The number one complaint I see on this sub about ventless gas units is the massive amount of moisture it puts in the air. That said, my in-laws have a ventless gas stove in their rather large family room and everyone loves it. It's great to stand in front of and warm your rear after you just came in from the cold. They don't run it for long periods of time so between that and the large size of the room, the extra moisture has no apparent effect. Edit: In summary, I'd say it's great for point of use (warming your buns, power out for a day) but I wouldn't use one for primary heat. More on reddit.com

Provided it has enough oxygen available for complete combustion, burning natural gas produces water vapor and CO2 but not CO. So monoxide usually isn't an issue for those. As far as consuming all the oxygen, chimneys don't provide that for wood or propane or whatever else, either. They're an exit not an entrance. Houses are not airtight, there's enough exchange to keep all but an unusually small or very well sealed room supplied with oxygen. Edit: as u/ridingbicycle pointed out, certain items like oil furnaces may need an intake, as they'll go through a lot of oxygen and are often mounted in the basement where soil provides a much greater seal than simple walls. More on reddit.com

Sounds equivalent to running your stove burners More on reddit.com

Post pictures of fireplaces and questions you might have about fireplaces. ... Currently have a superior insert that needs to be taken out (major safety issues). Going to convert it to a gas fireplace. More on reddit.com