The acpifan device driver supports fans possibly exposed
  by the BIOS. Even if a system contains only one physical fan, multiple
  acpifan instances may be present. In such cases each
  acpifan typically represents an
  acpitz(4) thermal zone, and the
  combined state of all instances reveals the speed of the fan.
The acpifan driver does not support
    controlling the fan. The current state (on or off) is made available through
    the envsys(4) API and the
    envstat(8) command.
The acpifan device driver appeared in
  NetBSD 6.0.
Jukka Ruohonen ⟨jruohonen@iki.fi⟩
The acpifan driver does not yet support additional
  functionality introduced in the ACPI 4.0 specification.