Just throwing this out there because I'd never seen it before.   I just stumbled across Empire heating's newer Ultrasaver 90 and 92% direct vent heaters.

Looks just like a Rinnai , but 10% more efficient.   Empire  PVS18 and PVS 35

http://www.homeclick.com/empire-heating-systems-pvs-18n-ultra-saver-90-plus-natural-gas-17-500-btu-wall-furnace-with-lp-conversion-kit/p-747993.aspx?adpos=1o1&creative=67837976825&device=c&matchtype=&network=g&gclid=COeG-b3BnckCFUo6gQod_XcOVQ

Could be a good application if a ductless mini-split doesn't work for certain locations.