The points which affect this are:

a.	the size of the building or room
b.	the purpose for which it is used
c.	the number of occupants
d.	the type of work they are doing
e.	heat gains from other sources, e.g., electric motors and equipment
f.	the amount of steam, dust and odours from production processes
g.	whether the customer has any particular temperature requirement for the building

The table of recommended ACH given in information leaflets, and the more extensive and detailed list given for different types of rooms and buildings is based on a combination of practical experience and theory, and can safely be used as a guide for all normal conditions in this country.

The figures should be doubled for work in hot climates or situations, and increased by 50% if there is a possibility of much tobacco smoke.

How to calculate the number of Ventilation units required for a room:

=	Volume of room in m³ × ACH required        Volume of air moved by one unit in m³/h
e.g.	A workshop 25 × 10 × 4.5m requiring 8 ACH
	Number of 300mm impeller units:	25 × 10 × 4.5 × 8 1415 (medium speed)	= 6.4 (say 6)
	This would be increased to 7 if Roof Mounted fans were used due to the lower output (1305 m³/h on medium speed).