... system used for central heating for nearby houses or office buildings. 40-60 degrees celsius through radiators makes sense. ...
Forced hot water heat temp is 82-98C. 40-60 for radiant heat (not really common).
Would have to be fairly special houses -- central heating (for apartments) usually means *steam* heat (convenient, because no return pipes with steam).
That's part of the problem. It's difficult to retro-install the systems, you basically have to build the neighborhood at the same time as you build the data center. I can't find where I got this from but i think they might have used underfloor heating.
I'd like to chime in on this. without an expensive series of closed loop systems or a constant source of cold freshwater, you're gonna need a system which utilizes the Latent Heat of Vaporization to get enough heat out to work and be cost-effective. That's going to be difficult to do when your coolant has a boiling temp of 100 degrees Celsius at 1 atm pressure. It's doable, but there's no money in it.
You'd need a secondary coolant system with some kind of refrigerant in it with a much lower boiling temp, a heat exchanger, pumps, valves and piping. Then you need a power source for the pumps. ROI on this kind of investment is calculated in decades, not years.
I was a nuke plant operator and you constantly have to explain to engineers that something being technically possible and being economically viable are two entirely different things.