I was thinking of making a fanless ( or almost ) water cooling system - putting copper pipes on my room floor ... What I need it a easy way to monitor the water temp and when need arises start the 120 MM fan that sits on the radiator.
If you've got enough surface area on your tubing, I doubt you'll need a fan. It never hurts to be safe, though.
I'm working on something very similar. I'll be using about 16 feet of 3/4" copper tubing attached near the floor of my computer room. I've got most of the parts and will fire up the torch tomorrow to solder it all together.
I'll post pictures in a couple of days.
My first attempt at a passively cooled radiator was the Tower of Cooling Power
. It has about five feet of 3/4" copper tubing with fins attached. With no fan it keeps the CPU temperature at 41-44 degrees C, depending on room temperature and CPU load. The thread includes a test I did with a fan pulling air through the radiator. It didn't change the temperatures much. I expected more of a difference.
2. If you are using Windows (you have my sympathy...) look at Speed Fan, I think it will give you software fan power control, however beware of the question of what happens to the temps and fan control if you get a BSOD or other crash.
I've used SpeedFan on a server with Windows 2000 Server. Anytime the OS is not running the fans default to full speed. Very nice program, and it's free.
3. I've found a couple of circuits here and here
that will give both on / off and temperature related speed control with a relatively simple setup.
I'm using the second circuit that Gooserider mentions to control a couple of fans on my desktop comp. One pot sets the minimum cold fan speed and the other pot sets the fan speed at operating temperature. It works great. Highly recommended.