Can I connect two 12v rails on a PC power supply?

Yes, provided the 12volt rails have a over-crrent protection charecteristic, otherwise one may supply the majority of the output and overheat.

For an ordinary power supply (not the switching types), paralleling the outputs is possible. As mentioned by Davegur, you need to put overcurrent protection on each 12V line. If one fuse (assuming you use a fuse) blows, the other PSU will be overloaded and blow its fuse as well. Alternatively, you could use a ganged circuit breaker that will trip both PSUs at the same time if one or both should become overloaded.

Why not the SMPS types? Provided they are both provided with overcurrent protection, you should be OK

Thanks!!! I had been trying to get an answer to that on pc forums for a couple of days. This is very good news :) If I couldn't have connected the rails, I would've had to scrap my TEC project. Thanks again!!

All power supplies bring out separate drive connectors that have the +12 volt yellow, +5 volt red and the black ground wires, yet inside the power supply all of the yellows, blacks and reds get soldered to the pc board in the same place, so ya... parallel them all you want.

Yes, That is easy to check as well. Just open it up and look to see if there are large common pads for the ground and +12, +5 etc. SOme power supplies have separate +12 loops

True...

However on a 320 watt unit, as mentioned in the original posting, it is unlikely. This feature is usually found only on higher wattage and better quality supplies. But, thanks for pointing out that some do have separate +12 supplies inside them, you are of course correct.

Well, the thermoelectric cooler is 320w, but the PSU is a high quality 550w PSU with dual 12v rails. I'm trying to freeze my processor with parts that will fit into a single pc case :)

"I'm trying to freeze my processor"

Don't forget to insulate the cold stuff, i.e. cold side of peltier, heat buss (if any) processor (BOTH sides) and some of the mother board around the processor.

Indeed... I'm using dielectric grease, silicone, and neoprene :)

The 2 rails are switched independently. Don't think you can parallel them.

Old low cost PSU have only 1 rail. Try to find an external 400W 12V power supply.

Pineapple

I disagree to the point that it is generalized.

I have had problems with some power supplies where in the voltage reading on different connectors - 5 volt in my case - varied by more than 1/2 a volt which let to frequent crashes of the system. This is on a system up and running, partially loaded.

Try re-routing the power from the forward shields. HeHe

LOL! :D

Everyone has given very good and correct suggestions. Apply it without any fear.

Thank you! And thanks to everyone for your help!!

Just make sure you're not using power which other parts of the system require. Some graphics cards need a lot of 12V.

Well, I'm actually going to be using 4 power supplies for my pc... A 550w for my pc components, an identical 550w for the 320w peltier, a 250w for my fans, and a 200w for my gpu TEC.

absolutely no problem. u can connect it like that..

As a confidence check use a DVM to check for continuity between the two 12V rails you want to common.