Blue, yellow, lazy or sooty: what a gas fireplace flame is telling you

Why a gas flame is blue in one place and yellow in another

A gas burner mixes gas with air before it burns. Air drawn into the burner through an inlet or air shutter, ahead of the flame, is called primary air. Where the mix has plenty of it, the gas burns fully and quickly, and the flame is blue and fairly small. Where there is less, some of the carbon in the gas forms tiny particles that glow yellow as they pass through the flame.

Decorative fireplaces and log sets are tuned on purpose. A blue flame looks clean but does not look like a wood fire, so burners for log sets are designed to produce yellow flames that rise off a blue base. The color you see is a balance the maker set, and changes from that balance are what tell you something has shifted.

Flames that are working as designed

A thin band of blue right at the burner ports is what a healthy gas burner shows. Above it, the picture depends on the appliance. Vented log sets are meant to flame yellow, and one maker's manual says so outright; their tall flames can climb around the logs much as a wood fire's would.

Vent-free sets look different by design. Their burners keep the flame away from the logs, so the flames are smaller, bluer and tipped with yellow. The pilot has its own look as well: one vent-free maker calls for a natural gas pilot flame that is blue and gently curved, showing no yellow or orange. Brief orange flecks that come and go in the main flame can be dust in the room air passing through the fire, and they are not a fault by themselves.

Too yellow, too tall or rolling over the logs

A flame that turns deep yellow or orange, grows taller than before or starts wrapping logs it did not touch has lost part of its air or gained too much gas. Check the logs first, because their position changes where the flame goes. Manuals warn that flame contact on a log leaves soot, and one tells owners to reposition any log that yellow flame touches.

The burner itself can be the cause. One log set maker notes that the flow of primary air into the burner tube must not be reduced, and that dirt, lint and other buildup around the air inlet, or a gas orifice that is out of line with it, will reduce it. A gas fireplace flame too high for its setting can also mean the gas pressure or input is off, and manuals give an allowed inlet pressure range for each gas. Pressure and air shutter settings are service work, not owner adjustments.

The pale, drifting flame of a starved burner

On a sealed direct-vent fireplace, watch for a flame that turns pale, blue, slow-moving and almost transparent, drifting instead of standing up. One maker's troubleshooting chart lists exactly that flame as the symptom of a blockage in the vent. When exhaust cannot leave properly, it mixes back into the air coming in, and the flame is starved of oxygen.

A unit in that state can shut itself off after a while. Turn it off and have the vent checked from the appliance collar to the termination cap outside. Look at the cap yourself from the yard: nests, plants, a new deck or anything piled against it belong in the notes for the service call.

Soot on the logs and the glass

Gas fireplace soot is unburned carbon, and it means the flame is not burning cleanly. One log set manual calls a continued build-up of black carbon on the logs an indication of incomplete combustion. A vent-free set maker goes further, stating that properly installed and maintained sets do not deposit soot on their logs, and that a sooted log is the cue to shut the set off and call for service.

Soot on gas fireplace glass has the same meaning. It is different from the white or gray film that forms on glass with normal use, which one maker attributes to sulfur from the fuel and which a gas fireplace glass cleaner removes. White film is upkeep; black soot is a symptom. Candles lit nearby and fans or return grilles blowing across the fireplace are named causes of soot as well.

What you can check yourself, and what to leave alone

With the fireplace on high, compare what you see with the photos in the manual. Are any logs touching flame they should not, or sitting out of their pins? Is a ceiling fan, an open door or a floor register pushing air across the fireplace? One vent-free manual suggests reversing a ceiling fan when it pushes the flame onto the logs.

If a log needs to go back into position, turn the unit off and let it cool, or use heat-resistant gloves, as one manual advises, since logs stay hot for some time after use. Do not add extra logs, pine cones or decorative material to the burner; manuals warn that added items cause sooting. Leave the air shutter, the gas pressure and the burner alone; those settings belong to a service technician.

When to stop and call

Switch the unit off and call for service if you see any of the signs below. When a carbon monoxide alarm goes off, get everyone out before anything else and phone 911 from outdoors; for a gas smell, get out first and make that call from outside, to the gas company or 911, without lighting or switching anything on the way.

• Soot on the logs or glass, or dark marks on the wall above the opening
• A lazy, pale blue flame on a direct-vent unit, or a unit that keeps shutting off
• Flames rolling out of the opening or reaching toward the screen instead of curling back into the firebox
• A yellow or lifting pilot flame, or one that no longer wraps its sensor
• An exhaust smell, or headaches while the fireplace runs