A growing system (e.g., greenhouse) is equipped with a mixing box that features a PLC-controlled cooling coil and a heating coil in series. PLC regulated inputs include air and CO2-containing exhaust. The cooling coils in series facilitate condensation to maintain relative humidity in an acceptable range. Substantial cooling is necessary to achieve sufficient condensation and temperature reduction. Condensate is collected for reuse. The heating coil heats cooled CO2-enriched air to an acceptable temperature for plant health. The output is a temperate low relative humidity CO2-enriched stream optimal for plant health.