Air changes per hour, or ACH, is a measurement of the air exchange rate in a specific room or space. It is an important factor in determining the air quality and ventilation in a room and is often specified in building codes and regulations. In this article, we will discuss the ACH calculation, how it is used in architecture and building design, and provide a table chart of common ACH specifications.
The ACH calculation involves determining the volume of a room and the rate at which air is replaced within that space. The resulting number, usually expressed in ACH, indicates how many times the air in the room is replaced in one hour. This is important for ensuring that fresh air is circulated and stale air is removed, helping to maintain good indoor air quality and prevent the buildup of pollutants.
In architecture and building design, the ACH specification is used to ensure that the ventilation system is sufficient for the size and usage of the space. Different types of rooms and spaces may have different ACH requirements, such as offices, hospitals, schools, and residential buildings. It is important for architects and engineers to consider the ACH specification when designing a building to ensure that the ventilation system meets the necessary requirements.
ACH Rate by Room Type
The ACH rate, or air exchange rate, is an important factor in determining the ventilation and air quality in a specific room or building. Different types of rooms and buildings may have different ACH requirements, as outlined in the chart provided earlier. The air change rate has units of 1/time. When the time unit is hours, the air change rate is also called air changes per hour (ACH). The ACH can vary widely depending upon what is going on inside the building. For example, it is generally considered that 4 ACH’s is the minimum for good indoor air quality.
Below is a table chart of common ACH rate specifications for various types of rooms and spaces:
| Room Type | ACH Rate |
|---|---|
| Residential | 3-5 ACH |
| Office | 6-10 ACH |
| School | 10-15 ACH |
| Hospital | 15-20 ACH |
HVAC contractors use “changes per hour” ranges to calculate the amount of airflow required in different rooms to maintain proper ventilation and air quality. It is recommended to have good air changes per hour for maintaining proper ventilation and air quality.
Some health care professionals suggest a good ventilation system will exchange the air at least 3-4 ACH in indoor settings, with 6 ACH being ideal. For cleanrooms that comply with ISO Classes 4-9, hourly air changes per hour are sufficient to express how many times the air in the cleanroom is replaced.
For example, offices typically have an ACH rate of 6-10, while hospitals may require a higher rate of 15-20 ACH. It is important for architects and engineers to consider the ACH specification when designing a building to ensure that the ventilation system meets the necessary requirements for the specific type of space.
Due to variations in state building codes, 15 or 20 air changes per hour (ACH) may be the minimum required. However, in practice, it is recommended to have more than the minimum required. For schools, ASHRAE and the CDC recommend having a portable HEPA air purifier that provides a minimum of 2 ACH in an hour, with the goal of 6 ACH.
It is worth noting that these are general guidelines and the specific ACH specification may vary based on factors such as the size of the space, the number of occupants, and the level of air pollution. These factors into account when calculating the ACH for a specific room or building to ensure that the ventilation system is sufficient to maintain good indoor air quality. It is important to consult with a professional or refer to local building codes and regulations to determine the appropriate ACH specification for a specific space.
Calculating ACH
To calculate the ACH for a specific room or space, you will need to know the volume of the room and the air exchange rate. The volume of the room can be calculated by multiplying the length, width, and height of the space. Once you have the volume, you can use the ACH specification for the room type specified in the chart above or determined by local building codes and regulations to determine the appropriate air exchange rate.
To illustrate, let’s say we want to determine the ACH for a residential living room. From the chart above, we know that the ACH specification for a residential space is 3-5 ACH. If the volume of the living room is 2,000 cubic feet, we can use the following formula to calculate the ACH:
ACH = (Air Exchange Rate / Room Volume) x 60 minutes
Plugging in the values, we get:
ACH = (3 ACH / 2,000 cubic feet) x 60 minutes
This results in an ACH of 0.9, which falls within the recommended range of 3-5 ACH for a residential space.
It is worth noting that the ACH calculation is based on the volume of the room, not the floor area. The floor area, or the total square footage of the space, does not necessarily reflect the volume of air in the room. Therefore, it is important to measure the dimensions of the space and calculate the volume when determining the ACH, rather than relying on the floor area alone.
It is also important to consider other factors that may affect the ACH, such as the number of occupants in the space, the level of air pollution, and the use of air purifiers or other ventilation systems. These factors may require adjusting the ACH specification to ensure proper air exchange and maintain good indoor air quality.
Room in ACH Calculation
The room or space being considered is an important factor in the ACH calculation. As mentioned earlier, the volume of the room is used in the calculation to determine the air exchange rate. The volume of the room is determined by the length, width, and height of the space, and is important in ensuring that the ventilation system is sufficient for the size of the space.
In addition to the volume, the use and purpose of the room should also be taken into account when determining the ACH specification. Different types of rooms may have different ACH requirements based on the level of air pollution and the number of occupants. For example, an office space may have a lower ACH requirement compared to a hospital room, as hospitals typically have a higher level of air pollution due to the presence of sick individuals.
The velocity of the air in the room is also an important factor to consider when determining the ACH specification. The velocity of the air refers to the speed at which the air moves through the space. A higher velocity can help to increase the ACH, as it allows for more efficient air exchange. However, it is important to balance the velocity with other factors, such as the comfort of the occupants and the noise level of the ventilation system.
In summary, the room or space being considered is an important factor in the ACH calculation. The volume, use, and velocity of the air in the space should all be taken into account when determining the appropriate ACH specification to ensure proper ventilation and maintain good indoor air quality.
Time (Per Hour) in ACH Calculation
The unit of time used in the ACH calculation, the hour, is an important factor to consider when determining the air exchange rate in a specific room or space. The ACH calculation involves multiplying the air exchange rate by the number of minutes in an hour to determine the number of times the air in the room is replaced in that time period.
For example, if the air exchange rate for a room is 2 ACH, this means that the air in the room is replaced twice in one hour. If the air exchange rate was increased to 4 ACH, the air in the room would be replaced four times in one hour, resulting in a higher level of ventilation.
The length of time used in the ACH calculation can also impact the efficiency of air purifiers and other ventilation systems. If the air in the room is replaced more frequently, it may be easier for air purifiers to remove pollutants and maintain good indoor air quality. On the other hand, if the air in the room is not replaced frequently enough, pollutants may accumulate, leading to poor air quality.
The hour is an important unit of time to consider in the ACH calculation, as it determines the frequency of air exchange in the room. The length of time used in the calculation can impact the efficiency of ventilation systems and the air quality in the space.
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