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How Vacuum Power Is Measured

An overview of how vacuum power is measured using Cubic Feet (of air) per Minute (CFM), Water Lift, and Air Watts (a combination of the two).


Central Vacuums in North America are measured by CFM and Water Lift. This article explains these measurements and how the Air Watts measurement - which is a calculation of CFM and Water Lift - is the best way to measure the actual cleaning power of a vacuum system.

Cubic Feet (of air) per Minute (CFM)

The CFM is a measurement of airflow, and is assessed by how much air is taken in through a round opening ranging from two inches to completely closed. There is zero CFM at no opening and maximum CFM at a two-inch opening. Increased CFM becomes more important as the air opening size gets larger. For example, you can feel the air rushing into a straw when you inhale, but you would not feel a thing if you inhale through a two-inch pipe. If you want to vacuum through a larger opening, you need more CFM. Therefore, high CFM is needed for deep carpet cleaning.

Water Lift

This is in contrast to CFM, described above. Although central vacuums do not literally vacuum water, this benchmark is calculated using water. It is obtained by measuring how many inches up a tube the vacuum motor's intake can pull up, or lift, the water.

For the Water Lift test, there is no air moving through the motor. It is a sealed suction measurement to find the pure suction force of the motor. Water Lift measurements will fluctuate based on the amount of voltage the motor is receiving, altitude, air temperature, and barometric pressure.

The water-lift measurement is not the only key to effective vacuuming. A vacuum may have amazing water-lift, but without the motor's blades spinning fast and pulling in a lot of air, it will hardly move anything toward the vacuum filter. So, CFM and Water-lift must work together. Water-lift is important to keep the air moving, especially through long pipes and hoses, or when the tool being used to vacuum has a small air opening, such as in a turbine brush vacuum attachment.

Air Watts

A combination measurement of Water Lift and Airflow is called Air Watts, which has become more standard for the central vacuum industry. Air Watts is recognized by the American Society of Testing and Materials (ASTM) as the best way to measure the actual cleaning power of a vacuum system.

Air watts is a calculated measurement which takes into account both CFM and Water Lift in real time at the operating end of the vacuum hose, tool, or brush. Each of these measurements are in themselves inadequate because a vacuum is working somewhere in between these opposite maximums: a vacuum is not operating with sealed suction or at a two inch vacuum opening, but somewhere in between.

When you read the CFM and Water Lift specifications of a vacuum, remember you are reading their maximum values. These values are never at that measurement at the same time in an operating vacuum. You have to know the curve created by both CFM and Water Lift as the opening changes from zero to two inches. Somewhere along this curve the calculation for air watts will maximize.

In practical terms, when using a non-electric, air-driven power brush, such as the TurboCat, the air watt peak should be for an opening at or below 3/4 inch. This will guarantee that the turbine in the brush will be spun with enough force to achieve good carpet grooming. This is most common for motors with three fan stages or for some modern, powerful two stage motors, such as the motors in our SilentMaster S44 and Flo-Master M85 central vacuums.