Ventilating [V in HVAC] is the process of changing or replacing air in any space to provide high indoor air quality to control temperature, replenish oxygen, or remove moisture, odors, smoke, heat, dust, airborne bacteria, and carbon dioxide.
Ventilation is used to remove unpleasant smells and excessive moisture, introduce outside air, to keep interior building air circulating, and to prevent stagnation of the interior air.
Ventilation includes both the exchange of air to the outside as well as circulation of air within the building. It is one of the most important factors for maintaining acceptable indoor air quality in buildings. Methods for ventilating a building may be divided into mechanical forced and natural types.
Mechanical or forced ventilation is used to control indoor air quality. Excess humidity, odors, and contaminants can often be controlled via dilution or replacement with outside air. In humid climates much energy is required to remove excess moisture from ventilation air.
Kitchens and bathrooms typically have mechanical exhaust to control odors and sometimes humidity. Kitchens have additional problems to deal with such as smoke and grease.
Factors in the design of such systems include the flow rate and noise level. If ducting for the fans traverse unheated space like the attic, the ducting should be insulated as well to prevent condensation on the ducting. Direct drive fans are available for many applications and can reduce maintenance needs.
Ceiling fans and table or floor fans circulate air within a room for the purpose of reducing the perceived temperature because of evaporation of perspiration on the skin of the occupants. Because hot air rises, ceiling fans may be used to keep a room warmer in the winter by circulating the warm stratified air from the ceiling to the floor. Ceiling fans do not provide ventilation as defined as the introduction of outside air.
Natural ventilation is the ventilation of a building with outside air without the use of a fan or other mechanical system. It can be achieved with open-able windows or trickle vents when the spaces to ventilate are small and the architecture permits.
In more complex systems warm air in the building can be allowed to rise and flow out upper openings to the outside, thus forcing cool outside air to be drawn into the building naturally through openings in the lower areas.
These systems use very little energy but care must be taken to ensure the occupants’ comfort. In warm or humid months, in many climates, maintaining thermal comfort solely via natural ventilation may not be possible so conventional air conditioning systems are used as backups.
Air side economizers perform the same function as natural ventilation, but use mechanical systems’ fans, ducts, dampers, and control systems to introduce and distribute cool outdoor air when appropriate.
Ventilation is the intentional movement of air from outside a building to the inside. Ventilation air is used for providing acceptable indoor air quality. It mustn’t be confused with vents or flues; which means the exhausts of clothes dryers and combustion equipment such as water heaters, boilers, fireplaces, and wood stoves. The vents or flues carry the products of combustion which have to be expelled from the building in a way which does not cause harm to the occupants of the building. Movement of air between indoor spaces, and not the outside, is called transfer air.
Types of ventilation
- Mechanical or forced ventilation: through an air handling unit or direct injection to a space by a fan. A local exhaust fan can enhance infiltration or natural ventilation, thus increasing the ventilation air flow rate.
- Natural ventilation: occurs when the air in a space is changed with outdoor air without the use of mechanical systems, such as a fan. Most often natural ventilation is assured through operable windows but it can also be achieved through temperature and pressure differences between spaces. Open windows or vents are not a good choice for ventilating a basement or other below ground structure. Allowing outside air into a cooler below ground space will cause problems with humidity and condensation.
- Mixed Mode Ventilation or Hybrid ventilation: utilizes both mechanical and natural ventilation processes. The mechanical and natural components may be used in conjunction with each other or separately at different times of day. The natural component, sometimes subject to unpredictable external weather conditions may not always be adequate to ventilate the desired space. The mechanical component is then used to increase the overall ventilation rate so that the desired internal conditions are met. Alternatively the mechanical component may be used as a control measure to regulate the natural ventilation process, for example, to restrict the air change rate during periods of high wind speeds.
- Infiltration: is separate from ventilation, but is often used to provide ventilation air.