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Are Vent Free Fireplaces Dangerous?

Are Vent Free Fireplaces Dangerous?

Are Vent Free Fireplaces Dangerous?
Fearful woman

Vent free heating appliances may be one of the more novel hearth inventions. Installing them doesn't require venting to pass through an outside wall or roof. But their novelty comes with a lot of questions. The first is usually, "How do they work?" But, even after learning that their near 100% combustion relieves the burden of venting. Another concern people have is, "Are they dangerous?"

This article will look into both the horror stories and the science to help you separate the facts from fiction as we explore the supposed dangers of vent free fireplaces and ventless fireplaces.

Here Are Some of the Most Common Complaints

There seems to have been a spike in the popularity of vent-free appliances about 20 years ago. But, that spike was accompanied by a spike in customer complaints about them. Some bemoaned soot deposits in their homes, condensation on windows, and mold growth. There were complaints about bad smells. People began to have serious concerns about what they were breathing and whether it was contributing to respiratory issues. It's easy to find reports of burning eyes and itchy throats. There's even been those who've complained of sinus problems, headaches, lethargy, dizziness, disorientation. In extreme cases, customers even reported heart and lung damage.

Hazard Concerns

Digging deeper, we find that the concerns about vent-free hearth appliances boil down to a) concerns about indoor air quality and b) concerns about property damage. In truth, everyone should be concerned with both. Let's look at each in detail.

Indoor Air Quality

Indoor air quality is all about what makes it into our bodies through our lungs. Obviously, we need oxygen (O2). But that's only about 21% of normal air. Another 78% is nitrogen (N2). And the rest is a soup of other gases and particles. Most are benign. But, unfortunately, some of them can cause us problems. As clever as we are, humans have identified many of the polluting flies in our soup. We group them into one of two categories: particles or gases.


Solids and most liquids that float around in our air are collectively called particles. Some particles occur naturally. Others are the result of manufacturing or fabrication processes. And some are released when things are burned incompletely. Particles are concerning because they can become lodged in our lungs.

Our lungs have natural defenses against normal amounts of particles. But, when those defenses are overwhelmed, the results can range from irritation to damage. Some kinds of particles are even known to be carcinogenic. Particles are classified and studied according to their sizes.

Are Vent Free Fireplaces Dangerous?
vent free fireplace particles
Coarse Particles

Ten times smaller than fine grains of beach sand, or about five times smaller than the diameter of human hairs, these are the largest airborne particles. They include dust, pollen, and mold spores. Even our beloved pets' dander is a kind of coarse particle.

Mold is another coarse particle we're all familiar with. Mold spores in the air can cause everything from a stuffy nose to a sore throat, from coughing or wheezing to burning eyes or a skin rash. Asthma can make things even worse, as can a mold allergy. It's even possible for people with weakened immune systems to develop chronic lung disease or get mold infections inside their lungs.

Fine Particles

Coming in at four to ten times smaller than coarse particles are fine particles. These can be formed from gases or combustion and are one of the more concerning types of air pollution. Some studies have shown that high enough concentrations of fine particles can decrease lung capacity.

Ultrafine Particles

This is the newest category of particles to be researched. They can be twenty-five times smaller than fine particles. This means they're smaller than the little filter hairs on your lungs' cells. Scientists want to know if they end up being absorbed right into your cells, and if so, what happens when they do.


Gases are free molecules that remain separate from each other without condensing to a liquid or solid. Like particles, they too become part of the air we breathe. But, while some gases are lung irritants, others make it into our bloodstream and affect how our bodies function. Let's take a look at the biggest suspects in the vent free case.

Nitric Oxides

Nitric oxides occur naturally in the atmosphere. They're produced when lightning strikes or when something burns in normal air. Collectively, they're called NOx. They aren't very abundant in normal air. But, in large quantities, they can irritate the lungs and the eyes. The two main ones are NO and NO2.

Carbon Monoxide

Carbon monoxide is a product of the incomplete combustion of fuel gases. It's nasty stuff, causing headaches, dizziness, and nausea. Carbon monoxide poisoning can even lead to death. It prevents your blood from carrying oxygen. You can't see it or taste it, which is why it's called the silent killer.

Are Vent Free Fireplaces Dangerous?
Carbon monoxide detector
Carbon Dioxide

Carbon dioxide is what comes out of your lungs when you exhale. You'll also get it from the complete combustion of fuel gas. It's always present in the air around you. At normal concentrations, it's pretty harmless. But, if the air you're breathing has too much of it, you'll feel unnecessarily sleepy and dull.


The point of inhaling is to get oxygen into our lungs. The concern over it, then, is about what happens when you don't get enough of it. Combustion consumes oxygen in the air. When the air around you has too little oxygen, it's hard to breathe. It causes confusion and a loss of coordination. Eventually, without adequate oxygen, you suffocate.

Water Vapor
Are Vent Free Fireplaces Dangerous?
vent free fireplace gases

The humidity of air affects how comfortable it is to breathe. It can also affect how dry or muggy we feel. The water vapor created in the combustion of fuel gases affects both. We also know that mold loves dampness. So, too much water vapor can promote its growth in corners and other hidden spaces.

Volatile Organic Compounds (VOCs)

There is also a whole circus of toxic chemicals that come from everyday items. They're called volatile organic compounds. There aren't any set standards for these because we don't know what levels of them cause what effects. But we do know they can cause everything from allergic skin reactions to cancer. In the middle, there are symptoms like nose and throat irritation, vision and memory impairment, headaches, and nausea.

The most notorious of them are methylene chloride, benzene, perchloroethylene, and formaldehyde. Their sources are all around us. They don't just come from obvious things like paints and solvents, permanent markers, and insect repellents. They also come from air fresheners, cleaners, copy paper, and furniture. In fact, one of the worst offenders is actually your dry cleaning.

Property Damage

Even when our health isn't in jeopardy, the condition of our homes is important. A home in disrepair isn't an attractive one. It's not a comfortable one. And remedying it can be a tiresome, timely, and expensive prospect. The two biggest property damage concerns from vent frees are soot deposits and mold damage.

Soot Discoloration

Soot can badly stain areas around a fireplace, like mantels and ceilings. But, it can also get into upholstery, clothes, and, in extreme cases, your lungs. Since it wafts into the air and can take several days to settle out, it can even end up in places far away from where it originated.

Are Vent Free Fireplaces Dangerous?
Soot on hands
Mold Damage

Not only can mold accumulate on surfaces you can see, but it can also grow on those you can't, too. Think about the reverse sides of your walls. When the damage gets bad enough, you'll end up having to rip out and replace surfaces like drywall instead of just cleaning them.


It's certain that some people have had real trouble with their vent-free appliances. And there are fair concerns to address their safety. But, to answer the question of whether vent free fireplaces are actually dangerous, let's look at where the risks come from and what size they are.

Risk Causes

Sitting by itself turned off a vent free appliance is at worst a tripping hazard. Any potential risks don't show up until we fire it up. In other words, what happens when we burn things or heat them up indoors? The answer is they either combust or they volatilize.


Combustion is the process of combining oxygen to a substance in a way that releases heat energy. Complete combustion adds all the oxygen to a burned substance it can hold. The complete combustion of natural gas or propane only produces three things: carbon dioxide, nitrogen dioxide, and water vapor.

Incomplete combustion is the name we give to the state where things have not burned all the way. Here, substances that oxygen could attach to aren't holding as much oxygen as they could. Incomplete combustion of natural gas and propane yields the nasty chemicals we fear. Among those are carbon monoxide and nitrogen oxide.

It's true. The natural gas or propane that makes it into our home isn't 100% pure. That said, the main "impurity" is an intentional one. Each fuel type contains a tiny amount of a form of either ethyl or methyl mercaptan. When burned completely, mercaptan produces sulfur dioxide. And when incompletely burned, it produces sulfur oxide.


It's even possible for other substances we wouldn't normally think of to produce air pollutants. When dust is heated, it can be a source of particle pollution. That smell when you turn on a heater the first time for the season? That's dust volatilizing. Other materials, too, like plastic TV housings, can release particles or gas into the air. You might notice this as a new appliance" scent.


There's no doubt that some locations have better air quality than others. But the truth is that sources of air impurities are all around us. It doesn't matter whether we're indoors or outdoors or whether we live in a rural or urban area. The air around us is imperfect. Let's look at where these imperfections might come from.

Vent Free Hearth Appliances

Vent-free hearth appliances rely on burning fuel gases nearly completely. But as we saw earlier, even complete combustion uses up oxygen and produces water vapor and carbon dioxide. And, we do have to account for what comes from that tiny portion of incomplete combustion plus the NO2 that happens anytime something burns in Earth's atmosphere.

Vehicle Combustion
Are Vent Free Fireplaces Dangerous?
Exhaust gases spilling from a red car

It's easy to forget about cars and trucks and motorcycles we depend on every day. They're also sources of the same kinds of air impurities we're examining vent-free appliances for. Sure, they're outside. But, not only do we spend time outdoors, but the outdoor air is ultimately what makes it into our homes. NOx even shows up inside the vehicles themselves when we operate them.

Other Vent Free Appliances

There's a host of other household appliances, too, that produce the substances we're looking at. Cooking with gas? That's vent free combustion. Have a water heater with a pilot light? Vent-free combustion. What about your gas furnace? Vent-free combustion again. All burn nearly 100%. So, all are doing the same thing as our vent free fireplaces.

What About Our Beloved Candles?

One of the most often overlooked indoor pollution sources is candles. It feels harsh to condemn them for sure. But, candle combustion is very incomplete. It produces not only soot but also VOCs like formaldehyde, in addition to carbon monoxide and sulfur oxides. Candles made from petroleum-derived wax are the worst offenders.

Did We Even Discuss Cooking?

We mentioned gas cooking ranges and stoves make combustion byproducts. But, the cooking process itself, even on an electric range, releases fine and ultra fine particles into the air. Scientists are just starting to look into which ones and how much. But we know the food and the heated cooking vessel both release them. Some plants even contain trace amounts of substances like sulfur.

Other Everyday Processes

We're evaluating vent-free hearth appliances for any irritants they produce. But sources of lightning strikes, campfires, barbecues, welding, metalworking, natural oxidizing processes in the atmosphere, electric heating — all of these things produce some of the same irritants. A hard pill to swallow is that outside of laboratory conditions some level of pollutants is always present. There's no absolute way to avoid them. So, the practical question becomes, "How little is still too much?"

Ventilation is Key

We live in an imperfect world. So, our bodies, by design, can deal with trace amounts of impurities. There is, of course, though, a limit to what we can handle healthily. If pollutants always exist, no matter where we are, what keeps them in check? The answer is ventilation. It's the process of fresh air sweeping in, diluting the concentration of pollutants, and carrying them away.

Are Vent Free Fireplaces Dangerous?
Air pollution from multiple factories

The Earth's atmosphere is a great ventilator. It's the source of "fresh" air. This is why it's important to protect outdoor air quality, too. When not overburdened, the outdoors does a great job of mitigating naturally produced levels of air impurities.

But ventilation happens indoors, too. Most buildings aren't perfectly airtight. It makes sense that outside air comes in through the cracks. By the same token, indoor air leaves through them. These are called infiltration and ex-filtration. It surprises some people to know that an adequate amount of this kind of ventilation is an intentional part of any well-designed building. When natural cracks aren't enough, mechanical help is required.

Insulation and weatherproofing make our homes more energy-efficient. But they also reduce natural ventilation. Without some help, improving our homes this way causes them to build up more irritants. To help find the balance, the U.S. Department of Energy divides the country up into climate zones. These zones are used to set standards for energy-smart insulation and healthy air exchanges per hour. By comparing a home's insulation to these standards, it's possible to classify it as loose, tight, or average.

Activities with Similar Risk Profiles

Potential risks are apparently part of everyday life. No matter where we go, there are things that threaten to cause irritation, severe health disorders, and, yes, even death. We ought to minimize our exposure to certain risks for sure. But, going so far that we aren't participating in living life isn't exactly "healthy" either. Let's take a look at some other activities we take for granted, which pose similar risks to the ones vent-frees are on trial for.

Going Outside

Most of us recognize how good getting out in the sun is for us. In fact, we need to do it for at least 15 minutes a day for our bodies to make the vitamin D we need. But, the UV rays from the sun do pose a real risk. Americans have a 20% chance of developing skin cancer by age 70. And, on average, two people die of skin cancer every hour in the States. The biggest contributor to skin cancer? Sun exposure.

Are Vent Free Fireplaces Dangerous?
A woman sunbathing beside a pool

Most of us wouldn't consider not using our automobiles. But, just in terms of what we're breathing, driving a car is a risky proposition. Both CO2 and NO2 build-up in the cabins of our cars while driving. In fact, the amount of CO2 in the cabin of a car in a 45-60 minute drive can reach 5 times the suggested maximum for indoor air quality. It's part of the reason you get sleepy on longer drives. The amount of built-up NO2 can also climb over the max, especially in urban areas.

Let's not forget either, that 36,000 people die every year in traffic accidents. That means four people every hour lose their lives while driving.

Walking Down the Street

When we think about the risk of walking down the street, it's usually as a joke or in reference to small children. But, the risk is real. Most of us don't realize that every 88 minutes a pedestrian dies from being struck by a motor vehicle. On top of that, approximately 137,000 people enter the emergency room for injuries from vehicles crashes a year while walking.

Having a Drink

Except for those struggling with AUD or a loss from drunk driving, the risks associated with consuming alcohol usually stay near the back of our minds. But, here again, an activity we certainly take for granted has its risks. Alcohol is the cause of over 30,000 liver disease deaths each year. Not only that, alcohol consumption increases the risk of all sorts of cancers. They include mouth, throat liver, and breast cancers.

Grilling Out
Are Vent Free Fireplaces Dangerous?
Outdoor grilling

Yes. One of the most American activities we can think of has a carcinogen risk. When meat on a grill is charred, cancer-causing HCA compounds (heterocyclic amines) are formed. They're suspected of causing colon, breast, and prostate cancers. And every time fat drips or flares up, PAHs (polycyclic aromatic hydrocarbons) rise up and deposit on our food. PAHs are also on the list of human carcinogens.

Hearth Industry's Position

The hearth industry can only exist by observing the most current safety regulations. That's why it works with agencies like the National Fire Protection Agency. That's also why it conducted emissions tests of vent-free hearth appliances. It only made sense to know, for sure, what they put into the air we breathe.

Relevant IAQ Agency Limits

Several agencies governing air quality publish standards about the amount of our "compounds of concern" in the air. The goal is to keep them within tolerances and keep us from harm. We've alluded to this before, but it's worth saying clearly. There are compounds we suspect might be harmful in some concentration but that we don't have any standards for yet. So, in the meantime, the best we can do is look at ones that we know vent frees produce and check the standards for them.

Are Vent Free Fireplaces Dangerous?
Smog air pollution

The Consumer Product Safety Commission (CPSC) sets a limit of 0.3 ppm/hr for NO2. For CO, it sets a limit of 15 ppm/hr over 8 hours and 25 ppm/hr over a 1 hour period. The Canadian standard for CO2, which is stricter than the limits set by OSHA in the U.S., is 3500 ppm/hr. The National Institute for Occupational Safety and Health (NIOSH) set a required minimum O2 level of 19.5%. And the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) define 40-60% as the appropriate range for indoor relative humidity (water vapor). There aren't currently any safety standards for volumes of VOCs or ultra fine particles.

Testing Description

The American Gas Association's Research Division developed a vent free appliance model. They then validated it in two laboratory test homes — one in Cleveland and one in Chicago — to make sure it predicted the emissions of vent-free appliances on the market. Then, connecting it to simulation software, they ran some 20,000 trials for each of the five DOE climate regions. They tested a host of variables, including room size, house size, outdoor humidity, air exchange rates, and the number of people in the room (to account for the effects of people breathing). In addition, they tested the BTU rating, the size of the connecting space, the number of walls in the room with outdoor contact, heat loss factors, and even took into account various other water vapor sources like cooking, showering, washing, and drying clothes.

Are Vent Free Fireplaces Dangerous?
Testing standards for compliance
Testing Results

So, what did the tests turn up? Vent-frees emit significantly less CO than the maximum levels set by the CPSC. It's worth noting that the CPSC uses children, pregnant women, and the elderly as the baseline for its standards. NO2 production was also lower than the CPSC limit. There is no health-based standard for CO2, but Canada's indoor air quality standard for it is 30% stricter than the U.S.'s. Vent-frees tested to produce less than half the maximum. Available oxygen remained well above the minimum NIOSH standard. And they resulted in a room humidity less than half that needed to grow mold. All other things being equal it was even less than the lower maximum recommended by ASHRAE.

Position on VOCs and Particles

We mentioned before that there aren't applicable safety standards for many VOCs and particles. As a result, they don't show up in the testing. But that doesn't mean they can't be accounted for.

We've covered all the results of fuel gas combustion. It turns out the VOCs weren't among them. It's reasonable to expect any VOCs from vent frees, then, to be the same as for any other piece of household furniture or appliance.

As for our particles, it's the ultra fine ones that may be of interest. However, the science on whether they are harmful, how much of them would be harmful, and how to accurately measure them is still in development.

Mitigation Recommendations

So, it turns out that when testing vent-free appliances, they don't output any of the things they're accused of. So, what's happening? Where did all the soot and mold and breathing problems come from? The answer lies in the fact that, during testing, vent frees are only installed and operated the way they're supposed to be.

Let's look at what the industry intends for them.

Proper Installation

All indoor vent free hearth appliances should be listed to the American National Standards Institute standard Z12.11.2. This certifies that they've been tested to perform within accepted IAQ limits. They should also be installed by qualified installers according to the standards laid out in the National Fuel Gas Codes (NFPA 54) and the International Fuel Gas Codes.

One of the most important things these codes do is determine whether a space is capable of providing enough fresh air. Not only does the fresh air supply ventilate, but it also ensures enough oxygen to guarantee complete combustion. Always check with your installation planner.

But, the general guideline requires 50 cubic feet of room space for every 1,000 BTUs of fuel the appliance consumes per hour. Bedrooms and bathrooms have even stricter requirements.

Just as critical as the air supply is the cleanliness of the installed space. Especially when installed in an existing fireplace, the area has to be clean of previous dust, soot, and creosote. The latter especially are subject to volatilization. Once airborne, they'll interfere with combustion, produce more soot, and spread throughout the home. This is why chimney cleaning is required by codes even when installing a vent free gas log set.

ODS Pilot Systems

Since 1983, every vent-free appliance has been required to have an oxygen deletion sensing pilot (ODS). It's a safety measure to shut the appliance down in case of incomplete combustion. The oxygen level in a room changes the shape of a pilot flame. If the oxygen level drops, risking incomplete combustion, the pilot flame's shape deviates from the one that keeps the unit on. So, a properly working ODS is crucial to minimizing nasty stuff in the air. In addition, there have been zero recorded deaths in the U.S. linked to an ODS-equipped appliance.

Are Vent Free Fireplaces Dangerous?
Oxygen depletion sensor
Proper System Maintenance

Sadly, one of the most overlooked aspects of hearth appliance ownership is maintenance. At a minimum, a hearth appliance should be inspected and serviced by a qualified professional once a year. Ideally, this should happen at the beginning of the burn season and at the end. But, be aware that the maintenance schedule of a given appliance may vary. Some models have recommended service as frequently as every 250 hours of operation. That works out to every few months. We always recommend service by an NFI Certified Gas Specialist or Master Hearth Professional.

Proper Operation

Using vent frees the way they're designed to be is also critical. For instance, they're only supposed to be burned for about 4 hours at a time. Plus, they're only designed to burn air and fuel gas. Anything else in the air — candle leftovers, coarse particles like pet dander, or nasty VOCs — can waft into and be burned by the burner. Problems from neglecting to maintain the overall air quality in a home are multiplied in the presence of a vent free.

Carbon Monoxide Detectors

CO detectors are recommended for every home, whether or not they have a hearth appliance. Their purpose is fairly obvious. What's not though, is the margin for human error. Not every CO monitor alert is the fault of your vent-free appliance. CO monitors should be calibrated or replaced every year. On top of that, some monitors add successive readings into a cumulative reading. So, what users see may be much higher than a room's current level. Lastly, other VOCs may also trigger CO alarms. Heated creosote is a common offender.

Other Agency Opinions

Are Vent Free Fireplaces Dangerous?
Business people holding word bubbles

Industry agencies continue to support investigating the impact of vent-free emissions on indoor air quality. They include the AHRI (Air-Conditioning, Heating, and Refrigeration Institute), the Vent-Free Products Alliance, the National Fireplace Institute, the American Gas Association, the National Fireplace Institute, and the Hearth Patio and Barbecue Association. They recommend only using appliances listed to the ANSI Z21.11.2 standard. This certifies that they operate within the safety tolerances already in place. They also acknowledge that, while residents of some regions traditionally use ventless heaters as their primary sources of heat, the appliances are NOT designed for this purpose. The industry makes it clear that it does not support its use this way.

On the other hand, the Environmental Protection Agency, the U.S. Department of Energy, the American Lung Association, the Consumer Product Safety Commission, the Department of Housing and Urban Development, the states of California and Massachusetts, and even some dealers and manufacturers within the hearth industry advise against the installation or use of unvented heating appliances at all.

Their concerns center around two main issues. First, many home and business owners don't understand the proper installation and operation that guarantee performance within accepted safety standards. In other words, the allure of an "easier" installation translates into a "planning free" installation and a "maintenance free" ownership. These, though, are the exact conditions which guarantee the problems we visited earlier.

The second concern stems from uncertainty over what we don't yet know. That is, given that the scientific community may, at some time in the future, revise its recommendations about the safe limits of indoor air quality constituents and that meaningful research about ultra fine particles is still too young for definitive claims one way or the other, these agencies, in the interest of the most vulnerable populations (i.e. those with existing breathing problems and those who are likely to overlook maintenance) advise against unvented appliances altogether.

This is the main concern of one committee of the American Society of Heating, Refrigerating, and Air-Conditioning Engineers, which yearly seeks to enact a ban on the installation and use of unvented gas-fired appliances. So far, each year ASHRAE has elected to forgo the ban recommendation on the grounds of lack of evidence. Instead, similarly to the NFI and HPBA, it recommends "a public information program be developed that improves the knowledge of owners of these appliances with regard to usage and the importance of professional installation and maintenance." Additionally, all three support research "on these appliances to answer remaining questions about their impact on indoor air quality."

eFireplaceStore's Opinion

Are Vent Free Fireplaces Dangerous?
Call center specialists
Is a Vent Free Appliance Safe?

At the end of the day, though, what we all want to know is, "Are vent-free hearth appliances safe?" Even though they have their detractors, they really are as safe as anything we can make. They comply with all known safety standards. And the hearth industry is committed to adapting as new science becomes available.

When science showed that it was better to face the male end of stovepipes down to control leakage, the industry followed. The industry continues to adapt its products as new environmental standards take effect. And, there's no reason to expect it to discontinue that trend.

Is a Vent Free Appliance Right for You?

That said, vent-free appliances are not right for every customer. In particular, if your planned space already has excessive moisture, a vent free appliance will cause problems. Vent frees simplify the physical installation because there's no venting. But, vent-free doesn't mean planning-free or service-free. If there are concerns about being able to keep up the maintenance, a vent free is not the right choice. And, in the same way, you'd eliminate pet dander and candles and aerosols around those with chronic breathing issues, so vent frees wouldn't be right in their homes either.

What Kind of Appliance is Right for You?

If you fit any of the bills above, or if you still have concerns, there are plenty of direct vent appliances that would suit your needs. The margin of error a direct vent appliance allows is much greater since all the combustion is sealed off from the room. We recommend working with an installer who can provide the due diligence of visiting your home before you select your appliance. The right installer working with the right installation planner is the best path to getting an appliance you'll be happy with for years.

About the Author

Cortney Owens

Cortney enjoys marveling at fire, whiskey, and keeping his dad's old Ford clean and running.

When he's not studying for his MHP (Master Hearth Professional) or ECBA (Business Analyst) certifications, you can find him reading to his two toddler nieces or making a mess in the kitchen tweaking his recipe for pepper steak.

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