Featuring America's Home Inspector: Nationally Syndicated Columnist, Barry Stone

Ventless gas fireplace safety

The House Detective by Barry Stone, Certified Home Inspector

Dear Barry: We are concerned about the safety of our ventless gas fireplace. We use it a lot, and a black film has recently appeared on the glass panel, as well as on our windows. What should we do? Barbara

Dear Barbara: The first think to do is stop using the fireplace and report the problem to a qualified fireplace specialist for evaluation and repair.

When a gas-burning fixture produces a black residue, that is a symptom of incomplete combustion and faulty exhaust venting. It means that combustion byproducts are venting into your home, and this is potentially dangerous, depending on whether these byproducts include carbon monoxide.

After your fireplace is professionally serviced, read the owners manual before resuming use. The manufacturer’s instructions may advise not using the firxture for periods of more than two hours. The manual may also recommend that a nearby window be kept open while the fireplace is in use to dilute exhaust with fresh air.

Ventless gas fireplaces are vigorously defended by their manufacturers as being incapable of abnormal combustion. In past articles, I’ve expressed the view that no manmade product is, or ever can be, 100 percent foolproof. Your situation appears to support that opinion.

Ongoing Concerns About Unvented Gas Fireplaces

The House Detective by Barry Stone, Certified Home Inspector

Dear Barry: We read one of your articles on ventless gas fireplaces and are concerned about the safety of the one in our home. We have a code-approved unvented gas log fireplace. It has a built in carbon monoxide detector and other safety back-up devices, as listed in the owners manual. Are there any known problems with this type of setup? Bill

Dear Bill: Unvented gas log fireplaces are actually misnamed. They should be called interior-vented fireplaces. Most gas-burning fixtures, whether furnaces, water heaters, or fireplaces, vent combustion exhaust to the exterior. “Unvented” fireplaces vent exhaust to the interior of the building. Fortunately, there have been very few reported problems with these fixtures. But conceptually, they are at odds with the common sense of gas safety, and their inherent safety should not be taken for granted.

When gas is burned, the primary byproducts are carbon dioxide and water vapor. If gas combustion is incomplete for any reason, soot and carbon monoxide are also produced. Carbon monoxide, commonly recognized as an odorless, lethal gas, causes no harm when vented to the exterior of the dwelling. When vented into a home, it can cause debilitating sickness or death. For this reason, unvented gas log fireplaces are specially designed to maximize gas combustion. They are also equipped with carbon monoxide sensors, an oxygen depletion sensor, and a complex system of safety shutoff devices. As long as these safety features function perfectly, unvented gas fireplaces can operate without posing a threat to the health and safety of occupants.

The main concern with unvented gas fireplaces is not that they are likely to create a problem, but rather that no amount of well-intended safeguards can make any manmade device totally foolproof. It is possible for well-designed backup systems to fail, no matter how carefully manufactured, no matter how scrupulously engineered, no matter how well conceived. Tragic examples of this fact are two failed space shuttles, probably the most well built technological devices in history.

Nothing in the realm of human invention is 100% safe. The risks inherent in unvented gas fireplaces may be miniscule, but they cannot be deemed as nonexistent.

Those who have unvented gas fireplaces in their homes should exercise caution in the following ways:

  1. Install carbon monoxide detectors in various places throughout the home.
  2. Never leave the gas logs burning while you sleep.
  3. Keep the fireplace doors open when the fixture is in use.
  4. Be sure the gas log unit is not too big for the size of room in which it is installed.
  5. Keep a window slightly open when the gas logs are burning.
  6. Make sure that unvented gas logs are legal in your area.
  7. Follow the manufacturer’s installation instructions and safety precautions to the letter.

In addition to air quality safety, remember that water vapor is also a byproduct of gas combustion. During dry weather, this may not pose a problem. But if humidity is increased to high levels, condensation and mold could be the results.

Problem such as these do not commonly occur, but again, it should not be assumed that unvented gas log systems are unconditionally safe.

Problems With Ventless Fireplaces

The House Detective by Barry Stone, Certified Home Inspector

Dear Barry: We built a new home and installed a ventless gas log fireplace. As we look back, this seems to have been a stupid mistake. Since using the fireplace, a film has been forming on our windows. Our suspicion is that it is caused by exhaust from the ventless fireplace. What must it be doing to the air we breathe? It is too late to add a vent? Deborah

Dear Deborah: Installing a ventless gas log fireplace may have been an “unfortunate mistake,” not a “stupid” one. Most homebuyers, having limited esoteric knowledge of gas fixtures, would have no reason to suspect that a fully approved gas fixture such as this could be problematic or potentially unsafe.

The film on your windows may in fact be a combustion byproduct, and this, as you suspect, could be unsafe to breathe. Until this can be evaluated by a licensed expert or by the gas company, use of the fixture should be suspended, and the pilot (if there is one) should be turned off.

Ventless gas fireplaces operate without a chimney to the exterior of the building. They are designed to produce combustion products that are safe to breathe and can thus be vented directly into the home. The guaranteed safety of these fireplaces has been a subject of ongoing debate between product manufacturers and other experts in the fireplace profession.

The general claim of manufacturers is that ventless gas fireplaces have been designed in such a way that they will automatically shut down in the event of any combustion or venting problem. The opposing view is that regardless of built-in safeguards, there is no such thing in the realm of human invention as a 100% failsafe device. Failure may be extremely unlikely, but it can never be deemed as impossible. When one considers the potential consequences of venting partially burned gas into a home (i.e. carbon monoxide), nothing less than “impossible” should suffice as an acceptable criterion.

Adding a vent to the existing ventless system is probably not possible. Therefore, replacement with a different type of system (such as a pellet stove) may be a prudent alternative.