Wow. I spend an evening and morning getting the house ready for a potential buyer to look at today, and come back to a ton of new posts this morning! Heat rejection isn't exactly as fun a topic of discussion as tubes or subs or LCD/DLP/LCOS, but what the heck -- it might be a useful thread for some folks out there. And we've had some mighty fine expert input from electricians, EE's, and industry insiders over the last couple years, so I may as well toss in my two cents on cooling.

As I mentioned, this is my job -- I'm a professional mechanical engineer and I design HVAC systems (and some fire protection) for commercial buildings. In the design that I developed those numbers for, we were able to throw <55F air at the area year-round, but in a house your furnace won't provide cold air in the winter, and I doubt you want to try to install an airside economizer just for the equipment closet, which is why the typical choice is to use a fan to just ventilate a hot equipment rack in a home system. These are also just general numbers. For example, while many people keep their houses around 72 or so, most residential (and commercial) HVAC systems are sized to maintain 75 on a design day. And for all I know, dfskms likes his house warm or likes to keep his utility bill low and has the thermostat set to 76 in the summer. As for the fact that most people don't move that much air through their gear, you are exactly right. Many people don't ventilate at all, and convection alone does the best it can for them. However, my numbers were for a sealed equipment closet -- something that very few people actually have. In such a closet, fan noise would be less critical as the fan is pretty remote and isolated from the viewing/listening area and heat buildup would be much more likely to become an issue. Besides, the fan that Keta recommends is extremely quiet (somewhere around 45dBA, although the conversion from sone to dB is pretty clumsy). For the average equipment rack, most can be operated without any fan (and without any fan noise), but as you and I both mentioned, those that are a bit enclosed or crowded should be able to get by with a well-placed small DC fan such as a 120mm PC case fan. I do recommend not using a salvaged PC power supply to power the fan(s), however.



Since this is a subject I spend a lot of time with (albeit at a much larger scale than a CPU), I just wanted to clarify some heat transfer here. The heat sinks on a computer chip are not based on airflow -- in fact, fans weren't added at all until the 486 and weren't common until the Pentium. A heat sink (whether in a PC or in an amp like Outlaw's 755 and 770) is a highly conductive material with as much surface area as possible. By using a conductive material, heat can be carried rapidly away from the heat source through the sink to the air (hence the widespread use of aluminum and copper in heat sinks and the complete absence of asbestos heat sinks ), and by providing a great deal of surface area in a small space the sink can reject more heat. (The five heat sinks in Outlaw's original amp, the Model 750, combined for almost two thousand square inches of surface area, all in a 7" high chassis with a 240 sq. inch footprint.) Adding a fan does accelerate the heat rejection as you note, but the heat sink itself functions with or without a fan to draw air across it. That's why most good amps increase the size of the heat sink and eliminate the fan -- they are rejecting the same amount of heat to their surroundings either way, but by eliminating the fan they are cooling the amps quietly and without the danger of a failed fan allowing the gear to overheat and malfunction.

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i would agree that a good heatsink can not need a fan, but a heatsink benefits a great deal from having a fan pull the heat away from it more quickly. if you had fins on a heatsink that were really long then you wouldnt need a fan as much. it obivously depends on how hot something is going to get whether you need both or just a heat sink.
i was a little hasty to post my idea about cooling, rereading that he is building his house i would totally agree that exhaust fans would be best and not that difficult as its new.
the power supply with fans idea (from computer) works very nicely though, but it is more for standalone closed racks where you could mount the ps to the back (where its heat doesnt matter) and cut out holes so that the fans can suck out the heat.