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Gas Venting 101

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As you contemplate installing a gas fireplace, perhaps you’ve come across several options for venting your new heat source. “Hmmm,” you say, “direct vent, B-vent, natural vent, vent-free . . . what does all that mean?” We’re glad you asked. At Croft Fireplace Center, we want to be sure you understand your options before making a decision about which fireplace best fits your needs. Let’s take a few minutes to investigate different venting options and discuss the pros and cons of each one.

As the name suggests, a vent-free fireplace requires no external venting, so there’s no chimney to build and no pipe to buy. Since there’s no exposure to external air to aid in combustion, there’s no loss of heat outside your home. Obviously, this makes a vent-free fireplace extremely efficient.
The greatest drawback of a vent-free fireplace is the very thing that makes it so convenient. A vent-free unit is precisely engineered for nearly 100-percent combustion, but a tiny bit of exhaust remains in the room. The fireplace has no vent, so the exhaust has nowhere to go. That exhaust consists primarily of water vapor and carbon dioxide. It is not dangerous, but sometimes a faint odor of gas can linger.
To guard against unsafe oxygen levels, all vent-free units are equipped with oxygen-depletion sensors. When the oxygen level in a room reaches 18 percent, a vent-free fireplace automatically shuts down. In a small room, you should expect a vent-free unit to shut down often.
Most do-it-yourselfers are not qualified to safely install vent-free fireplaces. These units come with specific, detailed assembly instructions, and failing to follow the instructions precisely can lead to problems with oxygen depletion and carbon monoxide.

B-Vent fireplaces (also called “natural vent” units) are similar to traditional wood-burning fireplaces. Combustion is achieved using the air in the room; the exhaust is vented outside the home through piping similar to a chimney. Because B-Vent pipe is smaller and much less expensive than Class A chimney pipe, it is a cost-effective option upon installation. However, a B-vent fireplace is unlikely to be energy-efficient. Most of the heat it generates will be lost through the exhaust piping, in a fashion similar to a brick-and-mortar fireplace. Today, most consumers install B-Vent fireplaces for their aesthetic value or use them as secondary heat sources because installation is cost-effective.

Direct Vent
Currently, the direct-vent gas fireplace is the most popular and efficient appliance available. The flame in a direct-vent unit is sealed inside a glass panel. As a failsafe, a direct-vent fireplace will not function unless the panel is installed and securely in place.
Direct-vent fireplaces achieve combustion using a dual-vent system.
Two options are available:

 Coaxial venting uses a pipe within a pipe. The smaller “inner” pipe is fixed inside the larger “outer” pipe and vents exhaust outside the home. The fresh air required for combustion enters the fireplace through the space between the two pipes.

 Collinear venting uses two separate pipes side by side. One pipe brings in air for combustion; the other expels the exhaust.

Direct-vent fireplaces allow greater flexibility than vent-free units when installing the heat source in a bedroom or bathroom. Vent-free units installed in bedrooms or bathrooms are limited to 10,000 BTUs because their exhaust remains in the room; however, a direct-vent unit has no such restrictions because its exhaust is vented outside.

In addition, direct-vent units offer more venting options than B-Vent fireplaces. A B-Vent appliance requires vertical venting through the roof and three feet above; a direct-vent unit can be vented horizontally or directly to the wall, requiring significantly less pipe and eliminating the need to install a chimney. If horizontal or direct venting is not an option, it is still possible to install vertical piping with a direct-vent fireplace. Because direct-vent pipe is almost always less expensive than Class A chimney pipe, it remains a more cost-effective option than a traditional wood-burning fireplace or stove.

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