We have seen that good ventilation must remove vitiated, contaminated and overheated air and replace it with fresher air at such a rate and in such a manner as to provide comfortable and healthy conditions. When dealing with a particular ventilation problem, the basic decisions to be made are:

1 Type of System required i.e., extract, intake, or a combination of both.
2 Ventilation Rate (ACH) necessary for the conditions.
3 Provision for air replacement. Will existing openings be sufficient, or are special arrangements
   necessary?

Extract System

An Extract system is designed to remove foul air, usually at high level unless the fumes are heavier than air, when extraction would take place near floor level. This extraction creates an area of low pressure around the fan causing the fresher replacement air to flow into the space through doors or windows, or through air intake grilles suitably spaced.

This is by far the simplest, most common, economical and effective system for normal ventilation work. It cannot be used if filtered air is required in the room, as any leakage through cracks around doors and windows, or through doors when opened, would be into the room, and it would not be possible to filter all incoming air at these points.

Intake System

An intake, or plenum system blows in fresh air, which mixes with the air already in the room and forces its way out to atmosphere through any available openings. Careful location and speed control of intake fans and evenly distributed air outlet grilles are necessary to prevent draughty conditions, even in warm weather; incoming air may need filtering and/or warming, adding to the expense and difficulties of location of the equipment. Note that if filtered air is required an intake system is essential, and the room would be under a slight positive pressure, so that any leakage of air would be outwards from the room.

Combined System

A combined system using both extract and intake doubles the cost of a scheme, but is more effective than extract only in large offices, say over 15m wide, as controlled mechanical intake can be used to give positive gentle air movement in warm weather to create a feeling of freshness. It also reduces the number and size of openings required in the structure for replacement fresh air.