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How It Works...

Differences between B-Vent, Direct Vent, and

Vent-Free gas heating fireplaces.



Sometimes referred to as Natural Vent. B-Vent fireplaces use pipe (b-vent) that must be installed through the home and terminate above the roof.  They also use room air for combustion, but the amount of air they use is minimal. 


How it works:

B-vent gas fireplaces draw interior air from the area around the fireplace to help fuel combustion inside the fireplace. Because hot flue gases are buoyant, the exhaust air moves naturally through its own pipe system, upward through your roof, where it is carried out the chimney and safely away from the home. As the fireplace continues to vent flue gases up the chimney, the chimney walls increase in temperature. The hotter the chimney walls, the better the flue gases retain their heat and buoyancy, which makes them exit the chimney faster.


PROS: Combustion fumes are vented outside. B-vent pipe itself is relatively inexpensive and simple to install. B-vent fireplaces are relatively efficient.


CONS: B-vent must be installed through the interior of the home and terminate above the roof which may pose extensive installation labor costs or difficulties depending on the home. B-vent could potentially be a source of cool air into the living area if a downdraft occurs. Some of the heat from the fireplace is lost up the vent.


         Direct Vent

In direct-vent fireplaces, you use special direct-vent pipe recommended by the supplier. The direct-vent system involves two parts, one part fo the exhaust and one for the combustion air intake. The appliance itself is a completely sealed combustion chamber and no air from the house is used for combustion. The exhaust vents can go either out through a sidewall or through the roof.


How it works:

In direct vent fireplaces, a pair of rigid flues that nestle inside one another is installed in sections ranging from 4" to 48" in length, with 90°- and 45°- angled elbows that allow for flexibility with placement. A 4" inner flue is surrounded by an outer flue that is either 6 5/8" or 8" in diameter.


When the gas fireplace starts up, exhaust passes through the inner pipe to the outdoors, while the outer pipe brings in fresh air for combustion, transmitting it directly to the bottom of the sealed firebox, where it ignites the unit. The outer pipe also helps insulate surrounding combustible components from heat traveling through the system.


The direct vent fireplace unit is sealed off from the home’s interior with a high-temperature glass panel and protective screen, so any flames or combustion byproducts are completely separated from the air in your home. 


PROS: Combustion fumes are vented outside. Allows simple installation in cases where the vent is going out a wall and terminates at the wall. No need to terminate above the roof line as is necessary with b-vent or wood burning applications resulting in relatively low venting costs. Direct-vent fireplaces are typically higher in efficiency than b-vent fireplaces and put out a lot of heat even with a low flame setting.


CONS: Direct vent fireplaces have a sealed glass front and typically a smaller flame than a b-vent fireplaces or a vent-free fireplace. The position and look of the logs in the fireplace cannot be altered or incomplete combustion will occur.



A vent-free fireplace uses no venting pipe at all which allows place of the fireplace in many places otherwise unavailable to a vented fireplace.  The burner of the fireplace or gas log set is designed to be 99%+ efficient, therefore there is no need to vent the fireplace. 


How it works: 

The ventless fireplace works in a U-shaped path. Oxygen is taken in from the room and typically enters through the bottom of the fireplace. The regulator mixes the air with gas to produce the fire. There's usually a vent at the top that releases air back into the room.


PROS: Ease of installation and lowered cost since vent pipe is not needed.  Vent-free products are typically the most efficient of all fireplaces or gas logs since all the heat is generated into the room or home.


CONS: Combustion fumes are vented inside the home which affect some people who are very sensitive to smell or allergies. Vent-free products expel a fair amount of moisture as a result of the burn process which also is expelled into the home and can be a problem especially in a room with a lot of windows like a sun-room since the windows may drip from all the moisture. Typically there is a low flame with vent-free products and the placement of the burner or logs are set by the manufacturer and cannot be changed or altered. 


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