Vent-free gas fireplaces: the safeguards, the limits and the trade-offs

First: if an alarm sounds or someone feels unwell

If a carbon monoxide alarm sounds, or anyone in the room has a headache, dizziness or nausea while the unit runs, turn it off if you can do so on the way out, get everyone outside into fresh air, and call 911 from outside. The manual for one vent-free model warns that early carbon monoxide poisoning resembles the flu, with exactly those symptoms, and that pregnant women, people with heart or lung disease and people who are anemic can be more affected than others.

If you smell gas, do not light anything, do not touch electrical switches and do not use a phone inside the house. Leave, and once you are outside, call the gas utility or 911. Keep the unit off until a technician has checked it.

What vent-free means

Nothing connects a vent-free unit to the outdoors: no chimney, no pipe. It burns in the room, and everything it produces stays in the room: heat, carbon dioxide, water vapor and, if the flame is not burning cleanly, carbon monoxide. Units of this kind are tested under ANSI Z21.11.2 as unvented room heaters, and the Florida Building Code, Fuel Gas volume, allows them under the conditions in its section 621.

Ventless gas fireplace safety rests on those conditions and on the unit's own safety system; together they separate a safe installation from an unsafe one. A unit that meets them, kept clean and serviced, is operating the way its listing intends. A unit that is oversized for its room, installed where it is not allowed, or dirty and poorly adjusted is not.

The oxygen depletion sensor

Every unvented room heater must have an oxygen-depletion safety shutoff under section 621.6. It must shut off the main burner's gas and the pilot's alike once room oxygen drops to the maker's trip point, a point the code does not let fall below a set floor. The code also forbids any field adjustment of that set point.

In practice the sensing is done by the pilot itself. Its flame is aimed to keep a thermocouple hot, and when oxygen runs low the flame lifts off it, the thermocouple cools and the gas valve closes. It is a sound design, but it responds to oxygen, not directly to carbon monoxide, and it relies on a clean pilot. Dust and lint at the pilot can shut the unit down early, and a damaged or tampered pilot takes the unit out of service. A carbon monoxide alarm covers what the sensor cannot.

The limits in the Florida Building Code

Section 621 of the Fuel Gas volume sets the outer bounds for any unvented room heater:

• Input no greater than 40,000 Btu per hour (621.3).
• A combined unvented input of 20 Btu per hour or less per cubic foot of the room or space (621.5).
• Not relied on as a home's only comfort heat (621.2).
• Not installed in a factory-built fireplace unless that fireplace is tested and listed for it (621.7).
• Kept out of bedrooms and bathrooms, apart from one wall-mounted unit with oxygen depletion protection, rated up to 10,000 Btu per hour for a bedroom and 6,000 for a bathroom (303.3).

Sizing the room: doing the arithmetic

The 20-Btu rule is easy to check. Multiply the room's length, width and ceiling height in feet to get cubic feet, then multiply by 20 for the largest total unvented input the space may have. A room 20 feet by 16 feet with an 8-foot ceiling is 2,560 cubic feet, which allows up to 51,200 Btu per hour of unvented input under the code, though the single-unit cap of 40,000 still applies.

The manuals can be stricter. One maker's worksheet counts the room plus any neighboring room joined to it by an opening with no door or by ventilation grilles, then adds up every fuel-burning appliance in that space, such as a gas water heater or furnace, leaving out only direct-vent appliances. If the total is more than the space can support, the manual calls for more fresh air, a smaller heater, or opening the space to an adjoining room. It also requires extra outdoor air in a house built to its definition of unusually tight construction.

Moisture, odor and soot

Burning natural gas or propane produces water. By that manual's figure, an unvented heater gives off close to an ounce of water each hour for every 1,000 Btu of input, and mildew is the risk it names when a room lacks ventilation air. Water beading on windows or walls while the unit runs is a sign the room is getting more moisture than it can shed.

Odor and soot are signs too. Manuals mention a smell during the first hours of use as manufacturing residue cures off, which should pass. Soot is different: at least one maker states that a properly installed and maintained set does not deposit soot on its logs, and that soot on a log means calling for service and stopping use. Candles burned in the room and fans blowing on the flame are named causes as well.

Keeping a vent-free unit within its limits

Vent-free manuals want a qualified service person to inspect the unit before its first use and yearly after that, and they ask for extra cleaning in rooms where carpet or bedding sheds lint. Keep the logs exactly where the manual's photos place them, and keep furniture, fabric and anything flammable well back.

Under Florida Statutes 553.885, each room used for sleeping in a building constructed since July 1, 2008 with a fireplace or fuel-burning appliance needs an approved carbon monoxide alarm no more than 10 feet away. Whatever the age of the house, an alarm near the room with the vent-free unit and near the bedrooms is a sensible precaution. If the room turns out too small for the unit, the answer is a smaller unit, more air, or a vented appliance, not a disabled sensor.