First, what do the specs say on the heater installation manual? My guess is maybe the 3/4" may be too small. Do not try to use less than they specify. If that is okay, I think pulling the plug and using that outlet should be okay, but wait for one of the pros to comment. Depending on the model selected, some of those tankless systems can require significant supplies. With a significant supply comes the need for air and exhaust, but having it outside helps. Wouldn't work where I live - too cold, but okay for some.