Allowing the cooler to do its own thing fan speed-wise, in our LGA1155 test system, we saw a delta T of 42°C, which matched Corsair's H75 connected via PWM and also SilverStone's Tundra TD03. It's a pretty good result, especially as the larger and more expensive Kühler H20 950 only managed 1°C lower.
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In our toasty LGA2011 system, there was more of a difference here, although it still matched the H75 on its PWM setting with a delta T of 51°C. However this was 4°C warmer than the Kühler H20 950 on its extreme setting and the SilverStone Argon AR01 also managed to beat it, with many of the other full-height liquid cooler shaving a few degrees off too.
Cooling in general, was good, then but there was a slight issue with noise. The fan spins up quite aggressively when the CPU is put under heavy load - it's not quite as deafening as the Kühler H20 950 on its extreme setting but you're certainly going to know it's there. We also tried using the large 120mm standoff mount to see if this made any difference to cooling. The difference was just 1°C, with the standoff performing better, although this is within our margin of error.
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Performance wise, the Kühler H20 650 performed better than expected given its diminutive size, although this does come at the expense of noise - it's louder than Corsair's H75 for example. It keeps up with more expensive coolers on LGA115X systems although betrays its skinny radiator on LGA2011, where there's more heat to deal with. However, the extra grunt provided by its powerful fan does keep things in check quite nicely so we have no qualms about recommending it as a cooler for both systems. If you've got a 120mm fan mount nearby, then on a size to cooling ratio, the Kühler H20 650 is mightily impressive and will likely appeal to those concerned about memory module or VRM heatsink compatibility, especially on mini-ITX motherboards or indeed in small cases.
However, there are better value options if you are prepared to go for air cooling - the SilverStone Argon AR01
performs similarly and costs less than £30 while Thermalright's Archon SB-E X2
costs the same and is much quieter under load. However both the latter are much larger and more difficult to install. As ever, the answer is likely to include some deliberating over price, noise and cooling; if your heart is set on an all-in-one liquid cooler and you're not too fussed about fan noise, the Kühler H20 650 certainly won't disappoint. However, it would have to be a little quieter and perform a little better to get an award.
Intel LGA1155 Score
Intel LGA2011 Score