I just use fan's to supply Co2. House is fairly small, like you say an 8x8 is going to get up there from a few hours work.. I personally overload the fans.. Gas bottles really don't do much when I tried Andy. They didn't work for friends either. It just cost more.. We don't have the heat here to use it as a gas like you mate.. Heating the grow all week to use it is nonsense.. Might as well just give them air. If they need anything they can get it from gaseous exchange.. With all the nasty critters where you are i'd have the room shut!