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Fan Slows Down...

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Nov 17, 2001
I've got the Hydraulic full tower. I've noticed that when my side panel is closed, my CPU fan slows down in speed a bit. When I leave it open, it will speed up. Is this because of my side panel or that the panel is touching something inside the tower? I've got 2 twist ties (plastic coated) holding together a 120mm Delta blowing against my CPU fan/HS. I'm not sure if there is anything else touching.

Any Ideas :eh?:
cyber mouse45 said:
Off the subject, but just how loud is a 120mm Delta??

These are pretty good. I've got a couple of them. It's the "WFB1212M" model. It spins at 2100RPM pushing around 72CFM@34dB.

Here's a pic off another website:

The way I understand it, your 120mm is hanging between the case door and the cpu? Puting on the case door is blocking some of the airflow through the 120mm fan, and the extra boost the cpu fan gets is dying down some. Do your temps go up with the door on?
The best fix in this case would be cutting a corresponding hole in the door to let the nice fresh air in.
Yes, it's hanging on the rail right over the CPU fan. However, how would the 120mm fan be the cause of slowing down my Delta38 HS fan? I tried stopping my 120mm fan and it still does that.

My temps are relatively the same with the case panel on or off, but my temps fluctuate so much it's hard to say.
It slows down because it creates turbulence and pressure. Your 120mm fan is pushing out way more CFM than your 80mm can pull from it, so it is creating turbulence and a lot more pressure inside the case. So when you take the side off you pull the 120mm further away from the 80mm and the pressure level in the case drops through the roof. And when the pressure level drops you don't have the extra CFM floating around pushing on the 80mm fan...thus, slowing it down.

Hope that made sense...Basically, its all the added pressure...it puts more strain on the 80mm fan, and on the 120mm fan too. So they slow down because of that. Whichever way they are set up...it correlates.
That's strange. Even if I stop my 120mm from blowing, my Delta still does the same thing, whether the panel is on or off.
Hmmmm. I know I wrecked my cpu fan header, but I'm using the 3-4 pin adaptor, so I should be getting top speed. Could it be my adaptor?
I run a 120mm fan in my case too. Heckaloud!!!
Antiheiss knows what ....he/she is talking about. Some of the little 38mm deltas move 80cfm of air. Poke some holes in your chassis.
Take a slot cover off. Some fans are temp sensitive. The hotter they get, the higher the RPM. Odds are..........ambient pressure is merely flexing alittle due to flow constrictions.
If there is no temp problem this may be OK. All fans slow
down a bit under load. You may have sub-optimal
placement of the fan, but unless it is slowing down
a lot this shouldn't be a problem.
Bonka said:
Hmmmm. I know I wrecked my cpu fan header, but I'm using the 3-4 pin adaptor, so I should be getting top speed. Could it be my adaptor?
Would seem perhaps more likely that it could be a PSU problem. Maybe you are getting voltage drops. Using a good PSU?
Hmmm. It could be, although when I'm running at my overclocked speed, it only dips to 4.92 at times on the +5 rail. Usually stays at 4.95.

It's some no name "Y2K" branded power supply. Here are the readouts:

+3.3 - 15A
+5 - 22A
+12 - 11A
-5 - 0.5A
-12 - 0.5A
+5VSB - 1.5A

How many other fan's do have in there, do the fan's speed up under Cpu load, then slow down when the
Cpu is Idle? I had a similar problem with a Amd system(with the HLT command disabled). After doing some research on proper case air flow, positive & negative pressure's I agree with AntiHeiss about turbulence and pressure, I unplugged a couple of fan's the problem went away, Still concerned it may be a PSU issue, I took out the extra fans, I must have had a good balance of air flow theTemp only went up 1 degree. I ran a molex splitter out the back of the case buttoned everything back up no problem's, after connecting the fans to the external connector with the fan's out side away from the case the problem was re-created, even trying fans from mild to wild, my result In-adequate PSU. It's a process of elimination, and finding a good balance for your system's configuration
I may be off base here with a Intel system, but it could be possible that the Bios uses a cpu idling HLT command, which can cause fans to taxi up or down depending on the Cpu load.