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 pulled from the side of the building or home. The B vent fireplace was commonly found when gas fireplaces were first being 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 exit the home 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 to not 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 oxygen. A Vent Free fireplace pulls it’s 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 are 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 outside is generally unlimited. This fireplace is also designed to withstand the harsh outdoor elements.