Carbon Monoxide and Furnace Safety: A Homeowner’s Guide

A furnace that burns gas or oil produces carbon monoxide (CO) every time it runs. When the furnace is healthy, that gas leaves through the flue and never reaches your living space. When a part fails, a vent is blocked, or the house pulls exhaust back inside, CO can build up with no smell, no color and no warning. This guide covers how that happens, the symptoms to take seriously, where alarms belong, what to do when one sounds, and what a real professional inspection looks at.

TL;DR

• Gas- and oil-burning furnaces and boilers make CO, an invisible, odorless gas, so you cannot rely on your senses to detect a leak.
• Put UL 2034 CO alarms on every level and outside each sleeping area. An alarm is a last line of defense, not a substitute for maintenance.
• Headaches, nausea or dizziness that ease when you leave the house and return when you come back point to a possible source. Get outside first, then call for help.
• Have a qualified technician inspect every fuel-burning heating appliance once a year, before the heating season starts.
• If an alarm sounds and anyone feels ill, leave the building and call 911. Do not stay to look for the cause.

Table of Contents

• Why a furnace can produce carbon monoxide
• Carbon monoxide and boilers
• Symptoms of carbon monoxide from a furnace
• CO alarms: how many, where and what they can’t do
• What to do if an alarm sounds or you suspect CO
• What a professional furnace safety inspection should include
• A seasonal routine that keeps CO risk low
• Our perspective: why we push annual inspections over gadgets
• Book a furnace or boiler safety check
• FAQ
• Sources

Why a furnace can produce carbon monoxide

CO forms when fuel burns incompletely. NFPA explains that fuels such as natural gas, propane, oil, wood and gasoline can all produce it, and that heating and cooking equipment are common household sources. A furnace or boiler is designed to burn fuel and send the combustion gases outdoors. CO becomes a hazard when something interrupts that second step or disturbs the first.

The scale of the problem is not small. In a Michigan review of CO poisoning cases from 2009 to 2014, furnaces were the source in 19.9% of identified cases, according to the CDC’s furnace fact sheet. That is a single state’s data, but New Jersey homes heat with the same kinds of equipment through similar winters.

The most common failure points

In our experience servicing homes and buildings, CO problems from heating equipment tend to come from a short list of causes:

• A cracked or corroded heat exchanger. This is the metal barrier between the burning gas and the air your blower circulates. If it fails, CO can enter the air stream and bypass the vent system entirely. The ICC-published inspection procedure includes a heat exchanger leakage test for exactly this reason.
• A blocked or damaged flue or chimney. Nests, debris, a collapsed liner, or snow and ice at a termination can trap exhaust. Public health guidance from the CDC notes that risk rises in cold weather and after storms, often from blocked vents or improperly used portable generators.
• Loose connections and poor combustion. A dirty burner, a poorly adjusted gas valve or a disconnected vent pipe can leave fuel burning incompletely.
• Back-drafting. Exhaust fans, a clothes dryer and a fireplace can pull air out of a tight house. If the furnace cannot get enough air or vent properly, exhaust can spill back into the room.
• Improper installation. Wrong vent sizing, bad pipe slope or an unpermitted replacement can create a hazard from day one. If you are weighing a replacement, our New Jersey furnace replacement cost guide explains why permits matter.

Carbon monoxide and boilers

Boilers burn the same fuels as furnaces and carry the same risk, but the CO does not travel through your ductwork. It travels through the boiler’s flue, so the danger sits in the boiler room, the basement and any space where the vent runs. A boiler with a leaking vent connector, a blocked chimney or poor combustion air can fill a basement with CO that then rises through the house.

Because boilers are often tucked into utility spaces, people go months without looking at them. Three habits help:

1. Keep the area around the boiler clear. Do not store paint, boxes or cleaning supplies against it, and never block the air supply openings.
2. Look at the flue pipe once a month. Rust streaks, gaps at joints, soot or a pipe that has shifted are all reasons to call someone.
3. Schedule an annual service before heating season. Boiler repair and inspection are part of what we do, and a checkup is far cheaper than an emergency in January.

The ICC procedure cited above is written for furnaces, boilers, room heaters, water heaters, fireplaces and dryers alike, which is a useful reminder: every fuel-burning appliance in the house should be on the same annual list, not just the furnace.

Symptoms of carbon monoxide from a furnace

The CDC lists headache, dizziness, weakness, nausea, vomiting, chest pain and confusion as symptoms of CO exposure. These are easy to mistake for the flu, which is the main reason people stay in a dangerous house too long. The New Jersey Poison Control Center adds sore muscles, irritability, sleepiness, shortness of breath and impaired vision or coordination to the list.

Patterns that should raise suspicion

The individual symptoms are vague. The pattern is what gives it away:

• Several people are sick at once. The NFPA safety toolkit warns that flu-like symptoms in more than one family member can be the first sign of a malfunctioning heating system. Pets can be affected too.
• You feel better away from the building. OSHA’s indoor air quality guidance points out that symptoms that improve outside and return indoors are a key indicator of a source that needs professional investigation.
• Symptoms start or worsen when the heat comes on. The first cold snap of the season is a classic time for trouble, because the furnace has sat idle for months.
• There are physical clues. Soot or scorch marks around the furnace or vent, a pilot light or burner flame that looks yellow or orange rather than steady blue, excess moisture on windows, or a flue that feels unusually cool or has visible gaps. None of these prove CO is present, and the absence of them proves nothing, but each is a reason to have the appliance checked.

If you are asking whether you can tell by yourself that your furnace is producing CO, the honest answer is no. Only a CO alarm or a technician’s analyzer can confirm it. For a plain-language walkthrough of what to ask for, see our post on four signs and tests homeowners can request.

How much exposure is too much

There is no single safe number to memorize, because the effect depends on the concentration, how long you breathe it, and who you are. Children, older adults, pregnant women and people with heart or lung disease are more vulnerable. The ICC-published inspection procedure treats readings of 70 ppm or more as a trigger to evacuate everyone and call 911, and readings of 30 to 70 ppm as a trigger to shut off possible sources and ventilate. Treat those as professional thresholds, not as a range you can safely live in.

CO alarms: how many, where and what they can’t do

An alarm is the one tool that can detect CO while you sleep. Used correctly, it buys you time. Used as the only protection, it leaves gaps.

Placement

The EPA and CPSC recommend UL 2034-listed CO alarms on every level of the home and outside each sleeping area. The EPA fact sheet also says to keep them about 15 feet from fuel-burning appliances to avoid nuisance alarms, and not to mount them in kitchens or directly above a furnace.

New Jersey adds its own rules. A township summary of the state’s requirements says that since April 2003, alarms must be installed near sleeping areas in buildings with fuel-burning appliances or an attached garage, within 10 feet of bedrooms so they can be heard, and not in garages or utility rooms. Combination smoke and CO alarms are allowed. Check with your local fire official for the current rule that applies to your property, especially if you are selling, renting or buying.

Putting that together for a typical two-story house with a basement furnace:

One tension to notice: you want CO protection near the furnace, yet the guidance says to keep the alarm away from it. The answer is a placement on the floor above or in the nearest hallway or stairwell, outside the 15-foot zone, so that gas escaping from the equipment still reaches a sensor.

Maintenance

The CPSC recommends replacing alarm batteries annually and testing alarms monthly. Alarms also expire, so check the manufacturer’s date on the back and replace the unit when it is due. Many newer models use sealed 10-year batteries.

What alarms do not do

This is the part most homeowners miss. According to CPSC staff, CO alarms are life-safety devices meant to warn before cognitive abilities are compromised, and they are not designed to protect against low-level chronic exposure. NFPA makes the same point: alarms respond to acute, high-level exposure, not every trace of the gas.

That means a quiet alarm does not guarantee a healthy furnace. A cracked heat exchanger can leak a small amount for weeks while the alarm stays silent and the family gets headaches. Annual professional maintenance is the primary defense, and the alarm is the backup.

What to do if an alarm sounds or you suspect CO

Act first and diagnose later.

1. If anyone feels ill, or the alarm will not stop, get everyone outside into fresh air. Include pets if you can do so without delay.
2. Call 911 from outside. Do not go back in to find the source, open the furnace or reset the alarm.
3. If symptoms are mild and you are unsure, call Poison Control at 1-800-222-1222. The New Jersey Poison Control Center staffs that line around the clock. It says to call 911 if someone is not breathing, is hard to wake or is having a seizure.
4. Let responders clear the building. Firefighters and the gas utility can measure CO and shut off the appliance.
5. Do not turn the furnace back on until a qualified technician has found and fixed the cause.

The ICC-published procedure describes a lower tier for readings of 30 to 70 ppm: turn off potential sources and open windows. That is guidance for trained inspectors working a controlled situation. As a homeowner with a sounding alarm and a family in the house, the safer default is to leave and call.

Outdoor ventilation and fast decisions matter with any invisible household hazard, not only CO. A guide to what to do when a radon system isn’t working covers a similar mindset for another gas you cannot see: check the basics quickly, ventilate, and know when to call for help.

What a professional furnace safety inspection should include

A good inspection is more than holding a meter near the register. Here is what a thorough one covers, drawing on the ICC-published procedure and the Department of Energy’s combustion safety checklist.

• Heat exchanger check. Visual inspection and a leakage test, because cracks and corrosion can send CO into the circulating air.
• Venting and chimney. Checking for blockages, disconnected or corroded vent pipes, correct slope and clearances, and a sound flue liner.
• Burner and combustion. Flame appearance, ignition, gas pressure and measured CO in the flue gases, so the appliance is burning cleanly.
• Loose connections. Gas piping and fittings, wiring, and the condition of the cabinet and panels.
• Combustion air. Making sure the furnace gets enough air to burn properly.
• Worst-case depressurization testing. The DOE checklist describes running the home’s exhaust fans together and confirming that the furnace and other combustion appliances still vent properly rather than back-drafting into the living space. A reading taken with the house in its resting state can miss a problem that only appears when the bath fan, range hood and dryer are all pulling air.
• Other appliances. Water heater, boiler, dryer and fireplace share the same house air and sometimes the same chimney.

If you are a landlord, property manager or facility manager, ask for the results in writing, record the date, and keep them with your maintenance files. Commercial equipment has its own failure patterns; our post for building teams on what to measure when a rooftop unit stops heating is a good companion.

Questions to ask any contractor

• Will you test for heat exchanger leaks, and how?
• Will you measure CO in the flue gas, and will you check combustion air and venting under worst-case conditions?
• What will you give me in writing afterward?
• If you find a cracked heat exchanger or a venting fault, what are my options for repair or replacement, and what permits apply?

A contractor who answers these without hesitation is one worth keeping.

A seasonal routine that keeps CO risk low

Most of what protects you is simple and repeatable. Our seasonal HVAC preparation checklist covers the broader picture. For CO specifically:

Early fall, before the first cold week

• Book the annual furnace and boiler inspection.
• Have the chimney or flue inspected and cleaned if needed.
• Test every CO alarm and replace batteries or expired units.
• Clear storage away from the furnace and water heater.

All winter

• Keep outdoor intake and exhaust vents clear of snow, ice and debris after each storm.
• Never run a portable generator, grill or fuel-powered heater indoors, in a garage or near a window. Public health experts link generator misuse to CO deaths during cold weather and after disasters.
• Do not use a gas oven or stovetop to heat the house.
• Replace filters on schedule. A clogged filter strains the system and can contribute to overheating, which is not a CO cause by itself but is a symptom worth fixing. If your system is running but blowing cold air, see our troubleshooting post.

Any time something changes

• After a new window, insulation or air sealing project, ask for a combustion safety test, since a tighter house can change how appliances vent.
• After a flood, fire or storm damage, have heating equipment checked before restarting it.
• If a pilot light keeps going out, do not just keep relighting it. Read why it happens in our pilot light guide and have it diagnosed.

Our perspective: why we push annual inspections over gadgets

We work on furnaces, boilers, heat pumps and commercial rooftop equipment across New Jersey, and the pattern we see is consistent: the households that get hurt are rarely the ones that skipped a gadget. They are the ones that skipped a look at the equipment for years.

Alarms are essential, and we want every home to have them. But an alarm tells you something has already gone wrong, and it was never built to catch slow, low-level leaks. A technician can find a developing crack, a loose vent joint or a poor burner setting while the house is still safe. That is why we treat the inspection as the main event and the alarm as the safety net underneath it.

We also encourage people not to wait for a breakdown. A furnace can run well and still be venting poorly, because comfort and safety are different measurements. If your furnace is old, if you have had a repair that involved the burner or vent, or if anyone in the house has had unexplained headaches in winter, call us before the cold arrives. We offer 24/7 emergency service for when something cannot wait, but the best call is the one you make in October.

— John

Book a furnace or boiler safety check

If you want a trained set of eyes on your heating system before the season is in full swing, we can help. Brighton Air Corp provides furnace inspection and heating services for homes and businesses in New Jersey, along with furnace repair and boiler repair. We also serve Westfield and Summit from our Cranford base. If you suspect a problem right now, get everyone out and call 911 first; call us once the building has been cleared.

FAQ

How do I know if my furnace is producing carbon monoxide?

You cannot tell by smell or sight, because CO is colorless and odorless. The reliable signs are a CO alarm sounding, several people in the house getting headaches or nausea at once, and symptoms that fade when you are away from home. A technician can confirm it with a combustion analyzer and a heat exchanger test.

Should I put a carbon monoxide detector near my furnace?

Not right next to it. The EPA advises keeping alarms about 15 feet from fuel-burning appliances and out of furnace rooms, kitchens and garages to prevent false alarms and sensor damage. Place them in hallways outside bedrooms and on each level, including near the basement stairs.

How long does CO exposure take to cause poisoning?

It depends on the concentration. At high levels, symptoms can appear within minutes, while lower levels over hours or days can cause headaches and fatigue that look like the flu. Because there is no safe way to guess, leave the building and call 911 if an alarm sounds and anyone feels unwell.

Can a furnace leak CO even if my alarm is silent?

Yes. Alarms are designed to warn of acute, high-level exposure rather than every trace of the gas, according to CPSC staff. A slow leak from a cracked heat exchanger can go unnoticed, which is why an annual professional inspection matters even with working alarms.

Are boilers as risky as furnaces?

They burn the same fuels and can produce CO through the same failures, such as blocked flues, poor combustion and loose vent connections. The difference is that the gas collects in the boiler room and basement rather than moving through ducts. Annual service and an unobstructed vent are the main protections.

Sources

• CDC: Furnace Safety Fact Sheet
• NFPA: Carbon monoxide safety
• NFPA: Keeping Your Community Safe CO toolkit
• ICC: Recommended procedure for safety inspection of an existing appliance installation
• CDC: Carbon Monoxide Poisoning in Michigan prevention guidance
• OSHA: Indoor Air Quality in Commercial and Institutional Buildings
• EPA: Carbon Monoxide Poisoning fact sheet
• CPSC: Carbon Monoxide Alarms standards
• CPSC: CO Alarms
• U.S. Department of Energy: Combustion Safety Checklist
• New Jersey Poison Control Center: Carbon Monoxide
• Mahwah Township: Carbon Monoxide and Smoke Detector Regulations

Recommended

• Furnace Carbon Monoxide Risk: 4 Signs and Tests Homeowners Can Request
• Why the Pilot Light Keeps Going Out (and How to Fix It)
• Furnace Blowing Cold Air? Causes and Fixes