Yes, a boiler can produce carbon monoxide (CO). It does so when fuel burns incompletely or when exhaust gases can't leave the house the way they should. A well-adjusted, properly vented, regularly serviced boiler is a safe way to heat a home. A neglected one can quietly become a hazard, because CO has no color, no smell and no taste.
This guide covers how a boiler ends up making or releasing CO, the warning signs to watch for, what to do if an alarm goes off, and what a professional safety inspection looks at. We're a New Mexico plumbing and heating company, so we also add what we see in local homes.
TL;DR
- A boiler makes CO when combustion is incomplete or when venting fails. A boiler that is switched off and not burning fuel isn't producing it.
- Common culprits: blocked or disconnected flue pipes, corrosion, a cracked heat exchanger, and a burner that is out of adjustment.
- Put CO alarms on every level and outside sleeping areas. An alarm is your backup, not a replacement for maintenance.
- If an alarm sounds, get everyone outside to fresh air and call 911 before anything else.
- Have a licensed professional inspect your boiler every year, before the heating season starts.
Table of Contents
- How a boiler can produce carbon monoxide
- The usual causes of a boiler CO leak
- Warning signs you shouldn't ignore
- What to do if your CO alarm sounds
- Carbon monoxide alarms: where to put them and what they can't do
- What a boiler safety inspection covers
- Our perspective: treat your boiler like a team member that needs a check-up
- Have your boiler checked before the cold sets in
- Making your home safer between inspections
- FAQ
- Sources
How a boiler can produce carbon monoxide
CO is a toxic gas made when fuels such as natural gas, oil and propane burn without enough oxygen. The CDC lists gas- and oil-burning furnaces among the common household sources, and boilers belong in the same family of fuel-burning appliances.
A boiler burns fuel to heat water, and that water then travels through baseboards, radiators or in-floor tubing. Burning fuel always creates exhaust. In a healthy system, that exhaust is mostly harmless and leaves through a flue or vent. CO becomes a problem in two situations:
- The burner isn't burning cleanly. The fuel and air mixture is off, so more CO is created than normal.
- The exhaust isn't leaving the house. The vent is blocked, cracked, disconnected or backdrafting, so combustion gases spill into the room instead of going outside.
The second case matters most for homeowners. Even a boiler that burns cleanly can put people at risk if its vent fails.
Does a boiler make CO when it's off?
No. Combustion stops when the boiler stops firing, so there is nothing producing CO. The risk comes when the boiler is running, or when a vent problem lets leftover gases or a neighboring appliance's exhaust drift into the home. If you smell something or an alarm sounds while the boiler is off, other fuel-burning appliances such as a water heater, dryer, fireplace or gas range need checking too.
Does every heating system carry the risk?
Only systems that burn something. An electric boiler has no combustion and no CO. Steam or hot-water radiators fed by a gas or oil boiler are different: the radiators themselves can't leak CO, but the boiler feeding them can. The EPA lists fuel-burning appliances such as gas stoves, woodstoves and unvented space heaters as combustion sources, which is why a CO alarm makes sense in any home with fuel-burning equipment.
The usual causes of a boiler CO leak
The CPSC's guidance on home heating equipment points inspectors to the same short list of problems. These are the ones we see most often:
- Blocked vents or flues. Debris, a bird nest, a collapsed liner or a poorly sloped pipe can stop exhaust from escaping.
- Disconnected or loose flue pipes. Vibration, age or a past repair can leave a joint open so exhaust leaks into the boiler room.
- Corrosion. Condensation and age eat through flue pipes and boiler components.
- A cracked heat exchanger. This is the metal barrier between the flame and your system. A crack can allow combustion gases to escape where they shouldn't.
- A burner that needs adjustment. Dirt, wear or a bad air and fuel ratio produce incomplete combustion.
- Not enough air for combustion. A boiler tucked into a tightly sealed closet or utility room can run short of air. A home health report from the New Mexico Department of Health summarizing housing codes also lists proper venting and an adequate air supply to avoid backdrafting as basic requirements for combustion appliances.
One thing you can often see for yourself without touching anything: a persistent yellow-tipped flame on a gas burner is a classic sign of poor combustion, while a healthy burner shows a steady blue flame. If you spot yellow, don't try to adjust it. Call a licensed professional. Air and fuel balance and venting are not do-it-yourself work.
Why age and the desert both matter
In New Mexico, boilers often sit in garages, utility rooms and closets that also collect dust and the odd nesting pest. Dry air and dust don't cause CO by themselves, but they can clog burners and vents when nobody looks at the system for years. If your boiler is old, ask whether the repair costs and the safety questions point toward replacement. We cover that decision in when to replace your home boiler.
Warning signs you shouldn't ignore
CO itself gives no warning. What you can notice are the things around it.
Signs on the equipment
- Soot, scorch marks or stains around the boiler, vent or flue
- A yellow or orange flame instead of a steady blue one on a gas burner
- Rust or water streaks on the flue pipe or vent connections
- A pilot light that keeps going out
- Excess moisture or condensation on windows and walls near the boiler
- A vent pipe that is loose, sagging or visibly separated
Odd smells in the boiler area are worth a call too. If you've noticed stale or musty air near vents, what causes vent odors explains the common reasons, including blocked flues, damaged heat exchangers and faulty venting. Note that CO is odorless, so a smell never proves the air is safe or unsafe.
Signs in the people
The CDC describes CO symptoms as headache, dizziness, nausea, chest pain and confusion, and at high concentrations loss of consciousness or death within minutes. Those early symptoms look like the flu or allergies, which is what makes CO dangerous.
The pattern gives it away:
- Several people in the house feel unwell at the same time
- Pets seem sick too
- Everyone feels better when they leave the house and worse when they return
- Symptoms show up mostly in the heating season
If that sounds like your household, get outside into fresh air and seek medical care. Tell the doctor you suspect CO. Don't wait to see whether it passes.
What to do if your CO alarm sounds
Have this plan ready before you need it. The NFPA's CO guidance says that when an alarm sounds, everyone should get to fresh air immediately and call 911, and nobody should go back in until emergency personnel say the home is safe.
- Get everyone out. People, then pets if you can do so without delay. Don't stop to open windows or find the source.
- Call 911 from outside. Say you have a CO alarm and tell them about any symptoms.
- Stay outside. Wait for responders to clear the house.
- Don't restart the boiler. Even if the alarm stops, the cause hasn't gone away.
- Call a licensed heating professional. Have the boiler, vents and other fuel-burning appliances inspected and repaired before you use them again.
A silent alarm after you air the house out is not a green light. Find the cause first.
Carbon monoxide alarms: where to put them and what they can't do
Alarms are worth every dollar. Here is how to place them well, based on NFPA guidance:
- Install them on every level of the home and outside all sleeping areas.
- Choose alarms that meet the UL 2034 safety standard.
- Use battery-operated or battery-backup models so they work in a power outage.
- Consider interconnected alarms so that when one sounds, they all do.
The CPSC advises against mounting alarms in kitchens or directly above fuel-burning appliances because that invites nuisance alarms. A hallway near the bedrooms is better than the boiler room itself.
In New Mexico, the state Department of Health suggests testing alarm batteries when you change your clocks, and reports that in 2020 eight people died from CO poisoning in the state and 93 were treated in emergency rooms. Make it a twice-a-year habit.
Two limits to know
An alarm is not a gas leak detector. CO alarms sense the byproduct of incomplete combustion. They are separate devices from detectors that identify a natural gas leak. If you smell gas, leave and call your gas utility or 911 from outside.
An alarm is not maintenance. The CPSC describes alarms as a secondary layer of protection. An alarm tells you a failure has already happened. A yearly inspection is what prevents it.
What a boiler safety inspection covers
The CPSC recommends that fuel-burning heating equipment be inspected by a professional every year, noting that more than 200 people die each year from non-fire CO poisoning tied to consumer products, with heating equipment among the top contributors. The best time is early fall, before you need the heat.
A thorough visit looks at:
| Area | What the technician checks |
|---|---|
| Venting and flue | Blockages, disconnected sections, corrosion, correct slope and termination |
| Heat exchanger | Cracks, rust and signs of overheating |
| Burner and flame | Flame pattern, cleanliness, proper air and fuel mix |
| Combustion air | Enough fresh air reaching the boiler room |
| Controls and safety devices | Limit switches, relief valve, pressure and temperature |
| Surrounding appliances | Water heater, dryer and other fuel-burning equipment sharing the space |
Professionals also measure combustion gases with calibrated instruments. A model procedure in the fuel gas code's safety inspection appendix is used as a reference, and the benchmark commonly applied is that readings above 400 parts per million air-free are considered unsafe. That figure is for trained people with the right equipment. A handheld gadget from the hardware store can't replace it, and converting readings to an air-free basis takes training.
For a broader view of why vents, flues and heat exchangers matter in any home, see this explainer on the role of HVAC in property safety.
Our perspective: treat your boiler like a team member that needs a check-up
We've been working on New Mexico heating systems for over 38 years. Here is how we think about boiler safety.
Most of the homes we visit don't have a dramatic problem. They have a boiler that has run quietly for years with nobody looking at it. That is the thing about CO: nothing announces it. A system that is working hard every winter in Rio Rancho, Albuquerque or Corrales deserves the same attention you'd give your car before a long road trip.
We'd rather you call for a routine check in October than for an emergency in January. A routine visit is calmer, easier to schedule and usually cheaper than a rushed repair. And if we find something, we explain it plainly: what we saw, why it matters and what your options are. You'll talk to a local person, not an automated phone menu.
Two rules we repeat to every homeowner. First, don't adjust burners or poke at flue pipes yourself. Second, never treat a quiet alarm as proof that everything is fine. If a boiler is acting up, whether it's short cycling, leaving soot or just not heating, have a licensed professional look at it. Our guide on what to check when a boiler isn't heating covers the safe, simple checks a homeowner can do, and where to stop.
— Aaron Waller, General Manager
Have your boiler checked before the cold sets in
If your boiler hasn't been inspected this year, or you can't remember the last time, now is a good time. Waller's Plumbing and Heating installs and services boilers for homeowners in Rio Rancho, Albuquerque, Corrales and the surrounding area, using licensed professionals who leave your home clean. See everything we handle on our plumbing, heating and cooling services page, or visit our home page to book an appointment online.
If you suspect CO right now, don't book anything first. Get outside and call 911.
Making your home safer between inspections
A few habits cost almost nothing:
- Keep the area around the boiler clear. Don't store paint, cardboard or cleaning supplies against it.
- Look at the flue pipe now and then for sagging, rust or loose joints, without touching the burner.
- Never use a gas oven or range to heat the house, and never run a generator or grill indoors or in a garage. The EPA warns that deadly CO levels build quickly in enclosed spaces and can linger for hours.
- Test your alarms monthly and replace batteries twice a year.
- Check the replacement date printed on each alarm and swap it when it expires.
- Schedule your annual boiler visit at the same time each fall so you don't forget.
Plumbing and heating problems often travel together, so while a technician is on site it's a good moment to ask about the water heater too. Our plumbing inspection checklist for New Mexico homeowners shows what else is worth a look.
FAQ
Can carbon monoxide come from central heating?
Yes, if the central heating system burns fuel. Gas, oil and propane boilers and furnaces can release CO if they burn incompletely or vent poorly. Electric systems don't burn anything and don't make CO.
Are boilers safe to use?
A properly installed, vented and maintained boiler is a safe, common way to heat a home. The risk rises with age, neglect and blocked or damaged venting. An annual professional inspection and working CO alarms keep that risk low.
What are two warning signs of carbon monoxide?
The most common symptoms are headache and dizziness, often with nausea or fatigue. A telling pattern is several people feeling sick at once and improving when they leave the house. If that happens, go outside, call 911 and get medical care.
Does a carbon monoxide detector detect gas?
No. A CO alarm detects carbon monoxide from incomplete burning, not natural gas or propane leaks. If you smell gas, leave the home and call your gas company or 911 from outside.
Sources
- CDC: Carbon Monoxide Poisoning Basics
- CPSC: Home Heating Equipment
- CPSC: Carbon Monoxide Questions and Answers
- NFPA: Carbon Monoxide
- EPA: Sources of Combustion Products
- EPA: Carbon Monoxide's Impact on Indoor Air Quality
- ICC: Fuel Gas Code Appendix D, Safety Inspection of an Existing Appliance Installation
- New Mexico Department of Health: Housing codes analysis