should I duct cold air in or hot air out?

jpsa · Post by **jpsa** » Wed Oct 27, 2004 3:00 pm

I want to duct my cpu HS, I can do this in two ways. duct cold air from the outside directly to the cpu or I can duct the hot air coming out of the HS to the exterior. what whoud work better?

Bluefront · Post by **Bluefront** » Wed Oct 27, 2004 3:23 pm

That question can't really be answered...there's no "best" way. Duct air in to the CPU and it runs cooler. Duct hot air off the CPU to the outside and the CPU will run hotter, but the rest of the computer will run cooler. Take your pick.

There are too many other factors to consider to give you an easy answer....

jpsa · Post by **jpsa** » Wed Oct 27, 2004 3:35 pm

thx anyway... think I'll just do both

alglove · Post by **alglove** » Wed Oct 27, 2004 4:42 pm

It also depends on what your current case temperatures are like. If your case temperature is only slightly warmer than your room temperature, then the disruption of your existing case airflow may prove to be a larger factor than what is happening with your CPU. For example, by ducting the CPU, I made it more difficult for the heat from my graphics card to exit the case. The temperature of everything, including the CPU, went up as a result.

That is what happened to me a few months back. I ended up shelving the experimentation for another time.

lenny · Post by **lenny** » Thu Oct 28, 2004 12:11 am

jpsa wrote:thx anyway... think I'll just do both

And it seems like the easiest way of accomplishing this is with the (unfortunately fairly expensive) Silverstone Temjin 6 case.

I don't see why ducting the hot air from CPU out would result in hotter CPU temps than no ducting. But bluefront definitely has way more experience than me with ducting, so I'll give that serious consideration.

But one thing we can all agree on is what you do depends on your situation. For me, I'll pick what I can do the easiest

DG · Post by DG » Thu Oct 28, 2004 12:26 am

How will the duct be? CPU-side panel or CPU-rear case?

mkruer · Post by **mkruer** » Thu Oct 28, 2004 12:37 am

Here is what I found out for my system, if I left the back fan exhausting out, my case temperature was lower, but my CPU temperature was higher. However if I reversed the fan, my CPU temperature was about 5C lower, but my average case temperature went up by 2-3C.

Now what you can do, and what I am planning to do for my next system is reverse the rear fan to blow into the case (not out of) over the CPU and then also reverse the front case fan to blow out. This is the fundamental idea of BTX. The only major difference is that the BTX form factors assumes that the air in the front of your systems is cooler then the back. This is true in a closed environment however if your case is out in the open with the back against the wall, then this is false, the reason why is because the wall is cooler then the ambient temperature in the room (usually) and as air comes in a pass by the wall, it will cool slightly. By maybe a whole whopping 1/2C most.

In the end use common sense, or take a course is thermodynamics.

lenny · Post by **lenny** » Thu Oct 28, 2004 2:20 pm

mkruer wrote:Here is what I found out for my system, if I left the back fan exhausting out, my case temperature was lower, but my CPU temperature was higher. However if I reversed the fan, my CPU temperature was about 5C lower, but my average case temperature went up by 2-3C.

Maybe that's what BlueFront meant, and that makes sense. I took it to mean : ducting in will result in cooler CPU than no ducting, ducting out will result in warmer CPU than no ducting.

mkruer wrote:Now what you can do, and what I am planning to do for my next system is reverse the rear fan to blow into the case (not out of) over the CPU and then also reverse the front case fan to blow out. This is the fundamental idea of BTX.

I'd think you'll be fighting against convection currents. But it's worth a try and it's easily reversed if it doesn't work. Please take some measurements and tell us your results. There might have been a couple of attempts in the past in these forums but I don't recall if anyone posted results.

alleycat · Post by **alleycat** » Thu Oct 28, 2004 5:47 pm

I prefer to duct hot air out, but it's important to take a whole system approach. If most of the case air is exiting via the CPU HSF, then you can expect CPU temperatures to rise. If you are ducting cool air in, how is it exiting? Probably through the PSU, causing it to get hotter instead. In a well-designed system, cool air should be able to easily enter the case by negative pressure. Ducting hot air out gives you more control and puts the heat where you want it to be.

josephclemente · Post by **josephclemente** » Thu Oct 28, 2004 7:12 pm

I prefer ducting fresh air straight to the CPU. This allows me to run the CPU fan as slow as possible for less noise. The CPU is as cool as it can be for any fan speed.

Since the air traveling through my P4 3.0C CPU is still very usable for the PSU (no fan speed increase and the exhaust temperatures are fine) I'll gladly stick with intake for as long as I can get away with it.

silvervarg · Post by **silvervarg** » Fri Oct 29, 2004 2:14 am

It your are just ducting one of intake or exhaust I would prefere to duct air from CPU out of the case.
This might not give you the coolest CPU temperature, but it will aid the cooling of all other components in your computer, so GPU, RAM, MOSFETS, HDD and PSU will be cooler. This means you can probably run other fans slower or remove them entierly.
If everything in the case stays relatively cool there will only be a few degrees difference between case air and ambient air.