The first sentence in the second paragraph explains that the author has a heat pump and what it does for them. The article then goes on to explain where the miners fit in.
Many heat pumps take heat from the ground to pre-heat (in winter) or pre-cool (in summer) the outside air before sending it to the heat pump but that would have been too expensive for me
The expensive part is laying all the required pipe underground to be able to do this. His heat pump is in place and working, it is how he pre-heats (or cools) the air going into the heat pump that is the issue in play.
The expensive option (digging up and installing underground piping) is what he doesnt use, he uses the cryptocurrency miners to do the pre-heating.