You are out of your mind to install a ventless gas fireplace. Have you read about all the problems, the people who put them in and get ill from headaches due to carbon monoxide poisoning. Better yet, talk to your homeowner's insurance who likely will cancel you if they find out you installed one. Look at electric instead and yes they have heaters. Read this article below.

http://www.energyvanguard.com/blog/48762/Bob-Vila-and-the-Vent-Free-Gas-Fireplace-A-Sorry-State-of-Affairs


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Better yet read the nightmare stories from people who deeply regretted installing one.

http://www.dukefire.com/articles/article/3941815/60120.htm

Where do you live? They are illegal in some states.

I have had ventless gas fireplaces before in other homes. Never had a problem with them.
I have carbon monoxide detectors and never had a problem with them going off..

Why gas when you can do electric?! Either do a proper zero clearance vented gas, or look into electric.