> 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
> 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?
I am not farmiliar with this particular boiler. However with any fossil fuel combustion unit it is always better to take air from outside as this is where the by-products (fumes) are being exhausted. Take combustion air air from outside and dump it's fumes outside * no internal change in building pressure. If you take combustion air from inside then dump it outside you will eventually have a building with negative pressure - not good.