Where the air comes from and where the exhaust goes

One question, three answers

A gas flame needs oxygen, and it produces exhaust: carbon dioxide and water vapor when it burns cleanly, and carbon monoxide when it does not. Every gas fireplace has to answer two questions, where the air for the flame comes from and where the exhaust goes. The three venting types are three different answers, and they shape where the unit can be installed, how it is serviced and what can go wrong.

Direct vent: sealed off from the room

Behind the glass of a direct-vent fireplace is a sealed firebox, and the glass is gasketed shut. Its vent is a pipe within a pipe. Exhaust leaves through the inner pipe, and outdoor air for the flame comes in through the space between the inner and outer pipes. Outside, the pipe finishes at a termination cap, either on a wall or up through the roof. When a direct-vent insert goes into an old masonry fireplace, the same job is done by two flexible liners run up the chimney, one for intake and one for exhaust.

The glass is part of that seal. A cracked panel or a torn gasket on a direct-vent unit is a venting problem as well as a cosmetic one, and the unit should not run until it is fixed.

Because nothing inside the house feeds the flame, a direct-vent unit does not use room air and does not depend on the house to create draft. The maker's vent table sets how far the pipe can run up and across and how many elbows it can have. Where the termination may go comes from the manual and from Table 503.8 of the Florida Building Code, Fuel Gas volume, which scales the gap between the terminal and any window or door that opens to the appliance's input, and puts the terminal 12 inches or more above finished grade. Units built to heat a room are listed to ANSI Z21.88, and decorative vented fireplaces to ANSI Z21.50.

B-vent: natural draft through the roof

A natural vent gas fireplace, the B-vent type, draws its combustion air from the room. The exhaust passes a draft hood and rises by its own heat through a Type B vent, a double-wall metal pipe with an air space between the walls, to a cap above the roof. Under section 502.1, Type B vent pipe must be listed and labeled after testing to UL 441. Its top must sit 5 feet or more, measured vertically, above the highest draft hood it serves, and how far the cap rises above the roof is set by the roof's pitch.

Natural draft depends on conditions in the house. A warm vent draws well. Strong exhaust fans, a clothes dryer or another appliance pulling air out of a tight house can work against it, and exhaust can then spill out at the draft hood instead of going up the vent. Some natural-vent units carry a spill switch at the draft hood that shuts the gas off when that happens; the manual shows whether yours has one.

Vent-free: no vent at all

With a vent-free fireplace or log set, the burning happens in the room and the combustion products stay there. They carry an ANSI Z21.11.2 listing as unvented room heaters, and each has an oxygen depletion sensing pilot that shuts the gas off when room oxygen sinks to the maker's trip point.

Section 621 of the Fuel Gas volume allows them with limits: a cap of 40,000 Btu per hour on input, a combined unvented input held to 20 Btu per hour per cubic foot of space, a ban on serving as the home's only comfort heat, and, under section 303.3, not in bedrooms or bathrooms except for a small wall-mounted unit with the oxygen safety system. One vent-free maker's figure for the water added is roughly an ounce an hour from every 1,000 Btu of input, and that maker notes that some states and localities prohibit vent-free heaters altogether.

Where each type can go

Direct-vent units fit the widest range of places. One can sit against an outside wall with a short horizontal vent, or anywhere a pipe can reach the roof, provided the termination meets its clearances. A B-vent unit needs a path for a vertical pipe up through the house and roof, since it relies on rising exhaust. A vent-free unit needs no vent, but it needs a room large enough for its input and a location the code allows.

For an existing wood-burning fireplace, the choices follow the same lines: a sealed direct-vent insert vented through liners in the chimney, a vented log set that relies on the existing flue with its damper clamped open, or a vent-free log set where the room and the fireplace allow it.

Section 303.3 of the Fuel Gas volume also treats the sealed design differently. It lets a direct-vent appliance installed as listed go into bedrooms and bathrooms, while vented fireplaces and log sets are allowed there only when the room passes the volume test in section 304.5, and vent-free units get only the narrow wall-heater exception.

Vented vs ventless fireplace: heat and where it goes

A sealed direct-vent heater controls both its air and its exhaust, so it can be built to put a steady share of its input into the room. A B-vent unit uses room air for combustion and sends that air up the vent along with the exhaust. A vent-free unit keeps all of its heat in the room, together with its combustion products and water vapor, and that is the reason its size is tied to the room's volume.

Signs a vent is not doing its job

On a direct-vent unit, a pale blue flame that drifts lazily and looks almost transparent appears in one maker's troubleshooting chart as pointing to a vent blockage, and a unit that starts and then shuts down can be starved of fresh air. A termination cap that is blocked, crushed or boxed in by a new deck or addition deserves a look from outside.

On a B-vent unit, an exhaust smell in the room, soot at the draft hood or a spill switch that keeps tripping all point to a vent that is not drawing. With any gas fireplace, a sounding carbon monoxide alarm means leaving the house and calling 911 from outside, and a gas smell means leaving and calling the gas utility or 911.