Abstract: A building's Heating Ventilating and Air Conditioning (HVAC) system is made more energy efficient and the indoor air quality (IAQ) of the building's circulating air is improved by incorporating a greenhouse as an integral part of the HVAC system and by utilizing a novel feed forward control strategy that maintains the proper levels of temperature, humidity and CO2 concentration in the building under varying conditions of day, time, use and occupancy. A portion or all of the exhaust air from the building is discharged into a greenhouse where the heat and humidity are recovered in the winter by heating the greenhouse and the air conditioning is recovered in the summer by cooling the greenhouse. Selected plants are used in the greenhouse to remove CO2 and pollutants from the building's exhaust air while enriching the air with oxygen and beneficial negatively charged ions.