Heating, Ventilation and Air-Conditioning (“HVAC”) are standard in virtually every modern building. Indeed, HVAC is often one of the largest parts of the entire energy budget of most buildings. This is particularly the ease in extreme climates, both hot and cold. One of the goals of HVAC systems is to provide comfortable and healthy environment for building occupants, in terms of temperature, humidity, composition and cleanliness of air.
Central HVAC systems in buildings typically include one or more central air handling unit(s) and an air distribution system, where supply air is directed to various parts of a building through a network of ducts, and return air flows from these spaces, through other ducts or a plenum, back to the air handling unit(s). In the air handling unit, the air is cooled and/or heated, as well as filtered and often dehumidified and/or humidified, as needed. Thus, HVAC systems constantly circulate air through the building while continually adjusting its temperature and humidity to maintain comfortable environment.
However, in order to maintain good air quality, not all the air is recirculated. Some of the air leaks out through doors, windows, etc. and some fraction of the circulating air is intentionally exhausted outside the building. This is referred to as exhaust air. The exhaust air is replaced by an intake of outside air, also known as makeup air, to make up for the exhaust air being taken out. This is also referred to as “fresh air” or ventilation. This replacement of air is done because occupants of the building and the equipment consume oxygen and emit carbon dioxide (CO2) as well as a variety of other contaminants that gradually compromise quality and safety of the air. Such replacement of the air maintains fresh air quality.
Oxygen represents approximately 21% of atmospheric air and that is normally the desired level of indoor air as well. On the other hand CO2 is present only in very low levels in the outside air, typically at a level of approximately 400 parts per million (“ppm”). Once elevated levels of CO2 or reduced levels of oxygen are created, a fairly significant amount of outside air is needed to bring their respective concentrations close to the desired level. Indeed, to hilly restore oxygen and CO2 concentration virtually all the air may need to be replaced.
The outside air represents an additional, and depending on the outside climate conditions often a significant, thermal load on the air handling unit. In the case of a hot and humid climate, for example, the outside air injected into the HVAC system can require additional energy for cooling and dehumidifying the outside air and can represent a significant fraction of the entire thermal load and energy usage of the HVAC system.
The amount of exhaust air and outside air can be adjusted to meet the air quality standards. Certain minimum amounts of levels of oxygen, CO2 and other contaminants, a variety of organic gases collectively referred to as volatile organic compounds or VOCs, are often set to maintain air quality. In the USA, the American Society of Heating, Refrigeration and Air-conditioning Engineers (“ASHRAE”) issues guidelines, including the ASHRAE Standard 62, for the amount of outside air ventilation recommended for a given space and number of occupants. However, the greater the rate of air replacement, the more energy is consumed by the HVAC system.