Direct-Vent, B-Vent and Vent-Free Gas Appliances

Follow the air

Keeping these three straight helps when you read a quote, a manual or an inspection report. It also explains why the same-looking fireplace can need very different chimney work.

Direct-vent: a sealed box with its own air supply

A direct-vent fireplace or insert is a closed firebox behind fixed glass. Outside air reaches the burner through the vent, and exhaust leaves through the vent. Room air is not used for combustion. A through-the-wall or through-the-roof installation usually uses coaxial pipe. The Valor 1800 manual, for example, specifies 4-inch by 6-5/8-inch coaxial direct vent pipe. Inserts placed in masonry chimneys often use two parallel flexible pipes instead, one for intake and one for exhaust; see fireplace inserts explained.

Each model has its own venting limits. The same Valor manual, as one example, requires a minimum 24-inch vertical rise directly above the appliance, allows no more than four 90-degree elbows or the equivalent, caps total vent length at 40 feet, and requires horizontal runs to rise 1/4 inch per foot. It forbids connecting the vent to a chimney that also serves a wood or other solid fuel appliance, and gives each gas appliance its own vent system.

Inside a coaxial vent

B-vent: room air and natural draft

A B-vent appliance burns room air and relies on hot exhaust rising through the vent on its own. The vent is Type B gas vent, a double-wall pipe. DuraVent's Model BV, for instance, has an aluminum inner wall and a galvanized outer wall with an air space between them, is listed to UL 441, and is made for natural gas or propane Category I and draft-hood-equipped appliances, including gas fireplaces, furnaces, boilers and water heaters. DuraVent lists a 1-inch clearance to combustibles in the U.S. for its standard sizes.

Because a B-vent appliance depends on draft and room air, the things that upset draft matter: a vent that is too long or has too many turns, a cold or oversized flue, a blocked cap, or a house under negative pressure from exhaust fans. Spillage of exhaust at a draft hood is a warning sign. Type B vent is not a chimney for wood or oil appliances.

Vent-free: no pipe, more rules

Vent-free logs and heaters burn without any vent. One vent-free log manual describes an oxygen depletion sensor (ODS) pilot that turns the burner off before oxygen in the room falls under 18 percent, a room-size rule of 50 cubic feet per 1,000 BTU per hour of input, prohibitions in bedrooms and bathrooms for units above the input limits allowed there, and warnings about excess heat or moisture in tight houses. It also cautions that a state or local code may permit the appliance to run only as a vented unit.

If vent-free logs are under consideration for a fireplace, read the manual's room-size and ventilation rules first and confirm with your local construction office. The decorative-log version of this choice is covered in gas logs vs. gas inserts. Whatever you choose, keep working carbon monoxide alarms; see carbon monoxide alarms.

What each type asks of a chimney

• Direct-vent through a wall or roof does not need a chimney at all. In a masonry chimney, the chimney serves as a chase for the vent pipes, and the exhaust travels in the pipe system inside it. The termination cap and the top of the chimney still need to keep water out.
• B-vent and other Category I gas appliances vent into a Type B vent or a properly lined and sized chimney flue. When a gas appliance is connected to an existing masonry flue, the flue has to suit that appliance. A large old flue that once served a fireplace or an oil burner may be much larger than a modern gas appliance needs. See the orphaned water heater problem and liner options.
• Vent-free uses no flue, but if installed in a wood-burning fireplace, its manual sets damper requirements.

Where the vent ends outside

A direct-vent or B-vent termination has to sit at set distances from windows and doors that open, soffits, inside and outside corners, gas meters and regulators, mechanical air intakes and walkways. The appliance manual lists these distances and references the National Fuel Gas Code (ANSI Z223.1/NFPA 54). The numbers differ by model and by appliance input, so use the table in your own manual. Pellet stoves, which also vent through walls, have their own tables and are covered in pellet stove venting.

Once installed, keep the termination clear. Shrubs, snowdrifts, a new deck or an enclosed porch can end up too close to a wall terminal that was placed correctly when the appliance went in.

Replacing a gas appliance in New Jersey

New Jersey's construction code ties the replacement of fuel-fired equipment to a chimney verification program. The state's version of the IRC fuel gas and mechanical chapters references it, and the Department of Community Affairs publishes the matching paperwork, form F370. On that form, the verifying individual signs off that the chimney or vent is sound, unblocked, and has a liner and flue size that fit the appliance going in. The form states it may not be submitted by a homeowner in lieu of the required inspection.

Two details stand out. The form is not required when a direct-vent appliance is replaced with another direct-vent appliance. It is relevant when a gas appliance is going onto an existing chimney or vent, which includes swapping a B-vent fireplace, a furnace or a water heater that uses the old flue. Your construction office explains how it fits with the permit for your project. Our inspection before a new appliance covers the chimney side.

The gas line is a separate job

Every one of these appliances needs a gas supply at the right pressure, with its own shutoff valve. The Valor manual, for example, calls for an individual manual shutoff valve and says installation and service must be performed by a qualified installer, service agency or the gas supplier.

Our gas work starts from a line that is already in place: Chimney 911 hooks up logs, fireplaces and inserts to it and does not install new piping. If there is no line at the fireplace yet, a plumber or your gas utility installs it first. Installation is covered on our gas appliance installation page, and upkeep on gas fireplace service.

Questions homeowners ask

Sources

1. Valor 1800KN/1800KP Direct Vent Gas Fireplace: Installation Manual
2. DuraVent: Model BV Type B Gas Vent (product overview)
3. Grand Canyon Gas Logs: Vent-Free Burner Systems Installation and Operating Manual (PDF)
4. New Jersey DCA, UCC Form F370: Chimney Verification for Replacement of Fuel-Fired Equipment (PDF)
5. N.J.A.C. 5:23-3.21, New Jersey one- and two-family dwelling subcode (adopts the 2021 IRC with New Jersey modifications), via Justia Regulations

This guide is general information. It can't diagnose a specific chimney; an inspection can.