chimney cap and spark arrestor aren't blocked by debris or nesting material. For homes with mechanical ventilation systems, balancing intake and exhaust airflow reduces the pressure differences that cause backdrafting in the first place.
Homes that have had recent weatherization work, like new windows, added insulation, or sealed ductwork, are worth a second look, since tightening a home's envelope without accounting for combustion air needs is a well-documented way to turn a previously safe heating system into a backdrafting risk.
Common Boiler Malfunctions That Lead to CO Emission
Most carbon monoxide problems trace back to a handful of recurring mechanical failures, and recognizing the early signs of each one gives you a chance to act before the situation becomes dangerous.
A dirty or misaligned burner is probably the most common culprit, since it disrupts the fuel-air ratio that complete combustion depends on. You might notice a burner flame that's yellow or orange instead of blue, or flickers unevenly.
A cracked heat exchanger is more serious, because it allows combustion gases to mix directly with the air your home's system circulates. This failure often shows no outward sign until an alarm trips or a technician's combustion analyzer catches it, which is part of why routine testing matters more than visual inspection alone.
Ignition problems, including delayed ignition where fuel accumulates before lighting, can cause small combustion puffs that produce elevated CO even in an otherwise well-maintained system. If you hear a boiler making a soft "whoomph" sound at startup, that's worth mentioning to a technician.
Loose or missing access panels seem minor but matter because they're part of the sealed combustion chamber; once removed or left ajar after a repair, exhaust gases can leak into the room instead of venting outside. Our guide to low boiler pressure covers another common malfunction that, while not a direct CO cause on its own, often shows up alongside the same neglect that lets these bigger problems develop. If you're troubleshooting a gas burner that won't ignite at all, this technician-backed guide covers common causes worth ruling out before assuming the worst.
Massachusetts Codes and Requirements for Boiler CO Safety
Massachusetts requires carbon monoxide alarms in residential buildings with fossil-fuel-burning equipment or an attached garage, a rule that came out of a 2005 law passed after a carbon monoxide death in the state. The requirement applies broadly to one- and two-family homes as well as multi-family buildings, and alarms must be installed within the proximity to sleeping areas specified under the state's fire code.
Massachusetts also enforces separate requirements around fuel-gas and oil-burning equipment installation, inspection, and permitting, administered through the state's Board of Examiners of Plumbers and Gas Fitters and local fire departments and building inspectors. If you're installing a new boiler or doing significant work on an existing system, a permit and inspection are generally required before the unit goes into service.
Because code requirements can vary by city or town in addition to state-level rules, and enforcement details change over time, it's worth confirming current requirements with your local fire department or building inspector rather than relying solely on general guidance. For the specific alarm placement and type requirements that apply to your address, your local fire marshal's office is the authoritative source.
Long-Term Health Effects of Boiler-Related CO Exposure
A single acute carbon monoxide exposure, especially one severe enough to cause loss of consciousness, can leave lasting effects well after the immediate poisoning resolves. Neurological symptoms including memory problems, difficulty concentrating, and personality changes have been documented in people who survived serious CO poisoning, sometimes appearing days or weeks after the initial exposure in what's referred to as delayed neurological syndrome.
Lower-level, repeated exposure, the kind that might happen from a boiler leaking small amounts of CO over months rather than one acute event, is harder to recognize precisely because the symptoms (headaches, fatigue, difficulty concentrating) look like ordinary tiredness or stress rather than poisoning. This is part of why the pattern of symptoms easing when you're away from home matters as a diagnostic clue.
People with existing heart or lung conditions face a higher risk of lasting cardiac or respiratory complications from carbon monoxide exposure, since CO reduces the blood's ability to carry oxygen and puts extra strain on an already compromised heart or lungs. Pregnant individuals and their developing babies are also considered a higher-risk group, since the fetus can be more sensitive to reduced oxygen delivery than the mother is.
None of this is a reason to panic over a well-maintained system. It's a reason to treat any suspected exposure, even a mild one, as worth a medical evaluation rather than something to wait out at home.
What I've Learned Inspecting Boilers in Older Homes
In my work inspecting heating systems, carbon monoxide problems rarely show up out of nowhere. They usually follow a small, overlooked change: a vent partially blocked during a remodel, a nest built over a chimney cap, a panel left loose after a quick repair. Alarms save lives, but they're a backstop, not a substitute for a trained eye catching a problem before it becomes dangerous.
— Matt
Get Your Boiler and Vents Checked by a Licensed Technician
A safety check costs far less than guessing. Our visits for boiler and water heater safety checks include combustion testing with a flue-gas analyzer, a full inspection of venting and flue connections, and a written summary of what we found with clear next steps, so you're never left wondering what "passed inspection" actually means. We also handle boiler and furnace repair, annual heating tune-ups, gas line leak testing, and no-heat emergency diagnostics across Massachusetts, with same-day and next-day availability for urgent concerns.
If your boiler is older, was recently serviced by someone else, or you just want peace of mind before winter, schedule a heating safety visit and we'll take it from there.
FAQ
How likely is a boiler to leak carbon monoxide?
A well-maintained, properly installed boiler that's inspected annually is unlikely to leak carbon monoxide, since properly maintained fuel-burning appliances generally produce little CO. Risk rises sharply with age, skipped maintenance, and specific failures like a cracked heat exchanger or blocked flue.
What are two warning signs of a carbon monoxide leak?
Two reliable warning signs are flu-like symptoms that improve when you leave the house, such as headache, dizziness, and nausea, and physical signs on the equipment itself, like soot stains or rust streaks near a vent or chimney. Either sign on its own is reason enough to test the air and call a technician.
Is it safe to sleep in a room with a boiler?
Sleeping near a properly installed, well-maintained, and professionally inspected boiler is generally considered safe, provided working carbon monoxide alarms are installed outside the sleeping area as recommended by CPSC and NFPA. If the boiler hasn't been inspected recently or shows any warning signs, it's worth scheduling a check before relying on that room for sleep.