Does a Gas Fireplace Need a Chimney Sweep?

Which gas fireplaces use the chimney

Gas fireplaces come in several forms, and the answer depends on which one you have. Vented gas logs sit in an ordinary masonry fireplace and send their exhaust up the existing flue with the damper open. Gas inserts slide into a masonry firebox and connect to a liner run up the chimney. Some direct vent units use the old chimney as a chase for their intake and exhaust liners.

Other units do not use a chimney at all. A direct vent fireplace on an outside wall vents through its own short pipe, which still needs its termination checked, and vent-free logs have no vent by design. If you are unsure which type you have, the rating plate, usually behind a lower grille or on the burner pan, and the owner's manual will say.

What actually goes wrong in a gas flue

The flame is clean, but the chimney it vents through is still a chimney exposed to weather and wildlife. In the brick housing common from Queens to South Jersey, these are the problems a technician finds, roughly from most to least frequent:

• Nests from birds and squirrels, and leaves blown in, on flues with no cap or a damaged one
• Clay tile and mortar flaking from years of condensation, piling up at the bottom of the flue
• Old creosote left from the years the fireplace burned wood, not cleaned before the logs went in
• Soot from logs knocked out of position or a burner that needs adjustment
• Rust on the damper, its clamp, the cap or a metal liner
• A cap screen clogged with debris, which chokes the draft

Why gas is hard on an old flue

Burning natural gas or propane produces carbon dioxide and a surprising amount of water vapor. In a warm, correctly sized flue, that vapor leaves as part of the exhaust. In a cold or oversized flue, it cools on the way up, condenses on the walls and runs down them.

Many masonry fireplace flues were built for wood fires and are much larger than a set of gas logs needs. The modest heat from the logs rarely warms that big flue properly, so condensation is common, especially in chimneys on outside walls. The water carries mild acids that soften mortar joints and flake the face of clay tile. The damage is quiet and slow, which is why it tends to be found only when someone looks.

Creosote versus soot

Burning gas does not produce creosote. If you find creosote in a gas fireplace, it almost always dates from when the fireplace burned wood. That old deposit is still flammable and gives off a smoky smell in humid weather, so a flue converted from wood should be cleaned before gas logs are installed, and checked if no one is sure it was.

Black soot is a different matter. Soot on the logs, the glass or the firebox means the flame is not burning cleanly. Often the logs were moved out of their set positions during cleaning and now interrupt the flame, or the burner needs service. Put the logs back exactly as the log manufacturer's diagram shows, and have the gas side looked at by a qualified technician if the soot returns.

A white or cloudy film on the glass of a sealed gas unit is something else again. It is a mineral residue from combustion that builds up over a season, and it should be removed with a cleaner the fireplace maker approves, with the unit off and cool, so the glass is not etched.

The damper question

Vented gas logs need the damper open whenever they burn. Many log manufacturers' instructions call for the damper to be fixed open with a clamp or a stop so it cannot be shut by mistake, because a closed damper with vented logs sends exhaust into the room.

Inserts and direct vent units follow their own manuals; in those installations the damper has usually been taken out or locked open to make room for the liner. Whatever you have, do what the appliance's installation manual says, and if you do not know which type of logs are in the firebox, find out before lighting them.

Signs you can spot at home

Some warnings are visible from the room without any tools:

• Soot on the logs, the glass or the top of the firebox opening
• A tall, lazy, yellow flame where it used to be steady
• Bits of tile, mortar, twigs or rust flakes on the logs or the burner pan
• A musty or smoky smell from the fireplace when it is off
• Water at the base of the firebox after rain
• A CO alarm that chirps an alert or sounds while the fireplace runs

When a CO alarm or gas smell is involved

A blocked gas flue can spill exhaust, including carbon monoxide, into the house without much smell. Keep working CO alarms on every floor and near sleeping areas. If one goes off, everyone leaves, and the 911 call is made from outdoors.

A smell of gas is a separate emergency. Do not light anything or switch anything on; leave the house and call the gas utility or 911 from outside. The chimney visit comes after the utility has made the line safe.

Timing the yearly check

Late summer and early fall work well for a gas fireplace. Spring and summer are when birds and squirrels move into uncapped flues, so a check after nesting season catches whatever arrived, and it leaves time for repairs before the first cold evening. Chimney swifts, which nest inside open masonry flues in this region during the warm months, are a protected species, so an active swift nest is left alone until the birds leave in the fall.

Weather is the other reason to look before the season. Winter freeze-thaw cycles crack crowns and loosen the top courses of brick, and a nor'easter can bend a cap or lift flashing. On a gas flue those problems let water in to join the condensation already working on the tile. Catching them in the fall means a small repair rather than a wet, crumbling flue by spring.

What a gas fireplace visit includes

The technician checks the top of the flue, the cap and its screen, the liner or tile over its full height, the damper and its clamp, and the base of the chimney, and clears any nests, debris or fallen tile. If old wood creosote is still in the flue, it gets cleaned out. If the tile is eroding or the flue is far oversized, relining with a liner sized to the logs or insert is explained as an option.

Work on the gas valve, pilot and burner belongs to a licensed gas technician, and many homeowners schedule both visits close together before the season starts.