1) Some manufacturers recommend/require a backdraft damper if you are installing in a freezing climate, this is especially important if there is only a short run to the exterior.
2) There's a lot of electronics and other assorted moisture sensitive componets in these heaters, if this crawlspace is "damp", I'd be leery of installing a tankless unit down there, in my Chicago climate I frequently see corroded equipment in crawl spaces, and it's often equipment such as conventional tank type water heaters much less sensitive than tankless units.
3) I've put together a webpage on my experience with tankless units (I like them, but they are intolerant of incorrect installation), you may find it useful:
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