A proper heat pump is never less efficient than an electric resistance. Soil, at a depth of only around a few meters, rarely exceeds 10C, even in summer. That

A proper heat pump is never less efficient than an electric resistance. Soil, at a depth of only around a few meters, rarely exceeds 10C, even in summer. That gives you a 45C differential for your boiler in the worst of worst cases with a ground heat exchanger, which is more than enough for best-case efficiency, year long. This will be less expensive than a crypto in material cost very rapidly.