It's made for indoor use. There are a couple of small, zippered vents near the ceiling in the Kodiak Flexbow tent that we always leave open anyway. Mainly to help with condensation. IIRC the heater even has a low oxygen shutoff. I know I had a different propane heater several years ago that I used for heating a crummy nylon shock-corded tent. It used to kinda give me a headache and probably wasn't designed for inside use so much. It didn't have a low oxygen shutoff either. Definitely wouldn't have left it running when I went to sleep. We've used the Big Buddy in our tent trailer as well as the Kodiak tent and haven't experienced any ill effects whatsoever, but I don't remember ever leaving it running overnight either. We count on our sleeping bags to keep warm while sleeping. Kodiak has some killer looking, but pricey, sleeping bags that I would probably already own if I did more cold weather camping.How did you vent the heater?