> I wonder how practical and safe it would be to build fusion pants close to city centers in order to harvest the excess heat for district heating
The cost/benefit for doing this seems pretty similar between fusion as gas power. We don't usually do this with gas, so I guess it's probably not viable for fusion.
Combined heat and electricity production is uncommon in the US, but much more so in Europe. Especially in the Baltics, Scandinavia and the Netherlands, non-CHP generation is rare. Related: higher energy cost, and elaborate local heat distribution networks.
Many cities/towns in the USA have small power plants in them (typically associated with a University, large hospital system, or central business district) which "sell" not just power, but also hot water, and steam. The steam is typically used to heat buildings. Google for "$CITY steam tunnels" or "$CITY CHP plant" to find these in your area.
San Francisco has[0][1][2][3] at least five combined heat and power plants that generate electricity and also sell steam to neighboring buildings via 72,000 feet of pipes.
I worked at a privately-owned for-profit "factory" in Santa Monica whose primary product was chilled water (their other product was warm water). They built pipelines to nearby large buildings and sold chilled water to them.
0: https://cordiaenergy.com/locations/san-francisco-3/ (2 for-profit CHP plants)
1: Skanska (for-profit)
2: San Francisco General Hospital
3: Apparently there are some "Muni" CHP plants scattered about SF as well (publicly-owned)