There are four main types of venting systems used by gas fireplaces. The four main types are as follows:

1. B-Vent
2. Direct Vent
3. Vent Free
4. Outdoor (Ventless)

The following describes each of the four major venting systems in more detail:

B-Vent:
The B vent always exits the exhaust of the gas fireplace vertically. This means the exhaust goes from the gas fireplace to the roof. The oxygen for this type of fireplace is typically pulled from the room. The oxygen can also be removed from the side of the building or home. The B vent fireplace was commonly found when gas fireplaces were first introduced.

Direct Vent:
The Direct Vent system is more common in newer homes. This venting system is also referred to as a sealed combustion system. This means that everything that happens in the fireplace box exits to the outside of the home or building. No oxygen is pulled from the room for this gas fireplace. The exhaust for the direct vent can leave the house either vertically or horizontally out the side of the home. In all cases, there is one tube or vent for the exhaust and another for the oxygen. The incoming oxygen is the combustion air for the pilot light and the main burner.

A Direct Vent system can also have a power vent. The power vent is installed externally or in line with the venting system. The power vent is also configured not to let the fireplace run unless a draft is being created.

Vent Free:
Vent Free gas fireplaces do not have a vent for the exhaust or combustion of oxygen. A Vent-free fireplace pulls its oxygen from the room and pushes the exhaust to the room, too. This type of gas fireplace also has a sensor to shut down the fireplace when the room’s oxygen drops below eighteen percent. If this happens, the pilot light will immediately turn off. In turn, the main burner will also turn off.
A Vent Free gas fireplace is illegal in many places due to safety concerns.

Outdoor Gas Fireplace – Ventless:
This type of fireplace typically does not have a vent for the oxygen or exhaust and is only safe to run outside. They differ from a vent-free fireplace in that they do not have an oxygen sensor to shut the fireplace off since the oxygen out is generally unlimited. This fireplace is also designed to withstand the harsh outdoor elements.