thanx for all the feed back. ive done a few ajustments to my cabnet so i get all the CO2 i can outa the heater i ran some new ducting, the wieght of CO2 is only slightly higher than just reg air. air, dry 1.2929, carbon dioxide 1.977 in the water heater runs a constant vaccuum of -0.02" on just pilot n -0.12" at full flame. so that should be more than enough to pull the CO2 up n out the exhaust. i turned the heater off and just ran the fan and it pulled a constant of -0.10" on high and -0.08" on low. if any of my math or anything else looks alil off please feel free to let me know.ill post some pics some of the set-up so any feed back is more than welcome. thanx