Well this is interesting

Heat output of a single PC is going to be too variable to be useful for anything other than basic space heating - your outlet temperatures won't get high enough to be useful unless it's running flat out for an extended period of time, the average temperatures will be so low that they'd only be marginally useful even for heating. Not to mention the challenges of managing the /inlet/ temperatures when you're trying to extract useful heat energy across a fairly wide range of temperatures.

This is another situation where you need to do things in bulk for it to be useful - the heat energy byproduct of a datacentre is going to be far more consistent, and therefore much easier to work with even if the actual outlet temperatures aren't really high.