- Vented log sets are designed for externally vented fireplaces (chimneys) and operate more closely to a real wood fire, than do vent-free logs. Operating with the damper fully open on a wood-burning fireplace, vented gas logs can be used in either masonry with an open flu and chimney or a prefabricated firebox with vent pipe. Vented gas logs do produce heat but are not as efficient as vent-free logs and use more gas.