Dave: Rules-of-thumb like this are great to know. The fan, I'm sure, was installed (like the IMSAI) blowing IN because the inside was caked with dust. So, I made it an exhaust fan (no filter). As I work on the IMSAI, which has no filter either, I will swap it around, too. To answer another comment at the same time, the fan does indeed have an arrow but the arrow itself doesn't tell me if it should be facing IN or OUT. Rich ============================= Richard A. Cini, Jr. Director Wachovia Capital Finance 1133 Avenue of the Americas New York, NY 10036 W: 212-545-4402 F: 212-545-4589 -----Original Message----- From: cctalk-bounces at classiccmp.org [mailto:cctalk-bounces at classiccmp.org] On Behalf Of Dave Dunfield Sent: Friday, June 17, 2005 7:01 AM To: General Discussion: On-Topic and Off-Topic Posts Subject: Re: Altair Fan At 21:31 16/06/2005 -0400, you wrote: Hi Rich, Both of my 8800s have the fan blowing out. The general rule is: If the fan has a filter, then it should blow in.: By maintaining positive internal pressure, and filtering the air that comes in, you keep dust from entering the box. If it does not have a filter, it should blow out: A fast moving stream of air will carry more dust than a slow one. If you blow the fan in, it will carry dust into the box which will drop out before it exits at slower speed (because of the much larger area of all the vents and other openings) - it also causes dust buildup to concentrate that certain points bacause of sudden changes in airflow (you can often see bands of thick dust in boxes with fans improperly installed). Going the other way reduces the entry speed (same reason) and thus the amount of dust that enters - what does get in tends to settle out in a more uniform distribution. Regards, Dave -- dave04a (at) Dave Dunfield dunfield (dot) Firmware development services & tools: www.dunfield.com com Collector of vintage computing equipment: http://www.parse.com/~ddunfield/museum/index.html