Water power

Originally published 2007 in Atomic: Maximum Power Computing
Last modified 03-Dec-2011.


The other day, someone asked me whether I thought it was a good idea to water-cool a PC power supply.

I mean, sure! What could go wrong?

It's not as daft as it sounds, though. People have, of course, done it. You can even buy a water cooled PSU off the shelf these days.

On the face of it, water cooling a car engine doesn't sound like a good idea either. Water in the combustion chamber, fuel tank or oil is very bad, after all. What kind of idiot would want to water cool an engine?

As with engines, though, it's perfectly possible to water cool a computer PSU if you do it properly. A water block on every heat sink does the trick. A bit of convection will keep the rest of the PSU components acceptably cool.

A catastrophic coolant leak in the PSU would be unlikely to make the computer dangerous, although it might well cause one of those situations in which a freaked-out PSU blows up some other components.

Despite all this, though, the water-cooled PSU is still not a very desirable project. It pays to understand why, even if you're not personally considering adding a garden-hose grommet to the back of your own PSU.

Besides the obvious leakproofing issues, you see, it helps to remember that "water cooling" is not really "water cooling" unless you're using an evaporative cooler somewhere, or a total loss system where the exhaust water goes down the drain (or into your indoor pool, or out into the river next to your power station, or something...) rather than being recirculated.

All normal small scale liquid cooling systems, be they for cars or computers, use the liquid only as a heat transfer fluid. The fluid's purpose is to move heat from a place where it's inconvenient to deal with it to a place where it's easier.

A CPU is relatively small, and buried inside the computer, and makes a lot of heat. So it makes some sense to put a water block on the CPU so you can pipe its heat to a nice big radiator cooled by a nice big fan which you can hang outside the computer case. Or even outside the building, if you like.

On the other hand, a computer power supply is relatively large, and has its own exhaust to the outside world, and does not make all that much heat, anyway. Modern PSUs can be expected to be better than 80% efficient at most load levels. If you've got a 500 watt PSU running at full power at only 80% efficiency, then there's 100 watts of heat to get rid of. But a recent firebreathing PC is only actually likely to need about 400 watts when it's working hard, and most people's computers most of the time need only a couple of hundred watts, tops.

If your PSU's losing more than 50 watts as heat when driving a 200W load, it's a very bad PSU. 25 watts lost should be perfectly achievable. And you can get rid of a few tens of watts of heat from a device the size of a PSU - plus all of the leftover heat from the extra components in a PC - with plain heat sinks and a fan that's considerably quieter than the ones that most water cooled PCs have on their radiators.

Which brings me to the next problem. Your computer needs something to get rid of the heat from those other internal components. If you delete all of the intake and exhaust fans from your water-cooled PC, including the one normally found in an air-cooled power supply, then you're left with nothing but convection to cool everything that doesn't have a water block on it.

That means you either have to use a case with very good passive ventilation - just leaving the side panel off may not be enough - or extend your water cooling system to not just the CPU and video card, but also at least one of the motherboard chips, any and all hard drives, and possibly even the RAM. You can get water blocks to fit all of these parts and more, but it'll take a lot of tubing.

If the aim of your water cooling exercise is just to make a very quiet PC, by which you mean that the computer shouldn't be audible over the sound of the big lazy fan on the radiator, then you'll do better to get one of the off-the-shelf PSUs that have their own big, slow, quiet fans. If the water rig takes care of the CPU and video card heat, and the case has reasonable ventilation, and the ambient temperature isn't alarmingly high, then one temperature-sensing fan in the PSU or on the back of the case (which you could use with a fanless PSU) should be able to keep everything that isn't water cooled at a decent temperature.

If you're a silent computing fanatic that insists on a fanless (and thus bigger) radiator, then even a PSU fan's noise may be unacceptable to you, and you may indeed need to water cool everything. If that requires you to use a bigger pump that makes more noise, though, don't come complaining to me.

A better solution to this problem is to stick with fans, but move them further away. Long water pipes to a remote fanned radiator can be a pain, but it's quite easy to use bendy HVAC ducting leading to a fan in another room, or outdoors.

This opens up another crazy-project option, too.

Rig a reversible outdoor fan with an in/out temp sensor. Now the fan can blow air into the computer when it's cool outside, and suck air out when it's hot!

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