Does Stacking Fans Double the CFM?

The idea seems simple - if more air is desired to move through a unit, stack another fan on top of the first one to increase airflow - but is this how it really works? This article will answer a common question we receive: does stacking fans double the CFM (cubic feet/minute)?

In short, stacking 2 (or more) fans will not double the airflow. Here are a few principles that explain why:

  • The same amount of air is going from the first fan to all subsequent fans, and the air is traveling off the first fan’s blades at approximately a 45-degree angle, which increases turbulence on the intake side of any extra fans.

  • Stacking fans increases the static pressure, which may cause difficulties in more closed systems like a computer case. Creating high pressure will make it more difficult for fans to fight this pressure to get air across the fins - this can be minimized by removing panels/increasing openings in the case.

  • To counter some turbulence of stacking fans, you would have to pair a clockwise rotating fan with a counterclockwise rotating fan (such as the counterclockwise-rotating EBM Papst 700F series) - note the performance yields from this are varied and typically aren’t any more than about 20-30%.

Stacking fans directly on top of each other generally increases turbulence, static pressure, and cost without having a definite positive impact on CFM and cooling.

Ideas to increase CFM would include:

  • Purchase a fan that has more CFM
  • Put two fans side by side, they should move roughly twice the amount of air that a single fan is rated at.

Here’s a video that shows some of the principles discussed above:

If you have any comments or questions about this article, feel free to post your responses below.

As a general rule, two fans in series (one behind the other) will nearly double static pressure capability but will not significantly increase air flow in a low impedance airflow system. Two fans in parallel (positioned side by side) will approach double the air flow capability but will not increase it very much in high impedance airflow systems.

However, in an enclosure system with high system airflow impedance (minimal venting, multiple obstacles, baffles, etc.) the higher static pressure provided by two fans in series can increase air flow relative to a single fan.

Here is a good source from NMB for further reading on the topic.