Ventless Gas Fireplace: Is It Safe?

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A ventless gas fireplace is one without a chimney or vent—and it’s an increasingly popular option for cash-strapped homeowners, since installing a fireplace vent can cost up to $3,000. But is a ventless gas fireplace safe and effective—and will the flames throw enough heat? Here’s what you’ll want to know before you buy a ventless gas fireplace for your home.

How a ventless gas fireplace works

In theory, a gas fireplace without a vent should be safe because of how propane gas burns (and there are no worrisome flames and extreme heat around your kids and pets).

“When you have natural gas combustion, you’re combining oxygen with methane,” explains Allison A. Bailes, who has a doctorate in physics and is the founder of Energy Vanguard, which guides homeowners on healthy, eco-friendly improvements.

In a process called “perfect combustion,” molecules of oxygen combine with methane’s carbon and hydrogen to create two harmless byproducts: water vapor and carbon dioxide (CO2).

As a result, “When they’re running correctly, ventless gas fireplaces do a pretty good job,” says Bailes. And gas fireplaces are better for the environment as propane burns cleaner than wood-burning versions.

Safety issues come into play, however, when your combustion in a gas fireplace becomes less than perfect. A lack of oxygen—which can be caused by fans, drafts, nearby candles, or an older chimney or improperly maintained fireplaces—can “change the combustion process, making it incomplete,” Bailes explains.

Incomplete combustion creates the dangerous molecular byproduct carbon monoxide (CO)—an odorless gas that causes brain damage and even death at high-enough exposure levels.

Ventless fireplace safety concerns

The problem is that since these ventless versions typically don’t come equipped with a CO detector, most people won’t realize their fireplaces aren’t functioning properly. This, in turn, could put their health at risk.

According to Bailes, most home CO detectors are designed to detect high levels of the odorless propane gas, and the probable emission from ventless propane fireplaces is quite low.

The popular Kidde CO detector, for example, will sound the alarm after 4 minutes of propane gas exposure to air with 400 parts per million of carbon monoxide.

But to sound the alarm at lower levels, the exposure has to be much longer. At 40 ppm, it takes 10 hours for a Kidde detector to go off (and it doesn’t measure below that concentration). Yet the World Health Organization’s guidelines for CO exposure recommend no more than one hour of exposure at concentrations as low as 26 ppm. In other words, low levels of CO coming from a ventless gas fireplace might fly under a detector’s radar entirely.

Ventless gas fireplace safety features

That said, these gas fireplaces do have several safety features that work to prevent potential issues with carbon monoxide. For instance, all gas fireplaces without a vent come equipped with oxygen depletion sensors. These monitor how much oxygen is reaching the fireplace and, if it’s too little, will automatically shut the fireplace off before incomplete combustion can create a hazardous situation.

Manufacturers offer a variety of guidelines and restrictions that aim to mitigate harm.

For one, these ventless fireplaces aren’t meant to be installed in bedrooms or rooms without ample square footage. Guidelines also advise that they shouldn’t be run for too long (typically no more than two hours). So if you were hoping for heat and flames for hours in this space, the ventless pick isn’t for you.

If you read the manual and follow the rules, you’ll probably be fine. Yet a study conducted at the Building Research Council at the University of Illinois suggests that many may not adhere to these ventless guidelines as much as they should. In this study of 30 homes with these particular gas fireplaces, half of the homeowners they observed ran them longer than recommended. Perhaps as a result, 20% of these homes had CO concentrations greater than what the Environmental Protection Agency considers safe (9 ppm over eight hours, a recommendation similar to the WHO’s).

The study also found higher-than-recommended levels of nitrogen dioxide, a pollutant that can irritate the lungs. Because of CO safety concerns, ventless gas fireplaces are banned in California and Canada.

Other ventless gas fireplace concerns

Beyond CO poisoning, another concern with ventless gas fireplaces is water vapor. Remember, one of the two byproducts of perfect combustion is water vapor, which is harmless to people but maybe not great for your home. For people in dry climates, a little extra water vapor might be a plus—your fireplace is also a humidifier. But it does present a potential issue: Vapor hitting a cold surface will create beads of water, which can eventually start to grow mold.

Should you buy a ventless gas fireplace?

So should you install a ventless gas fireplace? If you’re the better-safe-than-sorry type, you’re probably better off sticking with something vented, even if the cost is higher. But if you do decide to go ventless, make sure to properly size and install the fireplace and follow the safety measures in your manual carefully.

For extra peace of mind, consider investing in a low-level CO detector to hang near the fireplace. You can buy a detector that measures down to a single part per million, called a low-level CO detector, for $75 to $150. Then you’ll know for sure that your fire is as safe as it is snuggly.

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