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What is ACH and How Do You Calculate It?

What is ACH?

ACH, sometimes referred to as ACPH, stands for Air Changes per Hour. It is more commonly referred to as the air exchange rate. ACH is the measurement of the amount of times a volume of air within a space will be added, removed, or exchanged with filtered air. To put it simply: it is how many times the air enters and exits a room through the HVAC (heating, ventilation, and air conditioning) system within an hour.

ASHRAE (the American Society of Heating, Refrigerating and Air Conditioning Engineers) has guidelines for air changes per hour and they naturally vary based on the size of the room. The standards that ASHRAE has set are to ensure that the space is seeing a sufficient amount of air exchanges and providing it with clean air.

The amount of new air that needs to be introduced into a space is highly dependent on the activity level and number of occupants in the space. Your heating and cooling system (HVAC) delivers new air flow into rooms through supply ducts, while removing air through return ducts.

Something to consider with your home’s ventilation is knowing how old your home is. Older homes will tend to have some cracks or leaks which add to the air exchange rate. Newer homes will be more airtight and will need to be monitored more as their only form of air exchange is through the ventilation system. While this new airtight construction is great for managing the temperature of your home, it can make calculating your home’s ACH a little difficult.

How Do You Calculate ACH?

An HVAC technician will be able to determine the ACH using tools. However, you can calculate it on your own as well. Air Changes per Hour is calculated by first determining the volume of the space you will be exchanging air in. Beware: we’re about to get a little mathematically technical on you so break out those high school math skills. First you have to know the air volume of the space and that requires the room dimensions which consist of:

  • Length (L)
  • Width (W)
  • Height (H)

The calculation for the air volume of the space is: Volume = L x W x H (a quick tip just in case you do not know: x means you multiply). Once you’ve calculated the volume, you can determine the air changes per hour. The formula to calculate ACH is: ACH = 60Q / Volume. Q in this formula is the volumetric flow rate of air in cubic feet per minute, also known as cfm—usually the cfm of the filtration device being used. A quick note for this formula: “/” means to divide. To write it out for you fully and in simple terms: The ACH is 60 multiplied by the cfm of your air exchange device, divided by the volume of air in the room.

Ventilation and air change rates are calculated on a per-person basis. Therefore, when the amount of people in the space doubles, the necessary air change rate doubles as well. However, doubling the amount of people in the space does not change the ACH, this just means that you must have an HVAC system that takes into account the activity level of the space.

ASHRAE Recommended ACH

Below are some common types of locations and their recommended, approximate air changes per hour measurement. To clarify how these measurements are interpreted: If a home has an ACH of 1, that means that in that area that the ACH was calculated in, the air of that area was filtered out and changed completely within the hour one time. These recommended ACH measurements are, as I touched on earlier, based on a per-person basis. These measurements are approximate and should not be used to determine your own space’s required ACH. Nevertheless, here are the basic recommendations:

  • Home: 0.35-1
  • Hotel room: 1-2
  • Office: 2-3
  • Retail shop: 2-3
  • School: 5-6
  • Sports facility: 4-8
  • Restaurant: 6-8
  • Hospital room: 4-6

What Causes Insufficient ACH?

There could be a few reasons why the airflow volume into a certain room is not meeting the recommended measurement. These reasons could include:

  • Improper balance of system airflow. Certain rooms could be receiving an excess air volume, causing other rooms’ supply to decrease.
  • Your HVAC system’s blower is not working properly. A broken or damaged blower may not be moving enough air through the ducts so there is not enough for a good air exchange.
  • Damaged ducts. In order for there to be an accurate reading of the air changes, there must be working ducts that are moving the air. Some ducts may be leaking air.

Why is ACH Important?

The ACH is an important concept to know and understand as a homeowner. It’s a vital factor when it comes to your HVAC system’s heating and cooling abilities. If the air exchange within your home is not sufficient, allergens, pollutants, and irritants can build up and have serious effects on your indoor air quality, which can cause problems for you and the people living in your home. As ACH increases, the likelihood of exposure to airborne viruses and harmful pathogens decreases.

We spend so much time in our homes so it is best to keep up with how the air quality within your home is doing. It is crucial that you maintain your HVAC system to avoid any issues with ACH. The air change rate is managed through natural and mechanical ventilation. Some things you can do to restore and maintain the air quality of your home:

  • Eliminate (to the best of your ability) sources of pollution within the home, such as furnishings or air fresheners.
  • Open windows or doors to naturally increase air change rate within your home.
  • Regularly check to ensure all ventilation equipment is running efficiently and safely.

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