Should the chimney damper be open or closed when running ventless gas logs in an existing fireplace?

Closed is the design, not a workaround

Ventless logs earn their near-total efficiency precisely because nothing leaves the room — the burner is calibrated to combust so cleanly that the appliance is listed for closed-damper operation, with an oxygen depletion sensor standing guard. Leave the damper open and warm room air pours up the chimney the whole time the fire runs, dragging most of the heat you paid for with it. You haven't made the unit safer in any meaningful way; you've mostly made it worse at its job.

The exception: cracking it for moisture or smell

Some owners deliberately crack the damper during long burns to shed the water vapor and faint odor that come with ventless combustion — and that's a legitimate trade, efficiency for air quality. Sensitive noses and damp rooms sometimes call for it. But if you find you always need the damper open to live with the unit, that's a message: the logs may be oversized for the room, or the room too small for ventless at all. That's a sizing conversation worth having before another season — not a workaround worth normalizing.

First, be dead sure what you own

Here's where this question turns serious. Vented and ventless log sets look nearly identical to most homeowners, and the damper rules are exact opposites: vented logs must burn with the damper open — clamped open, by code — because they send their exhaust up the flue like a wood fire. Ventless logs burn damper-closed. Get them confused in the wrong direction and you're venting carbon monoxide into your living room.

We see mystery log sets constantly in Asheville real-estate closings — the sellers never knew which type they had either. If there's any doubt, don't light them. Have the set identified and inspected first; call (828) 545-0525. Our guide to [vented vs. ventless gas logs](/guides/vented-vs-ventless-gas-logs) covers the differences in depth.