Pos or Neg pressure cooling?

[Randy222]

Looking for some input on how you would place a fan in this scenario.

in-wall plastic leviton media box, 14x14 inch. In it will be a optiplex micro pc that has intel i5 and my apps will run the cpu heavy. In the box will also be the pc 90w brick psu. Media box has cooling slots/holes running down middle of the cover.

I have a 3" usb-a cooling fan that I will mount in upper right corner of the cover. PC is located bottom left. The micro will sit on it's side, so it's own cooling exit holes are upper right on this pc, it too has a internal cooling fan.

The box itself is mostly sealed off (holes and stuff taped over). One 1" knockout up top that brings in a few low voltage wires, and one receptacle box on the lower.

Would you place fan to pull out hot air (neg air pressure in box), or push in cool air (pos air pressure in box)?

[Whales]

Positive pressure is easier for dust management, you can put one filter on the intake fan and that's it.  Negative pressure pulls dust in through every tiny gap you missed.

> I have a 3" usb-a cooling fan that I will mount in upper right corner of the cover

That's only 75-80mm.  Use a 120mm or 140mm fan if you can.  Bigger fans can rotate slower than smaller fans to move the same amount of air, and noise is proportional to the rotation speed cubed. 

> optiplex micro pc

Is the front an air inlet and the back an outlet, or are there ventilation holes on other sides too?  Something I love about some SFF PCs is how simple their airflow is (front to back only) so you can duct them or fact them in convenient directions.  (Or stack them against each other in a pile).  Not sure if all micro format PCs are similar or not.

[David Hess]

Positive pressure is easier for dust management, you can put one filter on the intake fan and that's it.  Negative pressure pulls dust in through every tiny gap you missed.

That is what I always do, and how my current desktop PCs are configured.

A scotch-brite pad works well as a fan filter if you do not have something better.

[Randy222]

Leviton 49605-140

I taped off all the holes and knockouts from the backside of the box.

The vents and fan are on the cover (fan mounted on inside).

It will be on 24/7. I did look at filters, but I won't frequent the location and relying on others to clean the filter is as good as man living on mars, so no filter will be used.