Fresh air calculation as per ASHRAE 62.1

Thermal comfort is the basis of building homes from the beginning, mainly to protect oneself from the harsh climate conditions outside. Today, buildings are advanced enough to control the ventilation of a building. In order to maintain that, fresh air is required for any HVAC system. Fresh air is the air outside as opposed to that within a room or other enclosed space.

Why is it important?

Fresh air is a necessity for both physical and mental health. But unfortunately most of us, due to living and working in confined areas, don’t get the optimal amount of fresh air needed.
In some buildings, if indoor air is recirculated, due to which impurities may be breathed in again and again which include indoor pollution, and higher level of carbon dioxide levels. This creates poor indoor air quality. To avoid this, fresh air is supplied as per the number of occupants and the area of the space. 

Ever wondered how much air a human requires per second?

Average Fresh air requirement per person per second is 8 litres! This is how much air we breathe!
If we don’t get this much fresh air, we may experience fatigue, drowsiness & dullness of mind.
You may have heard of Sick Building Syndrome (SBS). This refers to a number of ailments that occur as a result of exposure to harmful chemical toxins at a home or work building. The main conditions for experiencing Sick Building Syndrome are spending long periods of time in well-sealed, poorly ventilated buildings that contain indoor air toxins.

Fig. Reference building

Fresh air calculation

The ventilation rate procedure is widely used, since it involves standardized calculations that are well-known in the HVAC industry. Based on air pollution research, ASHRAE has determined ideal ventilation rates for each type of building, which are presented per square foot and per occupant:

• The design airflow per unit area is expressed in cfm per square foot.
• The occupancy component is expressed in cfm per person.

Vbz = Rp · Pz + Ra · Az

Az = zone floor area: the net occupiable floor area of the ventilation zone ft2 (m2)
Pz = zone population: the number of people in the ventilation zone during typical usage.
Rp = outdoor airflow rate required per person as determined from Table 6-1
Ra = outdoor airflow rate required per unit area as determined from Table 6-1

Fig. Table showing cfm per sq. ft and per person values from ASHRAE 62.1

For example, the minimum ventilation rate for a restaurant is 0.18 cfm/sq.ft. and 7.5 cfm/person. If the area is 5,000 sq.ft and the restaurant is designed for 200 persons, the required ventilation rate is the following:

• Area component = 0.18 cfm/sq.ft x 4,500 sq.ft = 810 cfm
• Occupancy component = 10 cfm/person x 300 persons = 3,000 cfm
• Total airflow = 810 cfm + 3,000 cfm = 3810 cfm

This value is called the Breathing Zone Outdoor Airflow in ASHRAE Standard 62.1. It is then divided by the Zone Air Distribution Effectiveness, to obtain the Zone Outdoor Airflow that must be supplied by the ventilation system. If the distribution effectiveness for the example above is 0.8, the zone outdoor airflow must be 4762.5 cfm (3810 cfm / 0.8).