Sometimes referred to as Natural Vent - B-Vent appliances 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.
Pros: Combustion fumes are vented outside. B-vent pipe itself is relatively inexpensive and simple to install. B-vent appliances 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 appliance is lost up the vent.
In direct-vent appliances, you use special direct-vent pipe recommended by the supplier. The direct-vent system involves two parts, one part for the exhaust and one for 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.
Pros: Combustion fumes are vented outside. Allows a simple installation in cases where the vent it 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 appliances are typically higher in efficiency than b-vent appliances and put out a lot of heat even with a low flame setting.
Cons: Direct fireplaces have a sealed glass front and typically a smaller flame than a b-vent fireplace 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 placement 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 appliance.
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.