Chapter 3: Ventilation and Thermal Comfort

3.1 Introduction

The HVAC systems of commercial and residential buildings are designed to ensure both adequate ventilation and occupant comfort. The volume flow rate of air through the HVAC system of a building is found from the largest of either the ventilation requirement or the sensible heating/cooling load. The ventilation requirements usually govern the minimum supply air conditions in variable-air-volume systems at low sensible and latent load. The difference between the ventilation flow rate and the cooling/heating flow rate is met by reconditioning of the return air.

The consideration of ventilation relates to the health of the occupants, which is protected by adequate ventilation of the building with outdoor air. This is needed since indoor air can be contaminated by emissions from occupants, processes, building materials, and furnishings. The emissions result in elevated levels of contaminants, such as carbon dioxide, volatile organic compounds (VOCs), particulate matter, inorganic gaseous combustion products such as CO and NO, radon progeny, formaldehyde, pesticides, and aerosols. The indoor contamination is removed by either a mixing or displacement ventilation approach. In this Guide, the discussion is centered on the mixing ventilation approach. With mixing ventilation, contaminated indoor air is diluted with cleaner outdoor air brought in by the HVAC system. The mixing and dilution take place at the central air handler and near the supply jet outlet in the unoccupied zone of the room. A portion of the diluted gases is exhausted from the building, and the unexhausted gases are returned to the main air handler.