>875867
Resistive heaters are virtually 100% efficient (ignoring generation and transmission losses), but heat pumps beat that by absorbing heat from outside your house.
>In heating mode, heat pumps are three to four times more effective at heating than simple electrical resistance heaters using the same amount of electricity."
>https://en.wikipedia.org/wiki/Heat_Pump
For your idea to be effective, it would only make sense to make use of computers that are already going to be wasting energy, not installing new ones for the purpose of heating, unless of course the ROI from selling CPU cycles is >300-400%.
If powered by fossil fuels, it will only be environmentally effective until it reaches a point in obsolescence that it would be less polluting to replace your hardware with something more energy efficient. If it's using renewable energy it's not really an issue unless the energy could be better used to help others get off fossil fuels., or if the land could be put to better use.