Warm water: what W3 and W4 actually buy you

Every thermal engineer's instinct is that colder is better. In a data centre loop the opposite is usually true, and the reason has nothing to do with the silicon.

The chiller is the expensive part

Supplying 15 °C water requires mechanical refrigeration nearly all year. Supplying 40 °C water usually does not: a dry cooler rejecting heat to a 30 °C ambient still has a comfortable approach temperature. Removing the chiller from the majority of operating hours is the single largest efficiency gain available in the cooling system, and it is larger than any plate-level improvement.

What the silicon gives up

Raising supply temperature raises junction temperature one for one, all else equal. Whether you can afford that depends entirely on the headroom between your current junction temperature and the point at which the part throttles.

This is where cold plate quality converts directly into facility savings. Fourteen degrees of margin bought by a better plate can be spent on fourteen degrees of warmer supply water — and it is worth far more there than it is as unused headroom on the die.

Where it stops

Above roughly 45 °C supply, three things start to bite: the margin disappears on the hottest packages, elastomer and fluid life shortens measurably, and the room becomes unpleasant to work in. Heat recovery arguments push in the other direction, because useful heat wants to be hotter still.

The right answer is deployment-specific, and it is a joint decision between the silicon, the loop and the building. It should be made once, deliberately, at design time — not discovered during the first heatwave.