Earlier quoted context omitted.
For about the zillionth time, space heating with Bitcoin mining is about one-quarter as energy-efficient as using an actual heat pump and so the “we can heat buildings with it!” argument does not actually excuse the massive energy use at all.
Bitcoins use an entropy increasing resistance hash function, which makes it terrible for heating and cooling. There's a reason why we should use FedCoin instead of Bitcoin or cash or gold.
If you put 1000W of electrical energy into a Bitcoin miner, you get 1000W of heat out the far end. It's the same efficiency as a resistive element space heater, just has done some work (useful or not, it's still work) on the way through.
Same thing for a computer running BOINC or Folding@Home. Power goes in, heat comes out, but you've done something useful in the gap.