Asus P5W - Manual (Page 65)

ASUS P5W 2-
Do not forget to connect the fan cables to the fan connectors. Insufficient air
flow inside the system may damage the motherboard components. These are
not jumpers! Do not place jumper caps on the fan connectors!
If you install two VGA cards, we recommend that you plug the rear chassis fan
cable to the motherboard connector labeled PWR_FAN1 or PWR_FAN2 for
better thermal environment.
Suggestions for installing the chassis fan:
• If you install a fan at the back of the chassis, plug the fan cable to
PWR_FAN2 or PWR_FAN1.
• If you install a fan on the side of the chassis, plug the fan cable to
CHA_FAN1.
• If you install a fan in front of the chassis, plug the fan cable to CHA_FAN2.