A short-chain hydrocarbon combustor and carbon dioxide generator installable into an existing ventilation system venting an interior grow space effects combustion of a gas supply to produce carbon dioxide on demand and maintain a desired atmospheric concentration of carbon dioxide interior the grow space. Exhaust gas producible at a high-efficiency burner is drawn through a boustrophedonic flue and passed through each of a first and second heat exchanger wherein heat is conducted into a vented airflow traveling through the existing ventilation system for discard exterior to the grow space. Condensate precipitated during cooling is collectable at a water trap, and drainable for use in the grow space. Carbon dioxide enriched exhaust gas is thereby cooled by an at least 95% reduction in heat previous to emission into the grow space. Temperature and humidity are thereby controllable within desired ranges and yield maximization enabled.