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VFD on Constant-Volume Air Handler

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  rev. 2008-11-25        

Existing Conditions

Air conditioning and ventilation to xxx is supplied by constant volume fan system xxx.  There is a constant air flow regardless of heating, cooling and ventilation requirements of the space.

Retrofit Conditions

We recommend installing a variable frequency drive (VFD) and inverter duty motor on the supply and return fans.  The fan speeds will modulate to maintain adequate heating, cooling and ventilation rates.

Further Benefits


Application Details


Issues and Concerns

Slowing the fans will not necessarily result in a proportional reduction in airflow to all spaces.  This strategy will work on simple constant-volume systems where you are sure that all spaces will receive adequate airflow.

It must also be in coordination with the heat source, where a lower volume of air may reach temperatures above the acceptable limit.





When estimating air flow reductions, be conservative.  It is likely that existing site conditions will limit the extent of fan slowing that will be possible.  Slowing a fan to an average 80% of current volume will still result in substantial savings.

There are two parts to the savings calculation:

1.Thermal savings  based on changes in air flow as well as the source and efficiency of heating and/or cooling.
2.Fan savings based on standard Variable Frequency Drive calculations.

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