How to Calculate Air Changes Per Hour

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Proper air ventilation, as measured by air changes per hour (ACH), is essential for comfortable living inside buildings that are generally designed to be airtight, which includes most modern residential and commercial structures. This is particularly true in an era in which airborne viruses have been responsible for the COVID-19 pandemic that has sickened and killed millions of people worldwide since early 2020.


The American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) sets standards for indoor air circulation, and the ACH is higher for commercial buildings than it is for homes because commercial buildings tend to have higher occupancy rates. ASHRAE requires the highest ACH for restaurants and schools and the lowest for residences. It is calculated using a simple formula.

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Why Do You Need to Know Air Changes per Hour?

ACH is sometimes called the air exchange rate and is defined as the number of times per hour the entire air in a room or enclosed space is completely replaced with fresh or recirculated air. This important number is a measure of the efficiency of the ventilation system in a room, which can be affected by the use of filters that tend to slow air circulation.


Poor air circulation can cause a number of health conditions, such as respiratory problems, headaches, dizziness, nausea, and even heart disease and cancer in extreme situations. It also contributes to the spread of airborne pathogens, such as the coronavirus that causes COVID-19. The best way to guard against such health hazards is to design and maintain ventilation systems and equipment to guarantee the minimum ACH recommended by ASHRAE for the space.


The Air Change Calculation Method

The ACH calculation depends primarily on two parameters: the amount of air the HVAC equipment moves, measured in cubic feet per minute (CFM) and the volume of the space. CFM is a characteristic of the HVAC equipment in question and can be obtained in the product specifications, while volume can be calculated by measuring the room dimensions.


To calculate the volume (V) of a room, measure its length, width, and height in feet, multiply them, and express the result in cubic feet. You then multiply the CFM of the system by 60 and divide the result by the room volume. As a mathematical formula, this is expressed as: ACH = (CFM • 60) / V.

Sizing Bathroom Exhaust Fans

Bathrooms have higher ventilation requirements than other parts of a house owing to the high humidity and potential for moisture damage. The Home Ventilating Institute recommends about eight air changes per hour, and the size of the exhaust fan (measured in CFM) needed to achieve that depends on the size of the bathroom.


Instead of rearranging the general ACH formula to calculate CFM, the procedure for bathrooms is to calculate the floor space and base the size of the fan on the assumption that you need 1 CFM for every square foot. By this method, a smallish 8 x 5 bathroom with a floor space of 40 square feet would need a fan that moves at least 40 CFM, while a larger 10 x 10 bathroom would need a fan that moves at least 100 CFM. The calculation isn't always this simple in bathrooms larger than 100 square feet because allowances have to be made for each toilet, shower, and bathtub, but that's the general idea.



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