Vented vs. Ventless Gas Fireplaces
In a traditional vented gas fireplace, there are two vents that run to the outside of the home. One is a fresh-air intake that provides combustion air help the gas burn more efficiently. The other vent safely removes any exhaust gases created by burning the natural gas or LP to the outdoors.
On the surface, a ventless gas fireplace looks quite similar to a vented fireplace. It has a control panel to operate the pilot light and flames, and holes in ceramic artificial logs for the flame jets. There is a slight difference to the flame jets between the two styles, and as a result, ventless fireplaces tend to burn somewhat less realistically than vented units. As with vented fireplace, ventless units usually have blowers that circulate air around the firebox to heat the room.
However, ventless fireplaces have neither of the two outdoor vents found in vented units. Instead, combustion air for the burner is drawn into the fireplace from the air inside the home, and exhaust fumes also remain inside the home.
This may sound dangerous, but ventless fireplaces are engineered in a manner that minimizes the exhaust fumes. A special regulator creates a fine mixture of gas and air that burns extremely cleanly, and the units are carefully tested in approved laboratories before they can be sold. Ventless gas fireplaces are deemed to operate within the range of safety for cycling these combusted gases back into the home's interior. By contrast, vented gas fireplaces create a dangerously high amount of combustion exhaust and therefore must be vented to the outdoors.
Are Ventless Gas Fireplaces Safe?
The safety of ventless gas fireplaces is a subject of debate. According to the National Association of Certified Home Inspectors (NACHI), a patchwork of regulations across the United States controls the legality of ventless fireplaces. Roughly a third of states allow these units with no restrictions. California is the only state that outright bans all ventless fireplaces, and there are notable restrictions in Massachusetts. In the remaining states, a welter of regulations controls ventless fireplaces based on factors such population of the city, altitude, and surrounding geography. In many states, there are restrictions on where the home you can install a ventless fireplace—they may not be allowed in sleeping areas, for example.1
Low oxygen levels can be a concern with well-insulated homes that have a slow exchange of indoor and outdoor air. Ventless gas fireplaces have a feature called an oxygen detection system (ODS), which automatically turns off the unit if oxygen levels in the room fall below a certain level. The fireplaces may also have built-in CO (carbon monoxide) detectors that also automatically shut off the fireplace if high levels are detected. Still, hazards are present. Some manufacturers recommend leaving a window cracked open while running operating the fireplace to ensure there is a source of fresh air.1
NACHI also observes that although fumes are greatly reduced, ventless fireplaces still release small amounts into the home, increasing the risk of carbon monoxide exposure. And non-vented burning of natural gas or propane also produces water vapor as a byproduct, which can increase humidity levels and the risk of mold.1
It's therefore wise to do your homework on the units you are considering. Make sure its specifications meet safety standards, and that the unit is approved for installation by your building code authorities. Ventless fireplaces may well be a good choice for a decorative feature in a room where its use can be supervised, but shouldn't be used as a principal source of heat, especially in a sleeping area. And it is not a good choice if anyone in your home suffers from breathing problems, such as asthma or COPD.