Which way would cool a PC down the best with a fan?

draw cool air in, or blow hot air out?

thanks
Tom

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It depends entirely on the circumstance.

In order to understand the cooling situation, remember that:

• Heat needs to be removed from the case; that’s the physics. The energy produced by the various components will be transferred into the air. Air is a poor conductor of heat, so it will be very difficult for that heat to escape the case through whatever gaps exist without assistance; that is why we have exhaust fans (blowing out).
• When you’re pulling air out of the case, as a counter effect air will be pulled in the case by the displacement, but not very efficiently. Firstly, the exhaust fan will be working extra hard when the air pressure inside the case decreases and secondly, that volume of air has to be sucked in through numerous tiny little holes. Adding an intake fan reduces stress on the exhaust fan thus increasing the air flow, and keeps the internal air pressure higher making it a better conductor.
  

Volumes of high temperature in the case, such as around the CPU, graphics card, memory and core logic components is a priority. First choice is usually to install an exhaust fan in the top rear, right next to the processor. This ensures that hot air is removed before is spreads through the case. Next choice is to install an intake fan at the front bottom of the case in front of the hard drives if possible. Hopefully this ensures a reasonably body of air will pass over most of the components on its way out the exhaust fan.

It’s tempting for people to think that blowing air into something is the best option for cooling. But the simple and obvious fact is; as much air is sucked in to a fan as is blown out, it’s the placement of the fan that determines its best use.