I'm replacing my boiler for a Viessmann Vitodens 100-W (Category IV). I'd like your help deciding whether to take fresh air from inside or outside my house.

If it gets air intake from the surrounding room instead of outside, would there be any concern of CO backdrafting into my house?

Does it become a Natural Draft (Category I) boiler if I use air from the surrounding room? Or is it Category IV regardless of configuration?