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I just purchased an off-the-shelf 92mm case fan, plugged into the 4-pin sysfan header, and noticed right away after powering-up that the air flow is reversed – from outside to inside the case. My old (loud) fan spins correctly, but it's only 3-pin and shouldn't matter as all that's missing is PWM.

Now before you think I mounted it wrong, the flow directional arrow points towards the outside of the case, the concave part of the blades is on the case side, and the fan bracing is facing outward as well as the label on the rotor.

Now the weird part. Looking from inside the case, the blades should be spinning counter-clockwise in the direction of the rotation arrow on the fan. Instead, not only do they spin clockwise, the "teeth" portion of the blades are leading toward the counter-clockwise side, but instead the blades look to be going backwards. I tried some suggestions of forcing the spin in the opposite, but the fan doesn't want to go that way. Is this just a fan wired backwards at the factory or am I missing some fundamental and embarrassing point?

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migrated from Jul 28 '13 at 7:17

This question came from our site for system and network administrators.

Sounds like it may have been manufactured incorrectly. If you can, post a picture of it. :) – Ƭᴇcʜιᴇ007 Jul 28 '13 at 7:20
Not sure why this got migrated from How did the moderator know if this was for a enterprise server vs a home desktop? (Ok, it was my home desktop, but I guess the fact I'm a Cisco network engineer who occasionally deals with HP Proliants with 25+ years exp in IT means nothing. j/k) – generalnetworkerror Jul 28 '13 at 7:27
@generalnetworkerror we're psychic, plus the "off-the-shelf".. :-) – pauska Jul 28 '13 at 7:30
Are you using it with an extension cable or something like that? – gronostaj Jul 28 '13 at 8:27
@cpugeniusmv: Mounting backwards means the fan efficiency is diminished since the blades are not rotating in the direction designed for maximum airflow. The concave side of the blade with its tapering accelerates the air out; spinning backwards does move some air in the opposite direction from design, but not as it should. – generalnetworkerror Jul 30 '13 at 23:13
up vote 0 down vote accepted

As no one has served up an answer, and now I have it, I'm posting it. I went out and bought the exact same fan, a Cool Master Blade Master 92. The replacement fan spins counterclockwise as it should, so the wiring was reversed at the factory.

I've attached a picture below to show how the rotation was labeled on the fan casing, but with the actual rotation in the wrong (clockwise) direction.

reverse fan rotation

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