[Syonyk]

It... what?

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.

[imtringued]

Congratulations, you have reduced your heating bill by $x! You can now spend $x on mining and get more Bitcoin!

[Syonyk]

I knew someone last winter, during the spike in Bitcoin prices, who had done the math and figured out that heating on some older Bitcoin miners was profitable. Not only was the value of Bitcoin produced greater than his power cost (not everyone has CA power costs), it meant he wasn't paying anything separately for heat. I think he got them for very little because they "weren't profitable to run" when they were being sold.

I typically heat my office (off grid solar shed) in the winter on waste compute - I've got a few computers in here that run Folding@Home/BOINC and I just run them if I've got surplus power. I have propane backup for the really dark grey days ("generator days" if they extend past about a day - it's cheaper to run a gallon of gas through an old generator than to radically expand my battery bank), but most of my winter heat comes from running F@H/BOINC. To the point that on a clear, sunny winter day, I have my window open and fans sucking cold air in or I'll roast in here. I can hold about 1.8kW of load no problem on a clear day.

Now that I have solar on the house, I've considered adding an old compute rig or two in there for winter heat. It's less efficient than our heat pump on warm days, but we also get some cold, grey, foggy days in which the heat pump likes to ice up badly. Normally, if the heat pump isn't keeping up with things, the thermostat will call for the backup coils (which are just big resistors) to aid, and they're the same efficiency at turning electricity into heat as a couple computers. Old Xeons aren't amazingly power efficient, but you can also get them for free or nearly so and throw compute at your preferred projects.

One can optimize for a wide variety of things - energy spent, compute performed, Bitcoin mined, minimum cost, etc. It works better, and is a lot more efficient, if you look at everything as a system and integrate it. I don't run BOINC tasks on my homeserver when we're in air conditioning season, but I light up a couple cores of them in the winter.