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Sub_Umbra said:
It's a mixed bag.
Dirt is a real problem for 24/7 boxes. Since off the shelf boxes run in a partial vacuum they literally suck junk in through every tiny hole -- not just through the fans. I built a box with a slight positive air pressure and all incoming air is filtered.
As previously mentioned, running 24/7 can reduce thermal stress on all circuit boards present. Thermal stress can damage any circuit board after hundreds of on-off cycles. The condition usually manifests it self as a very nearly impossible to diagnose, spooky, intermittent problem as the board heats up and a broken trace moves out of contact. Everything seems fine for 90 minutes or so and then quirky things go wrong. I've seen this happen to circuit boards in other types of electronics.
Aside from cyclical thermal shock, consider what happens to your CPU when you power down. In probably the majority of machines when all of the fans stop ( including the CPU fan ) the CPU temperature will actually rise to a level higher than at any other point in its operation even though it is powered down, and not fall for a few minutes. If you shut it down often it's nice to use an OS that allows you to HALT the CPU and wait five minutes before powering down, to avoid this temperature rise.
If you run 24/7 you must expect more AC line events and worse yet, more phone line/cable/dsl spikes.
If you are going to run a machine 12-15 times longer each day than it was designed for you will eventually have some kind of problems. To run 24/7 IMO you should build it with that in mind. Don't forget a heavy duty power supply. In addition to a filtered positive air pressure I also built in about three times the CPFs I needed and then I can throttle back each fan with rheostats so it is more quiet than a stock box. I use big Paniflos and replace them all every two years. I had one fan fail around the time we were moving and I didn't have time to replace it. I just DCd it, slapped a piece of contact paper over the hole and was still able to pressurize the system by re-tuning the remaining fans with the rheostats.
If you run a standard box 24/7 the inside should probably be cleaned every 1-3 months, depending on your environment. This can also damage your box. Off the shelf boxes also collect dirt and fuzz in critical places that you can't see -- like in your CPU heatsink.
I run mine 24/7 -- have for years. I shut off the CRT whenever I get up. Of course there are pros and cons to each approach. Different strokes...
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Yes, dirt/dust is a REAL problem for 24/7 running. Easy solution is to get the darn thing off the floor. After that, yearly cleanings are likely fine.
I have to take exception on the cpu temp thing. The cpu core generates the heat, it is also the most temp sensitive part of the chip (as a whole), the rest of the chip being circuit board, connector pins and a copper/metal slug.
Right after power off, the core transfers heat to the slug which rises in temp. But this part doesn't matter. By the time the slug temp rises, the core has dropped (from its steady state temp) and we have no issues. Besides, core/chip temps run cooler than they have to anyway. At no time does the core temp rise above what it was while the pc was running.