Mechanical designers traditionally have modulated airflow through VAV systems with inlet vanes. The fan motor operates at a constant speed, while the vanes open and close to maintain a constant static pressure at the fan discharge.
Replacing the vanes with a variable-speed drive controlled by feedback from the static pressure sensor doesn’t alter the ability of the air handler to perform its designated task, but saves a lot of energy in the form of fan-motor horsepower. If the unit motor is old and nearing the end of its useful life, it’s worth considering replacing it too, with a high-efficiency motor and matching variable-speed drive.
The only problem with variable-speed drives is that they introduce harmonic distortions to the power distribution systems. This can cause problems, if the same branch of the power system (e.g., the same side of the step-down transformer) serves sensitive electronic equipment such as computers or electronic controls. In extreme cases the computer equipment may have to be protected by a special independent power source.
A similar project can be done for pump motors, where the pump output is controlled by throttling or by a bypass valve. Disabling these controls and replacing them with an adjustable-speed drive controlled by feedback from a static pressure sensor or flowmeter optimizes control and saves electricity.
A less costly alternative in either circumstance is to replace the existing motor with a high-effciency two-speed motor integrated into the controls circuit for two-stage operation of the motor according to an independent pressure sensor or flowmeter, the existing controls otherwise operating as at present.
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Excerpt from Energy Conservation Projects.