Balancing inside and outside air.
The interaction between a building and its climate should be dynamic; the building should be able to respond in a variety of ways to changing conditions. For example, in the San Gabriel Valley, a well-balanced building can take advantage of natural ventilation during eight months of a year using operable windows and filtered outside air circulated through the HVAC system.
In the past, HVAC systems were designed as closed systems – recycling the same conditioned air – and windows were rarely operable. This saved money, but it also created indoor air quality problems. Current systems have requirements to draw outside air into the building. Balancing the requirements for fresh air and interior comfort is complicated. There are several strategies to lessen the energy demand to condition the outside air.
When fresh air comes into the building at 100 degrees it can be slightly cooled if the air being exhausted (at about 80 degrees) can blow across (air-to-air heat exchanger) the fresh air. The incoming air might lose 10 degrees of heat.
Some HVAC systems include an air-to-air evaporatively cooled pre-cooler. In this case, hot fresh air is drawn through an air-to-air heat exchanger where the fresh air passes over the tubes of the pre-cooler and some of the heat is transferred to the pre-cooled air. The different streams of air do not mix.
During the work day in a commercial building, heat is being added from many different sources – the people in the building, the lights, the computers and printers, and the outside temperature. The “internal” loads are often enough to require cooling even when the outside temperature is pleasantly cool. If the HVAC system has an economizer cycle it can draw 100% fresh filtered air through the building when the outside air is below a set temperature, such as 65 degrees. This is natural ventilation that is controlled and distributed by the HVAC.
Depending on the size of the unit, economizers are required in the Energy Code.