Are Duraflame and Other Fire Logs Safe for a Chimney?

What a manufactured fire log is made of

A typical fire log is wood fiber, mostly sawdust, pressed into a log shape and bound with wax. The wax is the main fuel. It is what lets the log light from a single match through its paper wrapper, and it is why the flame looks tall, steady and bright next to a split-wood fire. Each log is designed to burn for a stated number of hours and then die out on its own.

A second family of products looks similar on the shelf: bricks or logs of sawdust compressed so hard that they hold together with little or no added wax. They burn more like dense firewood and are sold mainly for stoves. The two are not interchangeable, and the package tells you which kind you are holding.

The EPA's wood-burning guidance suggests that if you use manufactured logs, you choose ones made entirely of compressed sawdust. Whatever you buy, the wrapper or box is the manufacturer's instruction sheet for that product.

Open fireplace, stove or insert: which logs go where

Wax logs are generally designed for open fireplaces, masonry or factory-built, that burn with a screen in front. The same EPA guidance notes that many wax and sawdust logs are made for open hearth fireplaces only, and tells stove and insert owners to check their operating instructions before using any artificial log.

The reason is heat and air. Inside a closed stove, the wax melts and vaporizes faster than it would in an open firebox, the flame can climb into the baffle and flue collar, and the unit can run hotter than its design allows. With the door shut and the air turned down, a wax log can smolder and then flare in turns. If the stove manual is silent or says no, leave fire logs out of it.

A gas log set is a separate case. A fire log does not belong on a gas burner or on ceramic logs, whether the gas is on or off.

What the wrapper usually tells you

Read the wrapper whenever you buy a product you have not used before, even after years of burning others. Instructions differ between makers and even between sizes from the same maker. Points that commonly appear:

• Burn one log at a time, and let it burn out completely before lighting another
• Light the wrapper at the marked spots, with the damper fully open
• Do not poke, split or break the log while it burns
• Use it only in the type of fireplace the package lists
• Keep the screen closed, and follow the fireplace instructions on glass doors
• Do not cook over it unless the package says it is made for cooking

Misuse that turns a fire log into a problem

Stacking is the usual mistake. Two or three wax logs together release far more burning wax than one, and the flame can roll out of the opening or reach the damper and smoke chamber with real force. Tossing a fire log onto an established wood fire does the same thing, because the log starts melting from every side at once instead of burning down from the wrapper.

Breaking a burning log apart with a poker spills liquid wax onto the grate and hearth, where it flares. Using a log as a booster for damp firewood gives you the downsides of both fuels: the wax burns off quickly and the wet wood goes on smoldering. Closing the damper while a log still glows sends smoke and carbon monoxide into the room instead of up the flue.

A quieter misuse is burning logs back to back for an entire evening to heat a room. Each log is sized for a single, contained burn, and lighting the next one on a bed of hot wax residue makes it catch fast and burn hard. If the goal is heat rather than a few hours of flame, a properly installed stove or insert running on the fuel its manual names is the appliance built for that job.

Deposits in the flue from wax logs

Fire logs carry very little water, so they avoid the heavy, steamy smoke of wet firewood. They still make smoke and soot, and that residue settles on the flue walls like any combustion deposit. Burning only fire logs may keep a chimney cleaner than burning damp wood, but it does not keep it clean.

A sweep reads the color and texture of what is in the flue. Light, dry soot brushes out easily. Sticky or glazed creosote points to wood fires that smoldered, and a mix of both tells a story about how the fireplace has been used. In a flue that runs up a cold outside wall, as many do in older colonials and rowhouse end units, smoke cools faster and deposits form sooner.

Under NFPA 211, the national chimney standard, a chimney gets a yearly inspection whatever it burns, and a sweep whenever deposits accumulate. Switching to fire logs does not change that schedule.

Fireplaces that sat unused for years

A fire log is often the first thing lit in a fireplace that has been dormant: a brownstone parlor fireplace opened up during a renovation, a bedroom fireplace in a Philadelphia rowhouse, or a living room hearth a previous owner boarded over. The easy flame tempts people to treat the log as a test of the chimney. It is not a test, and a bad flue can fail quietly while the log burns normally.

Fireplaces like these are often unlined, or have clay tiles cracked by decades of freeze-thaw. Dampers rust shut or get wired open. Bird and squirrel nests fill flues that have had no warm draft in years. In attached houses, a chimney may sit in a party wall beside a neighbor's flue, and open mortar between them lets smoke travel where it should not. Simple checks from inside the room help you decide what to ask about:

• Open and close the damper and confirm it moves freely and stays open
• Shine a flashlight up past the damper for debris, nesting material or fallen tile
• Look over the firebox for missing brick or open mortar joints
• Find out whether a boiler or water heater shares the chimney

Around the hearth while a log burns

Keep rugs, cushions, books and decorations at least three feet from the fireplace, the distance the U.S. Fire Administration gives for keeping anything that burns away from heat sources. Leave the screen in place for the whole burn, since a wax log can spit small embers as the wrapper and wood fiber burn away.

Once the log has burned out and the ash is completely cold, scoop it into a metal container with a lid and set it outside on a surface that cannot burn, away from the house. Ash can hold heat for a long time after the flame is gone, so cold means cold to the touch, not merely dark.

A fire log that smokes into the room is telling you about the chimney, not the log. Check that the damper is fully open. If it is and smoke still curls out under the top of the opening, the flue may be blocked, undersized for the opening, or too cold to draft on a mild, damp evening. Let the log burn out with the screen closed, open a window near the fireplace, and have the chimney checked before the next fire.