HVAC Fan Laws - The Ultimate Guide (With Equations & Charts)

HVAC is a very important part of your home and you want to make sure that it’s always working properly. Fortunately, physics governs what happens to the fan curve when one factor, like speed, is changed. You may design your system to handle the change by using one of three fan law equations to forecast the impact of a potential or likely change.

What are the 3 fan laws?

Fan Law 1: CFM and RPM are directly proportional to each other.

CFM2 = CFM1 X (RPM2 ÷ RPM1) or RPM2 = RPM1 X (CFM2 ÷ CFM1)

Fan Law 2: Static Pressure changes with the square of CFM

SP2 = SP1 X (CFM2 ÷ CFM1)² or SP2 = SP1 X (RPM2 ÷ RPM1)²

Fan Law 3: Horsepower differs with the cube of CFM

HP2 = HP1 X (CFM2 ÷ CFM1)³

The Three Fan Law Equations


The fan law equations that best capture the relationship between volume and speed are the simplest and most direct. The volume change ratio (V1/V2) in this instance is the same as the speed change ratio (n1/n2). This implies that the volume will change in direct proportion to changes in speed.


The relationship between pressure and speed is seen in the second fan law equation. The pressure change ratio (P2/P1) is found to be equal to the square of the speed change ratio (n1/n2) in this case. Accordingly, a minor change in speed results in a proportionately bigger change in pressure.


The third fan law equation finally establishes the relationship between changes in speed and changes in horsepower. The horsepower change ratio (HP1/HP2) equals the cube of the speed change ratio (n1/n2)3, meaning that a little change in speed results in a significant change in horsepower. Because of this, it’s crucial to consider future horsepower requirements when sizing your fan motor.

What is the relationship between airflow, static pressure, and speed used in HVAC fan law?

The relationship between airflow, static pressure, and speed is governed by the HVAC fan laws. As the speed of the fan increases, so does the airflow. As the static pressure increases, so does the square of the speed. And as the airflow increases, so does the square root of static pressure. This relationship is often expressed as a curve.

In order to forecast how the performance will vary when other factors change, manufacturers test their products under various settings and plot fan performance on a “Fan Curve Chart.”

What is Fan Curve?

It is an assessment of the effectiveness of fans, which is governed by certain patterns (known as “fan laws”). The correlations between static pressure, air flow volume (CFM), and brake horsepower are the three main characteristics that we examine (BHP).

The appearance of fan curve charts varies from maker to manufacturer. Most like the graph below. For a certain model, the curve shows a constant RPM. Plot a horizontal line from the static pressure axis to the curve’s intersection. Plot a second line that goes straight down to the CFM axis. This is the CFM under certain circumstances.

Fan curve

This chart may also include a brake horsepower (BHP) curve from the manufacturer to demonstrate how much power is needed to operate the fan at a specific RPM and SP. The operational point is the name given to this junction. When a BHP curve is included, the amount of horsepower needed may be calculated by drawing a vertical line from our operational point that intersects the BHP curve.

Fan curve

How can you use the Fan Laws to improve the performance of your HVAC system?

The first fan law states that the performance of a fan is proportional to the square of the speed. This means that if you double the speed of a fan, you will quadruple its performance.

The second fan law states that the pressure generated by a fan is proportional to the square of the speed. This means that if you double the speed of a fan, you will increase its pressure output by four times.

The third fan law states that the power consumption of a fan is proportional to the cube of the speed. This means that if you double the speed of a fan, you will increase its power consumption by eight times. Fan curve

What are the consequences of not following the HVAC Fan Laws?

If you don’t follow the laws of fans, your HVAC system will not function properly. A variety of issues might arise as a result, such as:

  1. Your home not being heated or cooled properly
  2. Your energy bills going up
  3. Shortened lifespan of your HVAC system
  4. Voiding of your HVAC warranty
  5. Your HVAC system breaking down more often
  6. Oversizing your ductwork can be very expensive to fix
  7. If your ductwork is undersized, it can cause air leakage and create an uncomfortable environment.
  8. Properly sizing your ductwork is essential for ensuring a comfortable environment.

What factors influence the efficiency of a fan using Fan Law?

The efficiency of a fan is affected by the size and shape of the blades, the number of blades, and how they are designed in relation to each other.

The Fan Laws are predicated on the notion that the impeller in issue does not alter its rotational speed dramatically, leading to appreciable variations in the air’s density.

The power of a fan is determined by the size and rpm. A larger diameter fan will have more air moving but lower rpm, while smaller diameters fans will move less air at high speeds.

How to use the fan setting using Fan Laws in your HVAC system?

A typical HVAC fan setting is either on High or Low. However, the way you use these settings can vary depending on your heating and cooling needs. For example, in winter months, you may want to use the low setting on your HVAC fan. The low setting will circulate air in a room, which can help with heating up the space quicker. There are advantages and disadvantages to each setting.


Controlling the temperature in homes is made easier by continuously moving air. It’s best to evenly heat or cool rooms. Your HVAC system’s startup stress is significantly decreased, which can lengthen the life of some equipment.

Leaving fans on can lessen indoor air pollution when filters are replaced appropriately and systems are kept in good condition. From spring through early fall, this is especially beneficial for people with allergies and asthma.


It takes money to run fans continually. Due to the conditions in walls and attics during the warmest months of the year, air ducts may get overfilled with hot air. Air conditioning may work too hard if fans are left on.

The heater is used more frequently in the winter due to chilly air ducts. The more often fans are used, the more upkeep is required. For seasonal maintenance, especially if fans are left on the “on” setting all the time, call a specialist.


The auto setting is economical and clever since it helps households save a lot of money. By only turning on the heater or air conditioner when necessary, this option aims to maintain comfort in households. Fans operate at the slowest speed required to complete the task. Additionally, the air is dehumidified more efficiently.


Fan repairs may need to be made a little more frequently as a result of startup load on the HVAC system. Certain areas of a house may be colder or warmer than others. Every house is unique.

Bottom-line :

You may build your fan system to satisfy current requirements while also leaving room for any future requirements using the straightforward fan law calculations presented in the article. The calculations show that raising speed can help you add volume or pressure when you need it, but you must make sure your engine has the horsepower to handle the rise in speed. You may find out how many horsepower you need to have accessible by solving the third equation.

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