I really want a fireplace and after reading the things said about ventless fireplaces I have just about ruled them out. Now I need to know about zero clearance and where they can be placed. The wall that would work out best for the layout and flow of the room is interior and on the first floor of a 2 story. Can I run the vent under the house in the crawl space or should I run it up through the floor of the master bedroom upstairs? If it's up and out can I have another zero clearance up there on the same vent?

A vent pipe on a gas fired appliance can not run down hill. You have to have a rise on the vent pipe at all times.

No problemon the rise what about a second firplace upstairs on same vent?