best temperature for server room?

Clarifications

The Humidity is the problem ,if you put the Temp very low at 19 or 18 .

if your Airco doesnt have control of Humidity ,and has only Temp controll the best temp is 22-23 .so to keep Humidity at 45% .

So long as the system is able to run, and not overheat, either should be ok.

The problems you get into with higher heat, is that the system can reach a temperature that causes it to step down / slow the processor, or over heat and shut down, etc.

With having a lower temperature, you allow the possibility to over-clock the CPU, but, if you're running an office server you don't want to do that, as you can effectively lower the life of your processor by running it hotter, even if able to dissipate the heat. Over clocking then is not going to give you the stability and longevity the procssor was designed to support.

Yes, 21 should be just fine. I would recommend a small climate monitor (I use the MiniGoose II from ITWatchDogs - www.itwatchdogs.com) for alerts in case temperature goes high. I get a text message if temperature goes too high.

Here's a link to their video: http://www.youtube.com/watch?v=5Brv0vsHIlA&

Agreed 21C is the sweet spot. I'd recommend looking here: www.sensormetrix.co.uk they have a large range of options for environmental monitoring depending on your server room size and required sensors. With e-mail or SMS alerts you'll never have the heart-ache of walking into a melted server room on a Monday morning...