So a friend of mine that used to grow back in the day told me he used to run a airline from above his propane hot water heater to collect co2 from it? Anyone ever hear of this?
You have to be concerned about CO. Gas water heaters in a closet vent the gas outside, not in. If your grow room is connected to living space, I wouldn't do it. Get at least 1 CO detector in the main area/hall of your house either way.