I have a ventless kerosene heater in the basement, mostly for emergency use. I'd use it more, but Wife goes into a rage if she smells anything solventy in the house. I would not use a ventless heater w/o a CO detector, but the one in the basement has a digital readout and it barely moves when the kerosene heater is lit -- unless I adjust the wick too high and it starts to smoke (but even then the CO never gets dangerous, just detectable)
I wish I had a ventless NG heater down there. Just a small one would make a big difference. Electric heat is too expensive to run more than an hour or two per day, and electric is no help if the power goes out.