HVAC units aren’t “one size fits all.” When installing a new HVAC unit, it’s important to choose one that is appropriately sized for the property it will serve. A unit that’s too small will have to run constantly and will struggle to cool the space. Conversely, one that’s oversized may be prone to short cycling. Both of these issues can shorten the lifespan of the unit and may waste energy. Correctly assessing a residential or commercial space and choosing the right unit for that space is a key part of HVAC jobs.
HVAC technicians learn how to calculate cubic feet per minute (CFM) and how that relates to HVAC performance as part of their HVAC training and certification. This figure helps ensure even and consistent airflow, and good HVAC performance.
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What is CFM in HVAC?
CFM stands for cubic feet per minute, and refers to the volume of air that passes through a space each minute. The bigger the space, the more air flow is needed to ensure even heating or cooling, and healthy airflow. If the HVAC unit isn’t able to produce sufficient airflow, this could result in uneven heating or cooling, and may also lead to poor indoor air quality.
How do You Calculate CFM?
In general, an HVAC unit can produce about 400 CFM per ton. So if you have a large home that requires a 1,600 CFM, you’d need a 4 ton HVAC unit. There are exceptions to this rule of thumb, and individual manufacturers will provide information about the performance of their units.
When you’re calculating CFM, you’ll want to take into account the number of air changes per hour (ACH). This figure refers to how many times the air in the property is completely replaced per hour. The standard goal for residential properties is:
- 35 or more air changes per hour
- At least 15 CFM per person
- Higher ACH rates in rooms where humidity is a potential issue
This helps ensure good indoor air quality.
The recommended ACH varies depending on the room, too. A basement or other rarely-used room can have a lower ACH rate, while you’d want a higher ACH in the kitchen or bathroom to remove humidity and food odors.
The standard formula for calculating CFM is:
- CFM = Area of the room (sq. ft.) x Height of the celing (ft.) x ACH / 60
Once you know the CFM, you can choose an appropriate tonnage HVAC unit.
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The Environmental Technical Institute offers an HVAC training program that equips graduates to install, maintain and repair residential and commercial HVAC units. If you’re interested in becoming an HVAC technician, this program will give you both the hands-on technical skills and the understanding of key HVAC science concepts like CFM calculation you need to hit the ground running in a new job. Enroll in the ETI HVAC/R program today to gain practical skills and prepare yourself for a job in this growing industry.
Key Takeaways
What is CFM in HVAC?
CFM (Cubic Feet per Minute) measures the volume of air an HVAC system moves each minute and helps ensure proper airflow, comfort, and indoor air quality.
Why is calculating CFM important?
Proper CFM calculations help HVAC technicians select the right-sized system, preventing issues like uneven heating or cooling, short cycling, energy waste, and reduced equipment lifespan.
How is CFM calculated?
The basic formula is: CFM = Room Area × Ceiling Height × Air Changes per Hour (ACH) ÷ 60. This helps determine the airflow needed for a space and the appropriate HVAC unit size.
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