I had a ventless in my 24 x 36 garage right after I had it built. I went with the ventless because I didn't want a exhaust pipe through the new walls. The heater was a 30K also, and worked well. But, be prepared for lots of moisture since the heating process makes water. Mine was a natural gas ... More on garagejournal.com

They appear safe enough (blue flame usually means it’s outputting little to no CO) and of course install it per the instructions so it’s not at risk of tipping. I doubt whether moisture brought up by the other commenter would be much of an issue (over and above what would be there already in a Midwest winter). That said, whether this is the right heater for your space depends on what you’re wanting to get out of it. Aside from requiring a new gas line, the main consideration is that it’s a convection heater. So, it will heat the space relatively slowly and need to be operating most of the time via a thermostat. Contrast with an electric forced air heater like this one . This would heat the space quickly and be used mainly when you’re in the space. For example, turning it on 15 min before you’re ready to be in there, then letting it cycle on and off to maintain the set temp, then turning it off when you’re done for the day. We have one similar to this in a large garage/ceramics studio and use it in the way I’m describing, and we’ve been very happy with it and don’t have to worry about gas safety. The only downsides of the electric forced air heater are utility cost and noise. I wouldn’t say it’s loud but definitely would be more noticeable that the gas heater. Cost will depend on how big your space is, how often you’re using it and at what temp, and the price of gas in your area (probably cheaper than electricity, but also recognizing that the gas heater may be running more often than the electric one). There are also gas forced air heaters, which may be a good trade off. The other commenter mentioned one and here’s another example . These do require a vent, but that would probably reduce the safety (and moisture) concerns. More on reddit.com

Many HVAC dealers will have their own prefered brand of wall mountable ventfree LP or Natural Gas heaters · Many years ago when my Dad got to old to cut his own firewood… he replaced his wood stove with a ventfree natgas wall mount heater · We have the Mr. More on growingfruit.org

I personally wouldn’t do something gas & non-vented. I know this is probably an unpopular opinion, but I’ve started to care more about what toxic things I’m breathing as I get older. The research about the long term health impacts of poorly vented gas appliances is not good. The research is mostly around stoves, but I can only imagine an unvented heater would be worse. I’d personally try to go for something electric. More on reddit.com