the title Unvented Gas Heaters? with a heater in the background and a cautious logo

If you’re considering an unvented gas heater, this is the perfect article for you.

Here we will discuss how to make these appliances safer, explain their risks, and offer alternative options.

As a quick overview, unvented gas heaters can be dangerous, and though cautious homeowners may mitigate some of their risks, it is generally wiser to invest in an alternative heating solution.

The most common dangers that an unvented gas heater poses are:

  • Fire
  • Humidity damage
  • Risk of health due to carbon dioxide, carbon monoxide, nitrogen dioxide, and sulfur dioxide.
  • An unsightly residue on walls, flooring, ceiling, and all belongings in the space.

Now let’s dig into the nitty-gritty details about each of these aspects.

Why Do Consumers Use Unvented Gas Heaters?

Unvented heating appliances are widely sought after for their ability to heat living and workspaces quickly, efficiently, and with little effort. 

Unvented heaters do not require significant financial investments nor any home modifications for installation. Most unvented heaters also run quietly and do not need a blower.

a classic-looking unvented space heater sitting on the floor

These attributes can make them attractive solutions for many homeowners, but make no mistake, they are dangerous.

Even with careful maintenance and followed recommendations for safe usage, Unvented gas heaters pose several threats. Here are the three real dangers of using unvented gas heaters.

The Real Dangers of Using Unvented Gas Heaters

There are three primary risks associated with unvented gas heaters: fire, excessive condensation, and health hazards caused by the fumes, gasses, and pollutants that the heater produces.

Fire Risks

The most obvious drawback to these heaters is the danger of fire. Space heaters cause 43% of house fires in the United States.

Improper installation, positioning, and maintenance are the most common reasons why these heaters create housefires.

These heaters can still be a fire danger in smaller spaces if placed directly on carpets or rugs, too close to a wall, located near combustibles such as furniture, fabrics, and paper.

Allowing them to run for too long or running them unsupervised, especially with pets or children, are also dangerous causes for house fires.

three firefighters standing outside a red house with their firehose putting out a house fire
None of us want this happening to our home!

Humidity Damage

These appliances also create unwanted levels of humidity and condensation

This condensation directly results from the heater’s efficiency and the creation of methane that is released into the room.

Because there is no vent to collect the water vapors produced in the creation of heat, the water vapor moves into the open space. 

Adding too much humidity to an area can result in the following:

  • Wooden furniture may warp and rot.
  • Paint and wallpaper may begin to bubble and peel.
  • Mold may grow within the plaster. 
  • Severe health problems for homeowners, especially those who have allergies or asthma.

The heater’s efficiency has to be reduced from 99% to 90% (via improved ventilation) to reduce water vapor in the air.

Unvented Gas Heaters Cause Health Issues

Natural gas and propane heaters significantly impact indoor air quality and, as a result, can become a severe health concern.

Ventilation is needed to remove pollutants such as carbon dioxide and carbon monoxide. Unvented gas heaters do not have this option, though, which makes them not only dangerous but also falsely boosts their heating efficiency.

According to the National Fuel Gas Code, one air change per hour (ACH) would mean that a room that measures 10×20 feet would see a 25% reduction in the efficiency of a 3,800 Btu/hr unit.

Since air changes are less likely to happen with an unvented heater, they are seen as more efficient than they should be and are more dangerous.

Carbon Dioxide

Carbon dioxide, while not considered toxic, poses several health risks.

Increased carbon dioxide exposure causes headaches, dizziness, restlessness, tiredness, elevated blood pressure, asphyxia, and in severe cases, convulsions and coma.

Carbon Monoxide

Carbon monoxide, at low levels, causes long-term health issues. At higher levels, it is toxic and potentially fatal.

Carbon monoxide is a non-irritating, odorless, colorless, and unnoticeable gas. Only carbon monoxide deters can detect it.

Carbon monoxide is the most often lethal form of gas and is known to cause organ damage, permanent brain damage, and death.

Approximately two hundred people are killed by carbon monoxide that is produced by unvented fuel-burning heaters. 

Nitrogen Dioxide

Nitrogen dioxide can affect immune systems while raising susceptibility to respiratory infections, even when people are only exposed at minimal levels. 

Because some nitrogen oxide is always produced in a flame, it is credited with increasing asthma, cough, sore throat, even nausea, and vertigo.

Long-term effects of exposure to this gas can be chronic lung disease such as emphysema.

Sulfur Dioxide

When gas doesn’t burn completely, some carbon molecules become soot, which is proof of carbon monoxide in the space. 

If the unvented gas heater uses natural gas, then methyl mercaptan is added to the mixture. It causes that sulfuric rotten egg smell that allows you to notice the leak.

When methyl mercaptan burns, it creates sulfur dioxide that can irritate the eyes and the respiratory tract. 

In addition to this, a visible residue is left on the walls, showing evidence of pollutants coating the area.

Brown and yellow residues are effects of pollutants from the heater reacting to tobacco smoke, pet dander, or polyurethanes found in furniture and carpeting. 

Gray residues are effects of those same pollutants interacting with chemical formulations and wax of the non-organic air fresheners. 

Black residues are the apparent sign of soot caused by the incomplete burn in the heater itself but are also often caused by the incomplete burn by debris collected on the outside or top of the heater, such as lint, dust, and so on.

While unvented gas heaters appear economical and efficient, these qualities are just a facade resulting from improper ventilation.

The health concerns raised by chronic use of these style heat sources are why some states are beginning to prevent their installation within all new construction residential occupancies while requiring that existing units in older homes be carefully maintained in approved spaces or removed entirely.

yellow smoking sulfur dioxide spewing out of pipes onto the ground

Producers should inform all families, especially those with at-risk members such as pregnant women, young children, the elderly, and the immunocompromised, of the potential dangers of using an unvented gas heater.

How to Make Unvented Gas Heaters Safer

Most unvented gas heaters are small units intended for emergency use or in small areas, typically a single room. Some models offer options for minimal ducting or fans.

They often are made available as furniture and cabinet-style units.

However, they can also be wall-mounted or used as fireplace inserts to give a traditional wood fire’s realistic appearance and feel; most range in heat output between 5,000Btu/hour to 30,000Btu/hour.

This source of fuel-powered heat can be an excellent form of backup heat, making it a desirable option for some families.

If you decide to use an unvented fuel heater, please adhere to the following advice. 

Get Professional Installation

All unvented gas heaters should be installed by a professional. They are aware of the risks, have taken training, and know-how to spot hazards that the average homeowner may overlook.

Their expertise and judgment may protect you, your family, or your home.

Use Oxygen Sensors

It is recommended that only approved unvented gas heaters be used.

Approved heaters will have Oxygen Depletion Sensor (ODS) pilots, which shut off gas flow when oxygen levels in the space drop to 18.5% or lower. For reference, normal air levels linger around 21%.

Use Appropriate Spaces

Again, a professional installer would be able to determine the proper heating unit for your space; their knowledge is invaluable.

It is strongly recommended by the Gas Appliance Manufacturers Association (GAMA) not to use a heater that is too large for any given space.

As these are only supposed to be used for supplemental heat, they are not to be used for more than four hours at a time and are not to be used in small, unventilated spaces such as bathrooms or bedrooms.

Install a Carbon Monoxide Detector

A prudent homeowner would also be wise to install a digital display carbon monoxide detector that is listed as “sensitive.”

Carbon monoxide is deadly because it is odorless, colorless, tasteless, and otherwise undetectable to humans.

a person holding a carbon monoxide tester in their hand with fire in the background
A digital detector could potentially save your life.

Alternatives and Safer Heating Options

Now that you know the dangers of using unvented gas heaters, what are the safer options to consider? 

We did our research and here are several safer alternatives to unvented gas heaters:

Vented Gas Heaters

Vented gas heaters operate by drawing combustion air from outside the home and venting exhaust gasses outside as well.

This design prevents the buildup of carbon monoxide and other harmful byproducts within the living space. 

When using a vented gas heater, it’s essential to ensure proper installation by a qualified technician and regular maintenance to keep the venting system clear and functioning correctly.

Electric Space Heaters

Electric space heaters offer a safe and convenient alternative to gas heaters. They operate by converting electricity into heat and do not produce any combustion byproducts, making them suitable for indoor use without ventilation concerns. 

When using an electric space heater, go for a model with built-in safety features such as tip-over protection and overheat shut-off mechanisms

Additionally, avoid placing your electric space heater near flammable materials and never leave it unattended.

Radiant Floor Heating

Radiant floor heating systems utilize hot water pipes or electric coils installed beneath the flooring to provide warmth throughout a room. 

This method offers consistent and efficient heating without the need for visible heaters or vents. 

Radiant floor heating is particularly beneficial for maintaining comfortable temperatures in large, open spaces and can be controlled using thermostats to optimize energy usage.

Central Heating Systems

Central heating systems, such as furnaces or boilers, distribute warmth throughout a home via ductwork or radiators. 

These systems can run on various fuels, including natural gas, propane, oil, or electricity, depending on regional availability and preferences. 

Proper maintenance, including regular filter changes and professional inspections, is crucial to ensure the safe and efficient operation of central heating systems.

Wood-Burning or Pellet Stoves

Wood-burning or pellet stoves provide an alternative heating option for those seeking a traditional ambiance and efficient warmth. 

These stoves require proper ventilation, typically through a chimney or flue, to safely exhaust combustion byproducts outside the home. 

It’s essential to use seasoned wood or high-quality pellets and adhere to manufacturer guidelines for safe operation and maintenance.


While unvented gas heaters are compact and initially cost-effective, vented gas heaters are typically more economical, efficient and are safer in the long term.

If you decide to use an unvented gas heater in your home, hire a professional to install it.

Be sure to maintain the heater properly, install detectors, and follow its general safety protocols. Create some sort of ventilation for the space, and never allow the heater to run more than four hours at a time.

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