How a fireplace can put carbon monoxide in the house

When the alarm sounds, go outside first

A carbon monoxide alarm going off is not a moment for troubleshooting. Get everyone out of the house, including pets, and call 911 once you are in fresh air. The U.S. Consumer Product Safety Commission gives the same order: get outside to fresh air immediately, then call 911. Do not go back in to open windows, find the source or turn things off.

Tell the dispatcher which fuel-burning appliances are in the house, including the fireplace, a gas insert or log set, a furnace or water heater, and any generator. Once the responders have cleared the house, leave the fireplace and any suspect appliance off until someone qualified has checked it. If a gas appliance may be involved, the gas utility can also be called to check it.

Why carbon monoxide is easy to miss

Carbon monoxide is a gas with no color and no smell, produced when wood, gas or any other fuel burns without enough oxygen to finish the job. You cannot see it in a smoky room and you cannot smell it separately from the smoke. That is why alarms matter more than instinct.

The symptoms the CPSC lists are headache, dizziness, weakness, nausea, vomiting, sleepiness and confusion, which are easy to mistake for a cold or the flu. A useful clue is when several people in the house feel unwell at once, or when the symptoms ease after time outdoors and return inside. Sleeping people can be overcome without noticing any of it, which is where an alarm near the bedrooms earns its place.

How a wood fireplace spills it

A wood fire makes carbon monoxide for as long as it burns, and the chimney is what carries it away. Trouble starts when the exhaust cannot leave or is pulled back into the room. The clearest example is closing the damper while embers are still glowing. The fire looks finished, but the coals keep smoldering and producing gas, and with the damper shut it has nowhere to go but the room.

Other causes are a flue blocked by a nest, debris or heavy creosote; a cold flue that will not draft; and a house pulled into negative pressure by a kitchen hood, bath fans or a clothes dryer, which can draw exhaust back down the chimney. The Florida Building Code addresses the last one by asking for an outside air supply to a fireplace unless the room is mechanically kept at neutral or positive pressure. A wood insert with a liner that has separated from its collar can also leak exhaust into the smoke chamber and out into the room.

How a gas fireplace spills it

Gas burns cleaner than wood, but a gas fireplace can still produce carbon monoxide when the burner is starved of air or the flame is disturbed, and it can still spill exhaust. The risks depend on the type. A direct-vent unit is sealed from the room behind its glass, so a cracked panel, a failed gasket or a separated vent pipe is what lets exhaust in.

A B-vent unit draws room air and relies on the vent to draft, so a blocked termination or a depressurized house can make it spill. A vented log set in a wood fireplace needs its damper held open, and a damper that has been closed under it sends exhaust straight into the room.

Flame quality is a warning sign on any of them. Logs knocked out of their diagrammed positions push the flame into the logs, which makes soot and more carbon monoxide. A lazy, tall, yellow flame that leaves black deposits on the glass or the logs needs service. A vent-free unit puts all of its combustion products into the room by design; its manual sets the minimum room size, and its oxygen depletion sensor shuts it off if the room's oxygen falls. Those limits are part of the listing, not suggestions.

Where carbon monoxide alarms belong

Florida Statutes section 553.885 requires an operational carbon monoxide alarm no farther than 10 feet from each sleeping room in buildings permitted for new construction on or after July 1, 2008 that have a fireplace, a fossil-fuel-burning heater or appliance, or an attached garage. The statute accepts battery-powered, hard-wired or combination smoke and carbon monoxide alarms.

The CPSC advice goes beyond the statute and applies to any house: alarms with battery power or battery backup on every level of the home and outside sleeping areas. The U.S. Fire Administration and NFPA advise testing them at least once a month.

Every alarm has a service life set by its maker, stated on the unit or in its instructions, and it is replaced when that life runs out even if it still beeps when tested. A carbon monoxide detector in the fireplace room adds a warning close to the source. Placement details, such as distance from the appliance and height on the wall, are in the alarm's own instructions.

What a chimney visit checks for carbon monoxide

On a wood-burning fireplace, we check that the flue is clear from the smoke chamber to the cap, that the damper opens fully and stays open, and that the chimney drafts with the house operating the way it does day to day, including the kitchen hood and bath fans. On an insert, we check the liner connection at the collar and the top.

On a gas unit, we follow the vent from the appliance to the termination and check the glass and gasket on a direct-vent unit, the burner and log placement, and, where the design has a draft hood or relief opening, whether exhaust spills there at startup. Gas connections are tested for leaks with a leak-detection solution or instrument rather than an open flame, as NFPA 54 requires. Anything we find goes on the written estimate as a safety item, with the fireplace kept out of use until it is fixed if the finding calls for that.

Habits that lower the risk

Between visits, prevention lives in how the fireplace is used.

• Close the damper only once the fire is out and the ashes have gone cold
• Keep wood fires hot and the wood seasoned, since a smoldering fire is short of air
• Avoid running a strong kitchen hood or several exhaust fans while a wood fire is burning, or crack a window near the fireplace if you do
• Keep the vent termination of a gas unit clear of plants, debris and anything stored against the wall
• Stop using a gas unit that leaves soot on the glass or burns with a tall, yellow, wandering flame
• Have the chimney and any gas appliance inspected once a year, which is what NFPA 211 and the appliance manuals call for
• Test the alarms monthly and replace them by the date their maker sets