Hi Jean Disagree with the post above. We have used ventless logs in out old wood burning fireplace for two years and they are GREAT! You capture about 98% of the heat generated by the burning gas. Over the past two winters, our central gas unit has come on only a few times. We keep the entire house comfortably warm with this ventless set. Our set also has a little thermocouple that functions as a thermostat and turns the gas on to the logs even when we are not home. We were concerned about carbon monoxide, so we bought and installed 3 ot the Consumer Reports highest rated Night Hawk CO sensors.....they have never deflected above 0!
The one drawback to ventless gas logs is the moisture they produce. Complete combusion generates lots of water (moist air), which is one of the things that makes the warmth so comfortable in the dry cold weather......but too much moisture condenses out on cool windows and walls. So we run the fan on our central heater just to move the moist air over the cool aluminum fins of the air conditioning evaporator (in the plenum), which kind of acts like a whole house dehumidifier.