I was in a similar position, except not ventless and I went straight to "take your tank and GTFO" because sleazy gas company, and I didn't buy a house to keep renting crap.

I considered the same route but never bothered. Eventually I just ripped it out, and enjoyed some significant improvement in heating efficiency from trading a hole in my wall plus metal heatsink back into solid insulated wall. My family room got more comfortable and more spacious, even after (mostly) self-installing a mini-split to replace it.