The gas fireplace industry likes to use the term “vent-free” to describe fireplaces that keep exhaust gases in the house, but more descriptive terms might include “ventless” or “unvented” fireplaces. The chosen terminology is not criminal, but it carries the connotation that not having a vent is a positive convenience, when the reality is that a vent is an important part of any gas appliance. Additionally, gas appliances without vents are a liability that may cause headaches and other issues in some people. Determining whether a gas fireplace is vented or not can be as simple as poking your head in and checking for a hole that leads outside.
Read the full article here: A Ventless Gas Fireplace Doesn’t Belong in Your Home