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What we’ve learned…..

Vent-free /Unvented gas logs usually require the damper to be closed.  Some folks opt to just seal the throat of their fireplaces or the tops of their chimneys when these gas log systems are installed.  Vent-free or unvented gas log appliances are somewhat controversial.  There are some issues that have become evident in the last few years.

Vent Free Unvented Gas Logs - Spartansburg SC - Blue Sky Chimney Sweep


There are several scenarios that can cause the system to emit low levels of carbon monoxide.

  • Animal dander, pet hair, or dust on the burner
  • flame impingement from the logs being positioned incorrectly
  • air movement across burner

Any of these circumstances can cause the logs to burn at less than 100% efficiency and emit carbon monoxide.  Since these systems are sometimes categorized as “room-vented” appliances, the issues they raise can cause concern.

  • These low levels of carbon monoxide emission usually will not trigger your carbon monoxide detector, but are considered a health hazard.
  • The instructions on the logs say not to install in a bedroom or to use if you are pregnant, have emphysema, or other respiratory problems.

If they are not healthy under these listed circumstances, perhaps there is cause for concern across the board.


There is a concern about heat buildup in the firebox.  Fireplaces are designed to flush most heat up into the flue.  When logs are burned with the damper in the closed position or with the throat area completely sealed off, most of the heat is retained in the firebox.  This causes the walls of the firebox or the breast area of the fireplace to become hotter than normal.  Heat is then transferred through the firebox walls and into the wall cavity surrounding the fireplace.

If the builder did not observe proper clearances, or if framing lumber is in contact with the shell of the fireplace, pyrolization will occur.  When the wood repeatedly heats and cools as the logs are used, this causes the flash point or ignition temperature of the wood to lower.  Numerous home fires have been documented as starting in the walls, either next to or directly above the fireplace opening behind the mantle.


Many times the incentive to get vent free gas logs is to produce more heat for the living area especially in the event of a power outage.  Under these circumstances, the gas appliance may be operated for many hours without interruption.  This can be a problem because many manufacturers’ instructions stipulate a limited operation time.  Their specified time frame for operation insures that the levels of carbon monoxide that are emitted in the home are within the parameters considered nontoxic.  If those time frames are exceeded, then the levels of emissions in the home may exceed safety standards.

Because of these concerns, vent-free gas logs are against code in some states (including California, Colorado, Wisconsin, Massachusetts) and the entire country of Canada.  There are several websites that deal with carbon monoxide including and  

Our Recommendation

Our recommendation is to install vented or direct vent gas appliances. If vent free or unvented are chosen, we recommend a dual listed log set (one that can be operated with damper either open or closed) when installed in a wood burning fireplace/chimney.  If strictly vent-free/unvented are installed, we encourage operators to be familiar with manufacturer’s operating instructions and follow them strictly for safest operation and best results.  In all cases, we recommend that all gas appliances are regularly maintained and that carbon monoxide detectors are installed and checked regularly to make sure they are working.

Blue Sky Chimney Sweeps | Bless Your Hearth offers installation and service for gas logs, stoves, inserts and fireplaces.

What Does “Vent Free” Really Mean?

The latest fad in fireplaces, the vent free unit, has hit the market with a splash. Many people are raving over the high fuel efficiency and ultra low emissions. While vent free fireplace options do have a number of benefits, including these, they are not without their own challenges and setbacks. In fact, sometimes these units can pose serious danger to the health of the family in the home. Learn the truth about vent free gas sets.

Gas Fireplace - Spartanburg SC - Bless Your HearthVent free gas logs are specially designed to burn in a fireplace constructed for them. The design allows the fuel to burn at nearly 100 percent efficiently, which greatly decreases the amount of harmful emissions. Along with the low levels of emissions produced by the fire, a vent free gas log set also benefits the user by producing high levels of heat.

A number of safety features have been installed into vent free gas logs and fireplaces to ensure safe use in the home. The oxygen depletion sensor, or the ODS, measures the amount of oxygen in the room and shuts off the gas supply if the oxygen concentration ever falls below 18 percent in the room. Shutting off the gas supply then extinguishes the fire, allowing the oxygen levels to replenish to safer concentrations. In newer, more airtight homes, keeping up the oxygen level with this safety feature may require opening a window.

The safety pilot is another feature in which the gas supply is shut off if the flame ever goes out. First, this feature prevents highly flammable gas fuel from filling up the entire house, which creates a fire and explosion risk in the event of a spark. The safety pilot also saves the homeowner money by only allowing the flow of gas when a fire is lit.

While vent free gas log sets do have many advantages, they are not ideal for every home. They should never be operated for more than four hours, so homes needing long term heating from the fireplace should not rely on vent free options. Otherwise, the house will have low oxygen levels and excess humidity. For this reason, vent free sets should not be relied on in power outage situations either. Prior to vent free use, a new set must burn for three to five hours with an open vent to allow the set to properly cure. After this initial burn, the set can burn safely with the vent shut.

Choosing to invest in a vent free gas log set is a big decision. If you are unsure or if you have questions about the benefits and risks, get in touch with an expert to discuss your options. Blue Sky Chimney Sweeps, located in upstate South Carolina, can provide a professional consultation and guidance on making the best decision for your fireplace.