# How to estimate a required outside air (fresh air) for each space?

588 views asked Apr 20, 2015

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ASHRAE 62.1 (Ventilation) table 6-1 indicates the minimum required ventilation rate (fresh air or outside air-OSA) for acceptable indoor air quality (IAQ) for each space. Also ASHRAE 62.2 indicates the minimum ventilation rate for the low-rise residential buildings.

Indoor air quality (IAQ) includes Air temperature, Humidity, Noise, Lighting and Physiological factors.

The intension of providing OSA to space.is to minimize the adverse health effects in human. Indoor air quality (IAQ) requirement is based on chemical, physical and biological contaminates but may not all be achievable in all buildings.

Calculation of required OSA:

In Table 6-1 in ASHRAE 62.1 the amount of OSA is indicated per person (CFM /Person) and also per square foot (CFM /SF) of space. To achieve the required ventilation (OSA) amount, both of the factors in table 6-1 must be considered for estimating the total required OSA for space:

Total required OSA = N x (CFM /Person) + A x (CFM /SF)

N= Number of people in space

A= Area of the space in Square Foot

Example:  For 3,000 SF office with 20 people, how much OSA is required?

From Table 6-1 in ASHRAE 62.1 for office space:

The rate for occupants is-   5 CFM /Person and 0.06 CFM /SF

Total OSA = 20 x 5 + 3,000 x 0.06 = 280 CFM