yup yup... for a small grow i considered just burning a candle, oil lamp, or rubbing alcohol. with the water heater i just ran the exhaust into the grow room. its no good for maintaining optimum levels but its a big boost. and i havnt seen any negative effects so far, im worried the 4K+ ppm of CO2 might be stressful even though it doesnt last long, avg ppm is probably 800-1000. it spikes to 5K 1-2 times a day but comes back down to 2K within 2 hours and continues to drop to about 600
the water heater is 55 gallons and the total area of the basement where the CO2 is mostly spread out is about 16'x20'. area of plants consuming CO2 is about 45 square feet plus some leakage into the rest of the house