Yes, a blocked, cracked or poorly drafting chimney or vent can let carbon monoxide into the living space, and you cannot see or smell it. If a carbon monoxide alarm sounds, leave the house and call 911 from outside. After the emergency, a $49 chimney inspection looks for the cause before anyone uses the fireplace or appliance again.
If an alarm is sounding right now
Do not try to diagnose it from inside the house. Carbon monoxide is a colorless, odorless gas, and your senses will not tell you how much is in the air.
- Get everyone out, including pets, and stay out.
- Call 911 from outside and tell them a carbon monoxide alarm is sounding.
- Do not go back in to open windows, silence the alarm or look at the chimney.
- Tell the responders which fuel burning equipment was running: fireplace, wood stove, furnace, boiler, water heater or a gas log set.
- Do not relight or restart anything until a qualified professional has evaluated it.
If anyone has a headache, dizziness, nausea or confusion, say so when you call and get medical attention. Those symptoms are common and are not a reason to wait and see if they pass. This page is general information, not medical advice.
Signs inside the house that point toward venting trouble
Not every case comes with an alarm. These clues deserve attention, especially when more than one shows up together.
- Smoke or a sharp, burnt smell in the room when a fire is lit.
- Soot stains on the wall or ceiling above a fireplace, stove or furnace.
- Stuffy air or condensation on windows when fuel burning equipment runs.
- A pilot light that keeps blowing out, or a gas flame that burns yellow or orange instead of blue.
- Headaches or feeling unwell that improve when you leave the house and return when you come back.
None of these proves carbon monoxide is present, and a clean-looking room does not prove it is absent. Treat them as a reason to get a detector working and a professional looking at the system. The how-to on smoke backing into the house covers the most visible version of this problem.
Signs on the chimney and the appliance
Some warnings are visible at the equipment itself. A technician checks these, but you may spot them first.
On the fireplace or stove
- Soot streaks around the opening or on the damper.
- A damper that will not open all the way.
- Rust or white staining at the connector pipe from stove to flue.
On the chimney outside
- A missing, crushed or crooked cap.
- Heavy bird or squirrel activity, which can mean a nest in the flue.
- Cracked or loose masonry near the top.
Do not climb onto the roof to look. Binoculars from the ground are enough to note that something looks wrong, and a technician can get close safely.
How a chimney ends up leaking exhaust into the house
Combustion gases are supposed to rise and leave. Anything that interrupts that path can push them back into the room. The common causes look like this.
- Blockage. A nest, fallen debris, a collapsed liner tile or heavy creosote can narrow or close the flue.
- A cold flue. Cold outside air sits in an unused chimney and resists the first warm exhaust, which can stall draft at startup.
- Competing exhaust. Kitchen hoods, bathroom fans and dryers pull air out of a tight house, and the chimney can become the easiest way for air to flow in.
- Cracks and gaps. A damaged liner or smoke chamber can let gases escape into walls and cavities.
- Appliance problems. A furnace, boiler or water heater with incomplete combustion produces more carbon monoxide to begin with.
- A closed or stuck damper while a fire or appliance is running.
Modern, tightly sealed homes make the pressure issues worse, and older homes can hide cracked flues behind plaster. Both are common around Boston, where homes range from drafty antiques to well-sealed renovations.
Alarms: where they go and how to keep them working
A detector is your only reliable warning. Follow the manufacturer's instructions for placement and replacement, because designs and lifespans differ. As general practice, homeowners put alarms near sleeping areas and on each level, and test them regularly.
- Test the alarm on the schedule in its manual.
- Replace batteries as directed, and replace the whole unit when its end-of-life signal or date arrives.
- Never remove or cover an alarm because it keeps chirping. Find out why.
- Keep detectors away from direct drafts and cooking steam, as the instructions advise.
Massachusetts has rules about carbon monoxide alarms in homes, and they can change. Confirm the current requirement for your property with your local fire department or the state's official website rather than relying on a blog post. Landlords, condo owners and buyers should ask the fire department what applies to them.
What happens at the inspection, and what it costs
Once the emergency is over and the source is not in active use, the next step is a chimney inspection. It is $49, credited toward approved repair work, and the technician shows you photos of what they find and gives you a written price before any work begins.
An inspection of this kind looks at the whole path: the appliance connection, the damper, the smoke chamber, the flue liner, the cap and the chimney top, plus signs of blockage or backdraft. Related work you may be quoted, using Boston prices:
- Cleaning with inspection to clear deposits: $299.
- Animal and nest removal: $299 to $900.
- Damper repair or replacement: $199 to $1,200.
- Liner repair: $1,200 to $2,500.
Anything beyond those figures is quoted in writing after the inspection. The inspection cost page explains what the $49 covers, and the chimney repair page covers the fixes.
Help the technician by writing down what you noticed before the visit: when the alarm sounded, which rooms were affected, what equipment was running, whether a storm or strong wind was blowing, and whether anyone had symptoms. Patterns such as only on windy nights, only when the kitchen fan runs, or only in the first ten minutes of a fire point toward specific causes and save time on diagnosis.
One caution: a chimney inspection does not test your furnace, boiler or water heater combustion. If the alarm came from one of those, have a qualified heating or gas professional examine it too.
Habits that keep exhaust where it belongs
Most carbon monoxide trouble from a chimney is preventable with ordinary upkeep.
- Have the chimney inspected once a year, and cleaned when the inspection calls for it.
- Keep a cap on every flue so nests and debris cannot get in.
- Open the damper fully before lighting a fire, and leave it open until the ashes are cold.
- Burn dry, seasoned wood to reduce deposits.
- Crack a window when running a strong exhaust fan near a lit fireplace.
- Do not use a gas oven, grill or generator indoors for heat.
Set a reminder for your alarm tests at the same time you schedule your yearly chimney visit. If you want a professional to look at the venting, book the $49 inspection at our contact page or call (617) 404-9587.
Questions homeowners ask
Can I smell carbon monoxide coming from my fireplace?
No. The gas has no smell, so a smoky or burnt odor is a separate warning that exhaust is entering the room. Both deserve attention, but only a working detector can warn you about carbon monoxide itself.
My alarm went off once and then stopped. Do I still need to act?
Yes. Conditions can change as doors open or equipment cycles off. Leave, call 911 if it sounded, and do not run the suspect equipment until it has been evaluated by a qualified professional.
Can a clean chimney still leak carbon monoxide?
It can. Cleaning removes deposits, but a cracked liner, a blocked cap, a closed damper or a pressure problem in the house can still send gases the wrong way. That is why a full inspection looks beyond soot.
Is it safe to use my fireplace after an alarm if the alarm stays quiet?
Not until a qualified professional has checked it. A quiet alarm after you stop burning only tells you the gas has cleared, not that the cause is fixed.
Does a gas fireplace produce carbon monoxide?
Any fuel burning appliance can, especially when it vents poorly or burns incompletely. Gas fireplaces need their venting, burner and logs checked regularly, and the repair range for gas units is $299 to $900.