Oh, that's really easy. It must be a gas fired water heater. The exhaust pipe (flue) is
held on by sheet metal screws. Just undo the screws and take off the pipe. The unit now exhausts into the room. Natural gas burns very clean, produces CO2 and H2O (water vapor). When the tank turns on (when you take a shower or something), the growing room gets flooded with CO2, which will gradually dissipate of course. I keep a plug in carbon monoxide monitor in the room to warn myself if the tank is producing any CO, which of course is poisonous. The CO monitor has never beeped any alarm in over three years. By doing this, CO2 that normally goes up the chimney, goes into the growing room instead. Inbetween times, the pilot light is burning, and puts out a very small amount of CO2.