What is the Minimum Outside Air in an HVAC system?
The limit of fresh airflow into a recirculating system is known as the minimum outside air. This helps to ensure that interior air remains safe and comfortable to breathe. The amount of outside air required depends on the occupancy of the space, the activities taking place, and the ventilation rate.
How much outside air is required?
In general, the minimum outside airflow rates for different occupancy types are as follows:
- Assembly areas: 4-15 CFM per person.
- Classrooms: 3-8 CFM per person.
- Office spaces: 5-20 CFM per person.
- Storage areas: 0.5 CFM per square foot.
The American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE) provides standards for ventilation rates that take into account the above factors. These rates can be used as a guide when determining the minimum outside airflow rate for a space.
It is important to note that the minimum outside air flow rate is not the same as the fresh air intake rate. The fresh air intake rate includes make-up air that is used to replace the air that has been exhausted from the space, in addition to the minimum outside air flow rate.
Fresh air requirement per person ASHRAE
The information below shows the fresh air requirements per person as prescribed by ASHRAE Standard 62.1-200. Activity Level Outdoor Airflow Rate (CFM):
- Light – 20.
- Medium – 40.
- Heavy – 60.
The above rates are for general guidance only and may need to be adjusted depending on the specific application. For example, spaces with high levels of contaminants or odors may require higher ventilation rates.
Do all HVAC systems bring in outside air?
Yes, all HVAC systems are required to bring in some amount of outside air. The amount of outside air that is brought in can be controlled through the use of an economizer. An economizer is a device that controls the flow of outside air into the system. By controlling the amount of outside air that is brought into the system, the economizer can help to save energy and reduce costs.
What are the benefits of bringing in outside air?
There are several benefits to bringing in outside air:
- It helps to improve indoor air quality by diluting contaminants that are present in the air.
- It helps to regulate temperature and humidity levels.
- It provides fresh air for occupants to breathe.
- It helps to reduce odors.
- What are the risks of bringing in outside air?
- There are also some risks associated with bringing in outside air:
- It can introduce contaminants into the air.
- It can increase the amount of energy that is required to heat or cool the space.
- It can cause discomfort for occupants who are sensitive to temperature changes.
How do you calculate minimum ventilation requirements?
The minimum ventilation requirements can be calculated using the following formula:
- Ventilation rate = (Minimum outside air flow rate *Occupancy) /Air change rate.
For example, if the minimum outside air flow rate is 10 CFM per person and the occupancy of the space is 100 people, the ventilation rate would be 1000 CFM. If the air change rate is 4 times per hour, the minimum ventilation requirement would be 250 CFM.
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