b-venting vs. Direct-venting Fireplaces, Stoves, Furnaces...

There are two ways of venting new fireplaces and stoves, or ductless furnaces and other home appliances (besides the traditional chimneys): direct-venting and B-venting. Direct-venting is a more recent venting technology mainly used in gas stoves, wall furnaces, some heaters and modern wood fireplace inserts.

Direct venting

Free-venting
Be aware of free-vent or vent-less heating units. Even decorative units like gel fireplaces, or gas logs aren't entirely safe and without unhealthy emissions. Free-vent units degrade the indoor air quality and pose safety problems. Though some manufacturers claim that their free-vent units are risk-free, that’s not the full truth. Entities like the American Lung Association warn us for the risk of gas and wood fueled free-vent units. Only electric heating appliances do not require ventilation.

Direct Venting Fireplace A direct-vented fireplace (or stove, or wall furnace…) is a combustion sealed unit, that is, it gets the combustion air from outside and vents to the outside the fumes and pollutants (see image).

The direct-venting technology uses a direct-vent pipe, which can vent either horizontally through an outside wall or vertically through the roof, chimney or other top opening. The pipe may use elbows to go around possible obstructions.

This technology is particularly suitable for well-insulated homes. Direct venting doesn’t require a chimney, and when technically possible is cheap and easy to perform.

Horizontal Direct-venting of an appliance Image: An example of horizontal direct venting (wall furnace direct venting)

B-venting

Contrary to direct-venting, B-vent units do not get the combustion air from outside. B-vent appliances aren't entirely sealed units. B-Vent fireplace They use combustion air from the home ( in some few cases with the help of an outside air kit).

Like natural venting systems, B-vent units also draw the air needed for combustion from within the home. And like traditional chimney-based systems they also use the stack effect: since flue gases are hot they naturally rise up through the vent pipe to escape...

The difference is that they use a chimney liner (typically an approved stainless steel one) or another horizontal or vertical vent system to route the exhaust air.

Direct venting or B-Venting?

Direct-venting is a warranty of healthy indoor air and safety. With it, pollutants like CO, dioxins, arsenic or formaldehyde are all exhausted to the outside. On the other hand, direct venting is simple to install and relatively inexpensive (at least in new construction). The main “problem” with direct-venting is that it demands sealed units and a professional installation.

B-venting systems are better than traditional chimney-based systems, but they aren't as safe and clean as direct-venting...

Vent-free fireplace system Though many qualified stoves and fireplaces use the B-venting system, the best and most energy-efficient units use direct-venting.

See: Fireplaces & Health Threats

Image above: Vent-free venting (non-vented units) are particularly unhealthy and unsafe; there isn't outside exhaust and the combustion air doesn't come from outside.