A growing system is equipped with a mixing box that features a PLC-controlled cooling coil and a heating coil in series. Gaseous inputs include air and CO2. The coils facilitate condensation and maintain relative humidity and temperature in an acceptable range. An absorption unit or other chilled liquid source supplies chilled liquid for the cooling coil. Engine or boiler combustion provides hot liquid to the heating coil and CO2-containing exhaust. Condensate is collected for reuse. The output to a growing space is a temperate low relative humidity CO2-enriched stream optimal for plant health.