Functions & requirements for ventilation

From the viewpoint of working conditions & comfortable living, good and efficient ventilation should meet various functional requirements.

Strategies for ventilation

The first strategy for ventilation is to eliminate the pollutant source. Because the source control is the most effective and preferred method of providing good indoor quality for people in most cases.

And this is usually done via ventilation, i.e. usually increasing ventilation rates.

Rate of supply of fresh air

The quantity of fresh air to be supplied to a room depending on the use of a building.

The rate of supplying fresh air is decided by considering several factors such as a number of occupants, type of work, period of working, the age of occupants, etc.

Air movement (or air changes)

Air has to be moved or changed to cause proper ventilation of the space.

The rate of air change depends on the number of persons inside, room temperature, nature of the work, different room categories, and activities, etc.

Temperature of air

It is desirable that the incoming air for ventilation should be cool in summer and be warm in winter - before it enters the room.

Also the velocity of air is very important, including ventilation strategy, distribution pattern, etc.

Humidity

The air contains certain amount of humidity.

Air quality

It plays a significant role in the comfort of people affected by a ventilation system.

Natural or mechanical?

The air exchange from outside to inside of buildings happens in these ways - naturally or mechanically:

  • Unintentional via air leakages from windows, doors, and various holes such as joints and cracks in the building envelope.
  • Intentional via natural ventilation, through opening windows/doors and air vents for deliberate venting by people and for people.
  • Proper mechanical/hybrid ventilation, to add additional and controllable airflow within buildings to provide a good indoor environment.

 

Ventilation is typically described as separate from infiltration.

Infiltration is the circumstantial flow of air from outdoors to indoors through leaks (unplanned openings) in a building envelope. When a building design relies on infiltration to maintain indoor air quality, this flow has been referred to as adventitious ventilation.

Infiltration is often called air leakage. The leakage of room air out of a building, intentionally or not, is called exfiltration.

Infiltration is caused by wind, negative pressurization of the building, and by air buoyancy forces known commonly as the stack effect.

Controlling infiltration

Because infiltration is uncontrolled, and admits unconditioned air, it is generally considered undesirable except for ventilation air purposes.

Typically, infiltration is minimized to reduce dust, to increase thermal comfort, and to decrease energy consumption.

For all buildings, infiltration can be reduced via sealing cracks in a building's envelope, and for new construction or major renovations, by installing continuous air barriers.

In buildings where forced ventilation is provided, their HVAC designers typically choose to slightly pressurize the buildings by admitting more outside air than exhausting so that infiltration is dramatically reduced.