Outdoor instal is great as long as it doesnt freeze in your area and you instal it in a recess box to protect it and keep it away from rain dripping from the roof or dont instal it under a window. Is this a new house?
If it is an existing house you could save some demo work by puting it on the inside on an outside wall and use a sweet termination direct vent kit through the wall above the unit so you dont have to run alot of venting.
That vent kit is catagory 3 stainles steel, you must use cat 3 venting with tankless because of the high temp of the exhaust not regular b vent. This kit is a Heatfab product.
Thanks very helpfull. My problem is i dont have a good exterior wall to mount it on. Could i go up and then turn horizontal and go under a concrete slab (to be poured later) if i sleeve the vent pipe in 6" PVC or something?