I have a recipe for homemade CO2 production, if you want me to write it out just reply..... It's not free ( but it's pretty F@#$ing cheap! ) and it's only good for 7-10 days at a time. Of course it's no competition to tanks but in a chamber or even a closet it DOES help. The other thing I wanted to mention was your intake and exhaust. Beyond the possible CO2 loss from having the exhaust below the house is the threat of alien invaders! I live in a fairly wooded area and from time to time I have had to waste my time and money destroying entire populations of various little buggers. Try a find some form of screen/coarse foam/cloth filter to keep them at bay, if that "mite" be an issue for you. Another cool product that I noticed is for (exhaust) dryer vents. It's just a section of 4" pvc (?), straight or 90 degree angle, with a small plastic ball that rests inside a small cage at the exit........ air out, ball up, no air, ball down..... Oh shit, it went out, gotta go-