Ventless gas heater

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I am considering a ventless gas wall heater for my home .I curently have a wall forced air heater from 1982. It concerns me because of the age and I see black soot has formed on the out side vent/chimmny. I am being told that the ventless will cause carbon monoxide poisoning. The one I have sound like its getting dangerous too. It has been cleaned and the vent will be cleaned now to see if there is blockage. My mother and grandmother had ventless wall heat in there home and didn't have any problem. I am interested in any opinions.