IS MORE CFM IN HVAC AND VENTILATION ALWAYS BETTER?

by Suncourt Inc.

When choosing a ventilation product, many people assume that higher CFM (cubic feet per minute) is always better. After all, more airflow should mean better performance, right? Not necessarily.  A properly sized fan efficiently moves conditioned air or removes excess moisture, odors, and pollutants. However, an oversized fan can move more air than necessary, leading to higher energy consumption and unintended potential problems.

 

When using a duct booster with too much CFM in an HVAC system, several issues can arise:

Imbalance in Airflow: The booster fan may create excessive pressure in certain parts of the ductwork, leading to uneven heating or cooling in other rooms.

 

Increased Static Pressure: Too much airflow can raise the static pressure in the system, making it harder for the blower to function efficiently.

 

Reduced System Efficiency: Too much CFM can actually cause the system’s efficiency to decrease and in-turn cause it to work harder than necessary.

 

Potential Damage to Ductwork: If the ductwork is not designed to handle the increased airflow, it could lead to leaks, vibrations, or even damage over time.

 

Short-Cycling: If the increased airflow leads to rapid temperature changes, the HVAC system could turn on and off more frequently, reducing efficiency and possibly the lifespan of the system.

 

When it comes to exhausting, a powerful fan with too much CFM may create dangers by pulling combustion gases (from furnaces, water heaters, or fireplaces) back into the home instead of letting them vent outside, posing a serious safety risk. It may also cause unwanted air infiltration. Instead of fresh air coming through a designated intake, air may be pulled from undesirable sources, such as attics, crawl spaces, or gaps around doors and windows.

 

In both cases, higher CFM also often means a larger, more powerful motor, which will normally be significantly louder, creating an unnecessarily loud and disruptive environment.

 

Choosing the Right CFM

Larger spaces require more CFM to ensure proper air circulation and temperature control.  Smaller spaces require less CFM to avoid over-conditioning and energy waste.  Instead of opting for the highest CFM available, select a fan based on the room size and usage.

Something you will need to consider is Air Changes Per Hour (ACH).  This refers to the number of times the total volume of air within a space is replaced with fresh or conditioned air in one hour. ACH matters because it ensures proper ventilation and fresh air supply and

is a vital metric for ventilation and indoor air quality.

Formula for required CFM:

  • CFM = Cubic Feet per Minute (airflow rate of the HVAC system or component)
  • Volume = Room volume in cubic feet (length × width × height)
  • 60 = Converts minutes to hours 

Helpful Rule of Thumb

  • 1 CFM per square foot (for standard 8-ft ceilings)
  • Multiply by 1.25 for 10-ft ceilings
  • Multiply by 1.5 for 12-ft ceilings

While a high-CFM ventilation fan or booster might seem like an easy way to improve air quality and comfort, bigger isn’t always better. By choosing the right fan size for your space, you’ll get the benefits of comfort and safety without the downsides of excessive airflow.

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