Most office buildings are designed based only upon the outdoor air quantities required for suitable indoor air quality. However, a constant minimum volume of ventilation air is needed for VAV units depending upon occupant requirements. That is why interior zones are frequently air conditioned via an independent system with variable air volume for low or no-load conditions. The building's air filtration needs, pollution sources, service cost, and air resistance effect on energy costs should all be considered in system selection.
Economizer cycles can use outdoor air to cool the spaces, but dry bulb or enthalpy control should be considered in order to optimize this practice for energy cost reduction. Specific zones should also be defined so that the economizer cycle does not increase heating requirements. This is best achieved with separate air distribution systems for each major interior and exterior zone.
Ventilation must also be provided for elevator machine rooms, electrical and communications closets, and electrical and mechanical equipment rooms. Because these are not usually mechanically cooled like the rest of the building, some spot cooling may be needed. It should be noted that mechanical rooms must be thermally and acoustically isolated from any occupied office areas.