Reflections on watercooling

I've been watercooling my primary personal machine since 2020. It started off simple—a waterblock on my Threadripper 3970X, an EKWB reservoir+pump combo, and a Hardware Labs GTR360 radiator. A total of about $480 (not including fans), less than my motherboard cost. It is no longer so simple: the same machine in its current incarnation boasts a monoblock, a GPU block, a custom GPU backplane, four radiators, two reservoirs, four D5 pumps, two temperature sensors, two discrete inline sensors, and about twenty attachments of all kinds. There's at least $1000 worth of watercooling crap in there, and another $500 of fans. I guess you could say things are getting serious.

I've put together some thoughts having watercooled for over two years.

Generic cooling notes

• We don't use Fahrenheit, but it's worth knowing that each ℃ is 1.8℉, with an offset of 32. We don't really use Kelvins either, but Kelvins are equivalent to Celsius with an offset of -273.15. Rankines can fuck right off. Liquids boil at lower temperatures at higher altitudes (boiling occurs when internal vapor pressure equals atmospheric pressure). Water expands both above and below 4℃. A quick table:

℉	℃	Notes
212	100	Water boils at sea level. Very undesirable in computers.
140	60	Upper limit for pump + some tubing.
90	32.2	Thermal throttling begins around here
75	23.9
60	15.6
39.2	4	Water's volume is minimized (density maximum).
32	0	STP. Water freezes at sea level, expanding by 9%.

• Neither air- nor water-based cooling can get your temperatures below ambient (this can be achieved with phase cooling, thermoelectric aka Peltier, and cyrogenics). If it's 30 degrees in your room, your coolant and component temps are going to be at least 30 degrees. It is useless to report temperatures without reporting ambient temps. The effectiveness of your cooling system is measured by how close it gets to ambient, ΔT. Likewise, your heat-generating components are going to be hotter than your coolant.
• The goal is to move heat from the heat-generating components, and ultimately from the case. Effectively moving heat requires (a) thermal conductivity, (b) contact area and (c) a temperature gradient. The thermal conductivity of air is about two orders of magnitude less than water, which is about two orders of magnitude less than aluminum or copper.
  • A processor without a heatsink is depositing heat directly to stagnant air, which will heat up (reducing the temperature gradient) and can't take much heat anyway.
  • Adding a heatsink means the heat is deposited into a good conductor with more surface area...which in turn deposits into stagnant air. This air will heat up, and eventually the heatsink will, as well.
  • Adding fans moves the hot air (ideally out of the case), to be replaced by ambient air. The ejected hot air will diffuse into the ambient air, which is (ideally) a large convecting volume.
• I don't delid my processors, but it's an intriguing idea. Delidding is the removal of the processor's integrated heat spreader. The IHS is the metal cover atop your processor. The actual heat-generating die elements tend to be only a small portion of this total area (the remainder is mostly devoted to pins). Delidding is a bet that you have better thermal interface material than what was used between the die(s) and the IHS. This was absolutely true in the past, but modern processors often use an indium soldering solution that is just about as good, IMHO, as anything you're going to do.

Thermal interface material

TIM is thermally conductive viscous material applied between heat contacts. There's some between your dies and the IHS, and then some between the IHS and your waterblock/heatsink. Its purpose is to smooth out the conducting surface, which is dotted with microscopic pits that would otherwise preclude contact. TIM is one or two orders of magnitude less thermally conductive than metals making direct contact, but much more conductive than the air which would otherwise fill these regions. Thermal conductance is measured in W/Km (watts per kelvin-meter).

• Liquid metal is conductive thermal paste. It is more thermally conductive than non-metal pastes, which is Good. It will short out any electronics it touches, which is Bad. I don't mess with it. The place to use liquid metal IMHO is between the die and IHS when delidding. In this case it's contained, and you're usually replacing a crap TIM. Liquid metal's thermal conductance is no better than a good indium solder, so there's just no point with a good die-to-IHS TIM. Atop the IHS, it's unlikely that liquid metal will beat quality non-conducting paste by more than one or two degrees.
• No matter your cooling strategy, you'll be fucked without properly-applied high-quality thermal paste. It is easy to apply too little or too much. Application of the waterblock/heatsink will spread the paste for you; you want a very thin layer across the entirety of the integrated heat spreader. A thick layer will absolutely cause problems. Thermal paste ought be replaced every few years.

Air cooling

• Modern tower CPU coolers are pretty damn good, and can handle most processors without getting very loud or allowing the chip to get too hot. A top-of-the-line bequiet! or Noctua tower cooler can be had for $100, and will serve most people just fine. Get one, slap on some case fans, and you're done. GPU air coolers are not generally as capable (just look at the power draw of modern GPUs vs CPUs); if you intend to make heavy use of a powerful GPU, you'll be making a lot of noise. Air coolers are not generally well-suited for overclocking, but it depends entirely on how much power the die is drawing.

Water cooling

• I skipped over AIOs (all-in-ones, aka "closed loops") entirely, opting for a custom loop out of the gate. An AIO will consist of a waterblock with integrated pump, a radiator, tubing to connect the two, and fans on said radiator. AIOs are significantly cheaper, easier to install, simpler to work with, and don't end up requiring a bunch of small expensive attachments. You buy it, install it, and you're done. They have limited lifetimes due to coolant loss, though I wouldn't be at all surprised if someone brought out a refillable AIO. They're comparable to the best air coolers with regards to cooling capability, though they'll likely run quieter.
• This is an expensive hobby. I'm lucky enough to have essentially unlimited funds, as I'm a software engineer of twenty+ years with no dependents (this is a radically different situation from when I was growing up, and essentially had to collect components tossed by school). If you can't make a $100 purchase without thinking about it, you probably don't have the money for watercooling.