The application of variable-speed drives (vsd's) in hvac control is gaining popularity because it has proven to save energy when it is applied properly. Vsd's are used in variable air volume (vav) air distributions and in variable-volume chilled- or hot-water distributions.
The affinity laws are most commonly used to derive energy-savings estimates from vsd-controlled devices in comparison with other types of control devices. However, the affinity laws are the frequent subject of debates. The argument is that in an actual situation, the energy savings of a fan or a pump with vsd control are not proportional to the cubic relationship of the speed.