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Why Do Ventless Gas Logs Smell?
A discussion about the odor produced by burning vent free gas logs.

December 14, 2009
By: Greg Tillotson
Gas Log Expert & Webmaster for Hansen Wholesale


Similar Pages: Gas Log FAQs page! Vented vs VentFree Gas Logs

We have so many visitors reach our site that already own ventless gas logs and are asking why they produce a smell, how to get rid of the smell or other similar questions. Not only is this page meant to answer that question, but to provide valuable insight to those who are considering purchasing vent free gas logs now or in the future.

Keep in mind that this discussion DOES NOT apply to Vented Gas Logs. Vented gas logs are burned in wood burning fireplaces with the damper fully open, so the smell that is produced goes up the fllue. So unless you have a poorly drafting fireplace, you will not have the odor problem described below if you install vented gas logs.

So...on with the details!

For the sake of this discussion, I will be referring to Natural Gas. Although Liquid Propane and the additives used in its production have a different chemical makeup than Natural Gas, the basic premise below applies to both gases.

Ventless logs do in fact produce an unmistakeable odor. There is no way of preventing this, it is simply the nature of burning natural gas in your home without exhausting the fumes up a chimney. In a perfect world, the natural byproduct of burning "pure" natural gas is water vapor and carbon dioxide, and many people believe that burning natural gas in your home should produce no odor...so they think there is something awry with their ventless logs when they produce a smell.

The chemical formula that describes the combustion of Methane Gas (natural gas in it's purest form) shows how Methane gas is made up of Carbon and Hydrogen molecules. When burned, the Carbon combines with Oxygen producing carbon dioxide and the Hydrogen combines with Oxygen to produce Water Vapor. Both of which are odorless non-toxic gases. If the gas does not burn completely, then some of the carbon molecules remain as carbon (soot) and some of them combine with a single oxygen molecule and produce Carbon Monoxide (which is still an odorless gas). The final byproduct of the reaction is the production of heat (-890 kJ/mol). Here's the formula:

CH4(g) + 2O2(g) → CO2(g) + 2H2O(l) -890 kJ/mol

Side note: If you see soot accumulating on your logs, then you can assume that carbon monoxide is also being produced.

Unfortunately, we do not live in a perfect world and the natural gas that is supplied to your home is not perfect. It contains small amounts of impurities and additives, one of which is the rotten egg smell which is added so that you can smell a gas leak if it occurs (again, natural gas in its raw state is odorless). The most common rotten smelling additive is Methyl Mercaptan. The byproduct of burning Mercaptan is Sulfur Dioxide, which smells more like burnt matches mixed with rotten eggs. Include other odors produced by impurities in the gas supply and you have the distinct smell ventless logs produce.

Furthermore, Sulfur dioxide is an irritant to the eyes and respiratory tract in a more concentrated amounts. Although, gas suppliers claim that the amount of Mercaptan added to natural gas is so minute that it poses no health risk, many people do experience discomfort when burning ventless logs...particularly eye irritation or coughing spells.

What can you do if you have vent free logs and simply cannot live with the odor?

  1. Do not burn your ventless gas logs...or
  2. Open the damper in your fireplace (if you have a fireplace with a chimney...and/or
  3. Open some windows (and let fresh cold air in while you are trying to warm your house)...and/or
  4. Replace your ventless log set with a vented one designed to be burned with the damper fully open.

Bottom Line: We highly recommend that you purchase vented gas logs. You will be much happier with flame that they produce and less likely to experience discomfort from odors or irritation. We only recommend ventless logs when maximizing the heat output outweighs all of the other negative aspects.

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